GROWTH INCREMENT ANALYSIS OF ARCHAEOLOGICAL
FRESHWATER MUSSELS FROM INTERIOR NORTHERN
CALIFORNIA
______
A Thesis
Presented
to the Faculty of
California State University, Chico
______
In Partial Fulfillment
of the Requirements for the Degree
Master of Arts
in
Anthropology
______
by
Deanna M. Commons
Spring 2010 GROWTH INCREMENT ANALYSIS OF ARCHAEOLOGICAL
FRESHWATER MUSSELS FROM INTERIOR NORTHERN
CALIFORNIA
A Thesis
by
Deanna M. Commons
Spring 2010
APPROVED BY THE INTERIM DEAN OF THE SCHOOL OF GRADUATE, INTERNATIONAL, AND INTERDISCIPLINARY STUDIES:
______Mark J. Morlock, Ph.D.
APPROVED BY THE GRADUATE ADVISORY COMMITTEE:
______Frank E. Bayham, Ph.D., Chair
______David Ayers Eaton Jr., Ph.D. ACKNOWLEDGMENTS
First and foremost, I owe my deepest gratitude to Dr. Frank Bayham, my thesis committee chair, for his encouragement, patience, and guidance throughout this process. His early suggestions led me to develop this thesis, and his unending support and thoughtful insights helped me to produce this paper. Throughout my years in graduate school, I gained a tremendous wealth of information and important archaeological skills from his archaeology seminar on human behavioral ecology, lecture on cultural resource management, and zooarchaeology class. He has been a great friend and mentor, and I am forever indebted to his kindness.
Secondly, I would like to thank Dr. David Eaton for reading the many chapter drafts and offering me valuable advice. His “outside” perspective greatly improved the quality of my thesis.
I would like to thank Dr. James C. Chatters for taking the time to demonstrate his shell growth increment measurement method as well as providing me with a copy of his 1987 report. His technique has the potential to greatly benefit the archaeology community and I hope he receives the recognition he deserves for establishing a valuable alternative seasonality estimation method.
I would also like to acknowledge Dr. Eric Bartelink for his recommendations regarding my statistical analysis. In addition, my gratitude extends to Kevin Dalton and the Archaeology Laboratory for the many hours of use of the equipment.
iii I would like to thank Dr. Greg White of Pacific Legacy for allowing me use the mussel specimens from the two rockshelters. I am grateful for all the opportunities he offered and the experience I gained working under his leadership. He provided me not only with a wealth of knowledge, but guidance and generosity.
I especially would like to thank my husband Michael G. Commons and hydrologist Brian Rasmussen for all their time and effort put into collecting my mussel samples. If it was not for their eager willingness to trudge through the many creeks and rivers in northern California then my thesis would not have been made possible. In addition, I must thank Michael for his ending encouragement and support throughout this process as well as his cartographic contribution.
Lastly, I would like to thank all my friends and family for their love and support throughout this process.
iv TABLE OF CONTENTS
PAGE
Acknowledgments ...... iii
List of Tables...... vii
List of Figures...... viii
Abstract...... ix
CHAPTER
I. Introduction: Problem Orientation and Rationale ...... 1
The Potential Value and History of Chatters’ Increment Measurement Technique...... 3 Research Design ...... 6 Thesis Organization...... 12
II. Seasonality Studies and an Overview of Shell Growth Increment Measurement Research...... 14
Definition and Estimation of Seasonality...... 15 Shell Growth Increment Measurement Research...... 20 Chatters’ Increment Measurement Technique...... 29 Summary...... 33
III. Two Freshwater Mussels and Seasonality of Northern California...... 34
Freshwater Mussels in Northern California ...... 35 Environment ...... 41 Cultural Setting...... 42 Sentinel Bluff and Paynes Creek Rockshelters ...... 44 Summary...... 48
v CHAPTER PAGE
IV. Assessment of Chatters’ Increment Measurement Technique ...... 49
Step 1: Obtaining a Control Sample of Margaritifera falcata and Collection Plan...... 50 Step 2: Application of Chatters’ Increment Measurement Technique to the Control Sample...... 51 Step 3: Statistical Comparison of Growth Indices ...... 53 Results Summary...... 54 Summary...... 56
V. Application of Chatters’ Increment Measurement Technique to Two Archaeological Collections of Freshwater Mussels...... 58
Sample Selection and Approach...... 59 Results ...... 63 Summary...... 71
VI. Discussion...... 74
Potential of CIMT...... 75 Problems with CIMT...... 76 Determination of Season of Habitation ...... 79 Summary...... 82
VII. Conclusions ...... 84
Project Statistics ...... 84 Further Research...... 85
References Cited...... 86
vi LIST OF TABLES
TABLE PAGE
1. Growth Index Data from Gonidea angulata Control Sample ...... 55
2. Mean Growth Index and Standard Deviation for Control Sample and Chatters’ Study...... 55
3. Valve Count and Total Shell Weight Data for Unit 1 from Sentinel Bluff Rockshelter...... 61
4. Valve Count and Total Shell Weight Data for Unit 2 from Sentinel Bluff Rockshelter...... 62
5. Valve Count and Total Shell Weight Data for the Back Wall Complex from Paynes Creek Rockshelter ...... 63
6. Left Valves Used from the Excavated Units from Sentinel Bluff and Paynes Creek Rockshelters...... 64
7. Valve Count Totals and Percent Used from Sentinel Bluff and Paynes Creek Rockshelters ...... 65
8. Month of Death Death for Archaeological Margaritifera falcata Specimens from Unit 1 of Sentinel Bluff Rockshelter ...... 66
9. Month of Death Data for Archaeological Margaritifera falcata Specimens from Unit 2 of Sentinel Bluff Rockshelter ...... 67
10. Month of Death Data for Archaeological Margaritifera falcata Specimens from the Back Wall Complex of Paynes Creek Rockshelter...... 70
vii LIST OF FIGURES
FIGURE PAGE
1. Inner and Outer Structure of the Left Valve of a Gonidea angulata Specimen...... 22
2. Annual Growth Lines at the Terminal End of the Resilial Tuberosity of a Gonidea angulata Specimen ...... 31
3. Workstation for Applying Chatters’ Increment Measurement Technique...... 32
4. Taxonomic Classification of Two Northern California Freshwater Mussels: Margaritifera falcata and Gonidea angulata ...... 35
5. Illustration of Margaritifera falcata Right Valve with Shell Features Identification ...... 37
6. Illustration of Gonidea angulata Left Valve with Shell Features Identification...... 40
7. Archaeological Site Location Map of Sentinel Bluff and Paynes Creek Rockshelters...... 45
8. Approximate location of the Paynes Creek Rockshelter...... 47
9. Sentinel Bluff Rockshelter, Unit 1: Growth Indices Clustered by Month of Death for Margaritifera falcata...... 68
10. Sentinel Bluff, Unit 2: Growth Indices Clustered by Month of Death for Margaritifera falcata...... 69
11. Paynes Creek Rockshelter, Back Wall Complex: Growth Indices Clustered by Month of Death for Margaritifera falcata...... 71
viii ABSTRACT
GROWTH INCREMENT ANALYSIS OF ARCHAEOLOGICAL
FRESHWATER MUSSELS FROM INTERIOR NORTHERN
CALIFORNIA by
Deanna M. Commons
Master of Arts in Anthropology
California State University, Chico
Spring 2010
The purpose of this study is to evaluate the accuracy and reliability of a shell growth increment measurement technique that was developed by James C. Chatters for use on modern and archaeological freshwater mussel specimens. Freshwater mussels have been an important food resource for Native American groups in Northern Califor- nia for thousands of years. The freshwater mussels uncovered at archaeological sites can be used to estimate seasonality. While four other shell growth increment measure- ment methods have been developed and used more widely, these techniques lack appli- cability to archaeological specimens and result in inaccurate estimations.
To evaluate Chatters’ increment measurement technique, I applied the method to a sample of modern freshwater mussels. This was to determine if the collec- tion date of the sample of mussels aligned with the results from the application of the
ix technique. As the results were statistically similar, Chatters’ increment measurement technique was determined an accurate means for assessing seasonality.
This technique was then applied to a collection of archaeological freshwater mussels from two rockshelter sites in northern California in order to assess the applica- bility of the method on archaeological specimens, as well as to determine season of mussel harvest and site habitation. The results further validated the accuracy of Chat- ters’ increment measurement technique on archaeological mussels as well as demon- strated prehistoric mussel procurement to be almost exclusively limited to the late sum- mer and fall months. Season of site habitation also coincided with this data, although additional seasons of habitation were also possible and additional faunal material must be examined to gain a better estimation.
Through the analysis of both the modern and archaeological samples of freshwater mussels, Chatters’ increment measurement technique was determined an ac- curate and reliability means to estimate seasonality. While the technique lacks wide- spread publication and adoption, it has the potential to greatly benefit both the field of seasonality estimates and archaeology.
x
CHAPTER I
INTRODUCTION: PROBLEM
ORIENTATION AND
RATIONALE
Within the field of archaeology, seasonality refers to the time of year in which a particular event or activity took place. Seasonality is typically estimated through the analysis of faunal and floral material. Seasonality studies are then used to evaluate prehistoric settlement patterns, economic procurement activities, and mobility (Monks
1981). Freshwater mussels are ideal candidates for seasonality studies as the shell is not reabsorbed after deposition, providing a detailed life and environmental history (Eugster
1990:85). Season and even month of death can be determined through the analysis of the annual shell growth increments. While mussels have been used in seasonality studies since the 1960s, analysis of annual growth lines on mollusks have continued historically since the beginning of the twentieth century in biological, ecological, and evolutionary research (Claassen 1986:21).
There are four primary methods for measuring shell growth increments: shell annulus analysis or growth ring method; thin-sectioning; thick-sectioning; and acetate peels (Chatters 2007; Eugster 1990; Neves and Moyer 1988). The growth ring method measures the distance between the annually laid bands on the external surface of the shell, typically using a light to better illuminate the bands (Brousseau 1984: 233). The
1 2 other three techniques involve cutting a cross-section and, depending on the process, the shell is polished or acetone is applied to etch the surface and clarify growth lines. The thick-sectioning and acetate peel processes cut the shell in half, but thin-sectioning further slices the halved shell. These sections are then placed on a slide and observed under the microscope (Claassen 1998:155; Millstein and O’Clair 2001:158; Monks
1981:207-208; Rhoads and Pannella 1970:146; Ropes 1987:2-3).
Each of the commonly employed techniques have their strengths and weaknesses. The growth ring method is not destructive to the shell, but clarity of the annuli varies and accurate measurements cannot always be obtained if the shell is fragmented or of older age (Neves and Moyer 1988:179). While thin-sectioning is referenced as the more comprehensive and accurate method of the other three procedures,
Neves and Moyer (1988) have uncovered several shortcomings with the technique. The method is time consuming, where it takes between thirty minutes to a full hour to prepare one specimen, small shells often break as they are brittle, and larger shells sometimes required several cuts in order to fit onto the slide. All of these techniques are rarely applicable to archaeological specimen as they require a complete valve. Primarily fragments of shells are recovered during archaeological excavations, as a result of the thin and brittle nature of the shell. Casey (1986), for example, was unable to perform a shell growth increment measurement method on a collection of archaeological freshwater mussels from Kentucky as a result of the fragmentary state of the shells.
In the 1980s, Dr. James C. Chatters developed an alternative shell growth increment measurement technique, which I will hereafter refer to as Chatters’ increment measurement technique or CIMT. This technique is well-suited for the highly
3 fragmentary nature of archaeological freshwater mussel shells for it relies on and analyzes the relatively robust resilial tuberosity,1 a feature on the shell hinge that records the annual growth. Chatters introduced this technique in a series of cultural resource management publications and archaeological reports in Washington, Oregon, and northern California (Chatters 1986, 1987, 1995, 1997).
The technique itself if relatively straight-forward and effective. To prepare a specimen, the resilial tuberosity is lightly scraped to rid it of organic matter, and then a weak hydrochloric acid solution can be applied to accentuate the growth lines. The mussel is then propped onto a Unislide moveable stage with clay, which is attached to a magnetic encoder accurate to the nearest thousandth of a millimeter. The stage is positioned under a binocular microscope that is fitted with a cross-hair eye piece. The terminal year of growth is lined-up with the cross-hair and the width of this growth is measured along with the last full year of growth. The terminal growth is then divided by the last full year of growth and the resulting number multiplied by 100. This resulting percentage is known as the growth index and is compared to an annual growth curve for mussels within that climate zone to determine month of death.
The Potential Value and History of Chatters’ Increment Measurement Technique
This research report critically evaluates Chatters’ increment measurement technique. This method has the potential to change the way seasonality tests are performed on archaeological freshwater mussels. CIMT is not species-specific and can actually be used on mussel populations dispersed across the United States and throughout
1 The term “resilial tuberosity” was coined by Chatters.
4 the world, where archaeologists of several geographic regions could potentially benefit from this method. The method requires a feature of the mussel called the resilial tuberosity, which is attached to the resilium. The resilium is a component of the hinge ligament, a feature within the bivalves of the superfamily Unionoidea that contain roughly 800 mussel species worldwide (Bogan 2008:142).
Compared to thin-sectioning or acetate peels, Chatters’ increment measurement technique is relatively simple and easy to perform, without extensive preparation. It does require some specialized equipment yet provides clear results, and is ideal for archaeological shell. This method does not use the entire bivalve shell, but rather a small portion of the bivalve hinge. The majority of archaeological mussel specimens are fragmented due to the thin nature of the shell and the resulting taphonomic processes.
CIMT uses the feature on the dense hinge component that is more apt to be intact in archaeological freshwater mussels.
CIMT is a process that requires some knowledge in shell anatomy, and a degree of precision and focus in recording and identifying shell growth increments.
Through continued experience, a proficient understanding and comprehension of shell growth increments will produce a precise application of CIMT and thorough results.
Applications and citations of CIMT for analyzing shell growth increment patterns began in the 1980s and continued into the following decades (Bayham and
Johnson 1990; Broughton 1994; Chatters 1986, 1987, 1997, 2007; Chatters, Butler et al.
1995; Chatters, Campbell et al. 1995; Eugster 1990; Kelly et al. 1987; Tyree 1998).
While the application of this technique has been effective in determining seasonality, widespread knowledge and a critical evaluation of the technique is nearly non-existent.
5
I examined the cultural resource management reports at the Northeast
Information Center (NEIC) in Chico, California to determine where most of the literature regarding this method resided and the extent of use and publication of Chatters’ increment measurement technique. The NEIC has roughly 10,000 cultural resource management reports and over 7000 are listed in a database. An intensive search in the database revealed only three reports that had commented on or used Chatters’ increment measurement technique (Bayham and Johnson 1990; Chatters 1997; Kelly et al. 1987).
Primarily, the references or applications of CIMT reside in cultural resource management reports of the Pacific Northwest and a few scholarly articles and lacks widespread distribution. Even in Claassen’s 1998 book Shells, CIMT was not referenced under the “Incremental Growth Technique” section. While Chatters (1987) outlined his increment measurement technique in moderate detail, only two individuals have published using this technique, Dr. James C. Chatters and Susan Elizabeth Eugster
(Chatters 1986, 1987, 1997, 2007; Eugster 1990), although Chatters (1986) indicated that
Debbie Dove performed the shell growth increment measurement method on the archaeological collection from the Wells Reservoir project.
Prior to beginning this project, I had conducted a preliminary analysis on several archaeological Margaritifera falcata shells from the Paynes Creek Rockshelter site in the spring of 2008. After following the procedure Chatters (1987) and Eugster
(1990) outlined, an initial synopsis of site seasonality was formulated. This initial study led to further research on Chatters’ increment measurement technique and the development of this project.
6
Research Design
This research report is designed to assess CIMT. While Chatters has developed and validated his technique, there has been few that have adopted and used the method. The goal of this study is to determine the accuracy and reliability of Chatters’ increment measurement technique and the applicability to archaeological specimens, as well as to better understand the technique and possibly propel the use of it in the field of archaeology. The first part of this thesis is an assessment of CIMT through the application to a control sample of modern freshwater mussels. The second portion of the thesis is an evaluation of CIMT through an application to an archaeological collection of mollusks in order to determine site seasonality. Additionally, throughout both phases, I will also examine the strengths and limitations of the method. In the next few pages I will present the research questions and the proposed methodology that will address and answer the questions.
Phase I
The first component and purpose of this research project is to: Assess the accuracy and reliability of Chatters’ increment measurement technique and growth curve that he developed and employed in a series of archaeological reports and papers.
To accomplish this, CIMT will be applied to an independent body of mollusks. Samples of freshwater mussels will be collected and the growth index configured for each collection date. These indices will then be compared to Chatters’ annual growth curve graph for Margaritifera falcata (Chatters 1987:15). Chatters (1987) developed this growth curve by taking monthly collections of Margaritifera samples from the Yakima River, in Washington. He calculated the mean growth index for each
7 collection date and plotted this data on a graph that shows the expected percentage of new growth throughout the year. Next, the growth indices from the collected sample and the growth curve will be analyzed to determine if they are statistically similar and whether or not CIMT is an accurate and reliable method. These steps are outlined in further detail in the following pages.
Step 1: Obtaining a Control Sample of Margaritifera falcata Specimens and
Collection Plan
The first step in evaluating the accuracy and reliability of CIMT is to obtain a control sample of freshwater mussels. The control sample is a collection of modern freshwater mussels with a known collection/death date. By having this known date, the growth indices of the control sample can be calculated and then compared to the growth indices from Chatters’ growth curve (Chatters 1987:15).
I originally intended to employ a more unbiased sampling procedure with mussel collection each month throughout the year, but safety concerns dealing with water flow deterred that plan. Instead, I will collect the mussels by convenience sampling, a nonprobability sampling method, to maximize the time and effort placed on identifying productive beds of mussels. Convenience sampling involves grabbing whatever specimens are available (Bernard 2006:192). I acknowledge that the sample will be biased, but “nonprobability samples are always appropriate for labor-intensive, in-depth studies” (Bernard 2006:186). Convenience sampling can produce an adequately representative sample for the research purposes in this thesis. While this convenience sample differs from an ideal, random sampling procedure, I am not working with a diverse population. I am selecting mussels of one species and these mussels, as well as
8 freshwater mussels throughout the Pacific Northwest demonstrate similar growth patterning (Chatters 2007). Therefore, I assume that the selected mussels are an accurate representation of a larger population.
Twenty Margaritifera falcata specimens will be collected in one month intervals from June through September from the same mussel bed in one of the water concourses of northern California. The mussels will be collected in the summer months as the water levels drop making it easier to access the habitat of the mussels. The collection of the control sample will be focused in the Pit River in northern California, as it has been documented to contain populations of Margaritifera falcata (Taylor 1981:143).
Furthermore, Chatters (1987) collected a sample of Margaritifera falcata specimens from this river and the growth indices aligned with his annual growth curve for the
Margaritifera specimens from the Yakima River, in Washington.
Step 2: Application of Chatters’ Increment Measurement Technique to the
Control Sample
Once a control sample of freshwater mussels is collected, CIMT will be applied to each mussel specimen. This process will follow the previously described procedure as outlined by Chatters (1987:10-12) and Eugster (1990:89).
Step 3: Statistical Comparison of Growth Indices
The growth index, the percentage of new growth represented in the final year of growth, will be calculated for each mussel and the mean growth index of the mussels from each collection date will be determined. Once the mean growth indices are calculated for each collection date they will be compared to Chatters’ mean growth indices of the respective dates (Chatters 1987:15). Independent t-tests will be performed
9 determining the statistical significance between the mean growth indices from the control sample to Chatters’ of the respective dates. I hypothesize there should not be a difference between the means. This result would demonstrate an accurate application and replication of CIMT and would determine it to be an accurate and reliable method for assessing seasonality with freshwater mussels.
Phase II
Phase I assesses Chatters’ increment measurement technique through the analysis of a control sample of modern freshwater mussels. Positive results from the control sample will validate CIMT as an accurate and reliable means for assessing seasonality and support the use of the method on archaeological samples of freshwater mussels. Unlike modern samples, archaeological freshwater mussels are fragmentary, fragile, and have been exposed to post-depositional processes for potentially thousands of years. While the other four methods for measuring shell growth increments are often not suitable for archaeological specimens, CIMT is specifically designed for archaeological freshwater mussels. Phase II is an assessment and application of Chatters’ increment measurement technique on an archaeological collection of Margaritifera falcata specimens. The first question of Phase II is:
1. How applicable is Chatters’ increment measurement technique to archaeological freshwater mussels?
In order to evaluate this question, Chatters’ increment measurement technique will be applied to archaeological freshwater mussels from two rockshelter sites in northern
California. The seasonality analysis of the archaeological mussels will also answer the second question of Phase II:
10
2. What time of year was the prehistoric population harvesting the freshwater mussels and correspondingly occupying the sites?
For thousands of years, freshwater mussels have been exploited by Native
American peoples of California. Seasonality studies on freshwater mussels provide pertinent information regarding settlement practices and season of economic procurement. The archaeological material from the two sites has been analyzed minimally and many questions lay unanswered pertaining to prehistoric settlement and season of mussel procurement.
Susan Eugster (1990) studied a collection of Margaritifera falcata specimens from an archaeological site along the Sacramento River near Chico, California, in relatively close proximity of the two rockshelters. Her results indicated that prehistoric mussel collection occurred in the late summer and fall months when conditions for harvesting were optimal.
I hypothesize the mussels at the two rockshelters were harvested in the summer and autumn months when mussels were more easily accessible. In Northern California, water levels rise considerably during the winter and spring months when heavy rains plague the region. These high water levels inhibit accessibility into freshwater mussel habitats. As water levels drop in the warmer, drier times of the year, procurement of the freshwater mussels is more convenient. In order to evaluate the questions from Phase II, there are three steps involved in the process.
Step 1: Preparing the Archaeological Freshwater Mussels
Two archaeological rockshelter sites, Paynes Creek, dating to roughly 5100 years
B.P., and Sentinel Bluff, dating to roughly 2100 years B.P., in northern California,
11 contain samples of Margaritifera falcata. Excavated in 2005 and 1999 respectively, by
Dr. Greg White then of California State University (CSU) Chico, the archaeological shells have been sorted, catalogued and bagged by ten centimeter levels. The number of individual specimens that contain a portion of the resilial tuberosity and correspondingly have CIMT applied to them will be determined for right and left valves separately for each level. All levels with shell material will be examined. The left valves will make up the sample that will have CIMT applied.
Step 2: Application of CIMT to the Archaeological Freshwater Mussels
Chatters’ increment measurement technique will be applied to the archaeological shell specimens that contain an intact terminal end of the resilial tuberosity following the same procedure that was conducted on the control sample, as described earlier. The method will be applied only to the left valves in order to eliminate the chance of a mussel being analyzed twice.
Step 3: Aggregation of the Results
Once CIMT is performed on all useable mussel specimens, the results will be evaluated and compared to Chatters’ growth curve (Chatters 1987:15). (CIMT will not be successfully applied to all valve fragments that contain the resilial tuberosity, as post- depositional processes may have damaged the growth lines beyond recognition.) The valve count for analyzed specimens as well as the count of specimens used will be calculated. The valve count data will provide information to better understand the applicability of CIMT on archaeological freshwater mussels.
For each mussel specimen examined, the growth index will be calculated. The data from Paynes Creek and Sentinel Bluff Rockshelters will be tabulated and graphed to
12 determine the time of year the prehistoric populations were harvesting the freshwater mussels and correspondingly occupying the sites. While Paynes Creek Rockshelter does not have any distinguishable cultural components, Dr. Greg White noted three distinct components from Sentinel Bluff Rockshelter (White 2005). The results from Sentinel
Bluff Rockshelter will be compiled and analyzed per cultural component.
Thesis Organization
In Chapter II (Seasonality Studies and an Overview of Shell Growth Increment
Measurement Research), I discuss archaeological seasonality primarily focusing on the principal direct methods for estimating seasonality. I then discuss the history of shell growth increment measurement research and detail the four main measurement methods: growth ring method; thin-sectioning; thick-sectioning; and acetate peels. Finally, I describe the methodology and applicability of Chatters’ increment measurement technique.
Chapter III (Two Freshwater Mussels and Seasonality of Northern California) begins with a description of two mussel species found in northern California and to which
CIMT has been applied: Margaritifera falcata and Gonidea angulata. Next, I discuss the environment and cultural setting of a particular region in the lowlands of northern
California that the freshwater mussels inhabit. This is followed by a description of the age, deposition, and location of two archaeological rockshelter sites found within this region and the presence of freshwater mussels in the assemblage.
Chapters IV (Assessment of Chatters’ Increment Measurement Technique) and
V (Application of Chatters’ Increment Measurement Technique to Two Archaeological
13
Collections of Freshwater Mussels) contain a detailed overview of the materials and methods as well as the results of my analysis. Chapter IV focuses on my assessment of
CIMT as I describe the approach to sampling a control population of mussels, the application of the technique, the statistical analysis, and conclude with the results.
Chapter V describes the application of CIMT to the archaeological collection of mussels from the two rockshelter sites and concludes with the results from the analysis.
In Chapter VI (Discussion), I first describe the potential advantages and value of Chatters’ increment measurement technique. Next, I discuss the limitations I encountered while performing CIMT concerning growth line visibility and the resulting reduction of the sample size. Lastly, I discuss the seasonality of site habitation at the two rockshelter sites by incorporating the principles of human behavioral ecology.
In Chapter VII (Conclusions), I summarize my findings and discuss future research that may provide a clearer overall understanding of site seasonality at the two rockshelters.
CHAPTER II
SEASONALITY STUDIES AND AN
OVERVIEW OF SHELL GROWTH
INCREMENT MEASUREMENT
RESEARCH
Seasonality studies provide important insight to prehistoric economic and settlement behaviors through the analysis of archaeological remains of flora and fauna material (Eugster 1990; Leigh 1998; Monks 1981; Pike-Tay 1991; Pike-Tay and
Cosgrove 1992). The measurement of annual growth increments in mollusks is a valid direct method for estimating seasonality (Monks 1981). The annual deposition of shell creates distinguishable growth increments that are visible on the outer shell. These growth increments can be measured and compared to the previous years of growth to establish the season and month of mussel death.
This chapter begins with a focus on the definition of seasonality, followed by the common direct methods to assess seasonality. I then address the early literature and studies conducted on freshwater mussels to demonstrate the historic use and evaluation of mussel growth and morphology. Next, I describe the four main methods for measuring shell growth increments. Lastly, I explain Chatters’ increment measurement technique.
14 15
Definition and Estimation of Seasonality
Seasonality, in an archaeological context, refers to a particular time of the year in which an event or human activity takes place. Seasonality studies provide information in regards to subsistence strategies, mobility, and settlement patterns of prehistoric populations. The analysis of archaeological faunal remains is one of the more widely adopted approaches in investigating seasonality. Living organisms and the environment have an intricate relationship where seasonal fluctuations are reverberated among the flora and fauna (Monks 1981).
Season of death is measured through either calendrical or sequential dating.
Sequential dating will date an activity according to a season: spring, summer, fall, or winter. The use of seasons can be vague and ambiguous as season length and time of occurrence vary in different regions of the world. Therefore, calendrical dating, the use of the twelve month calendar, is typically used. Calendrical dating provides a more standardized framework when estimating the time a particular event took place.
Knowledge of a species cycle of growth and development is pertinent when attempting to date season of death (Monks 1981:178).
Monks (1981) divided the means of estimating seasonality using floral and faunal material as well as cultural and environmental information into two categories, indirect and direct methods. Indirect methods focus on the interpretation of the
“integration of cultural and environmental variables” (Monks 1981:217). These methods include matrix granulometry, soil analysis, mortality patterns and burial practices, settlement patterns and habitation structure orientation, artifact types and frequencies, and coprolite analysis. Direct methods include: the presence or absence of remains;
16 physiological events like epiphyseal fusion, tooth eruption and wear, antler growth, osteoporosis, and deposition of medullary bone; population structures of animal species; oxygen isotope analysis; and analysis of incremental structures in mammalian teeth and bone, fish, mollusks, and wood that distinguish annual cycles of deposit.
Indirect Methods
Unlike direct methods, indirect methods infer seasonality not from faunal or floral material, but from cultural and environmental data. These methods are an alternative to direct methods when seasonally indicative material is not present in the archaeological assemblage. Indirect methods are not widely practiced due to a lack of scientific testing and their assumptive nature. As a result, these methods should be employed in combination with or as a testable guide for direct methods.
Direct Methods
The direct methods are the more widely used approaches for estimating seasonality. These methods assess seasonality directly from the faunal and floral material.
Presence/Absence Method. The seasonal presence or absence method is the oldest and most common means for estimating seasonality among available species. This method investigates species biology and environmental relationships including such topics as migration, hibernation, and tolerance (Pike-Tay 1991). Primarily, the presence- absence method examines what resources are available during particular times of the year. If these resources are present in the archaeological record, inferences about seasonal activity in a particular region can be construed (Reitz and Wing 2008:262).
A local example would be the presence of the Bullock’s Oriole. This bird is present in northern California in the spring and summer months (Kemper 2001:21). The
17 presence of bones from this bird in an archaeological assemblage would indicate a spring or summer activity.
The “presence-absence data masks quantitative variation among seasonal indicators” and can potentially misconstrue seasonality assessments (Monks 1981:182).
For example, if within an archaeological collection there is a single element representing one season and the remaining components of the collection represent another season, forthcoming inferences that include the incorporation of both seasons should be tentative.
On the other hand, some species that are assumed present during a particular season may actually be available throughout the entire year (Reitz and Wing 2008:263). Furthermore, seasonality studies that include the presence-absence method must take into consideration that faunal material may have been transported (Pike-Tay and Cosgrove 2002:104).
Physiological Events. A Physiological event is often a seasonal or annual change that occurs within or on the skeleton. The development, maturation, and ossification of bones, the development of antlers, and the deposition of medullary bone are all examples of physiological events. The evidence, or the lack there of, of these events on the skeleton are used to estimate seasonality (Monks 1981:185). In the following paragraphs, the more commonly examined physiological events are explained.
Antler growth is a physiological event that can provide some means for estimating seasonality. Males of the Cervidae family grow and shed antlers annually.
Antlers are typically shed in early to mid winter and begin new growth sometime in the spring. Within an archaeological assemblage, seasonality can be inferred if the cervid remains have or are missing antlers. Although due to the great variation in the seasonal
18 shedding and growth of antler, this method is not the most reliable form for determining seasonality (Pike-Tay 1991).
Another physiological event is the deposition of medullary bone by females of certain bird species. The bone is deposited in marrow cavities particularly in the femur, tibiotarsus, and ulna. This occurs prior to nesting and provides added minerals for eggshell production. The medullary bone is then reabsorbed after nesting (Leigh 1998:
23; Lentacker and Van Neer 1996:491; Pike-Tay 1991:27). Not all birds deposit medullary bone and the timing of bone reabsorption varies, which can limit and misconstrue seasonality estimates.
Population Structure. The population structure method for assessing seasonality examines the sex and age of faunal remains and then compares these data to the sex and age of a living population (Monks 1981:211). For example, a high presence of adolescent deer femurs may suggest spring and summer seasons of deer hunting, when the young deer are maturing. Population structures falter in assessing seasonality if sample size is too small to be representative of the hunted population and if taphonomic forces or sampling techniques hinder the sample leaving a greater percentage of one age group than another (Pike-Tay 1991:24).
Oxygen Isotope Analysis. Oxygen isotope analysis is another method for estimating seasonality which measures oxygen isotopes in marine mollusks and compares these to seasonal water temperatures. If performed correctly, Monks (1981) proclaimed this to be one of the most effective ways to estimate seasonality. Yet, it is one of the more difficult procedures to accurately perform and requires sophisticated and specialized equipment to which many researchers do not have access.
19
Incremental Structures. Incremental structures are a form of growth patterning where the new addition of growth rests on the previous episode of growth. These structures follow cyclical patterning that provide for an estimation of season of death by comparing the terminal structure to the previous laid down ones (Monks 1981:193).
Incremental structures in mammalian teeth, bone, and antlers, as well as in fish and mollusk have been used to estimate seasonality
Mammalian teeth have been used to determine season and age of death (Leigh
1998; Pike-Tay 1991; Thomas 2003). The analysis includes the examination of seasonally deposited translucent and opaque increments in the cementum and comparing this to annual deposition cycles. Teeth have become a more reliable tool for studying seasonality as they are subject to little remodeling or resorption (Leigh 1998:26-27).
The use of mammalian bone as an indicator of seasonality is relatively rare.
While bone exhibits structural patterning, it is unclear whether this is from annual growth or other variables (Monks 1981:198). Bone is consistently remodeling and variations exist among species and skeletal elements of the rate of bone deposition (Leigh 1998:26).
Antler pedicels are the bony protrusion on the skull of cervids where annual antler growth takes place. A dark band of bone is deposited on the pedicel when the antler begins growing and a lighter band of bone is laid down as the antler matures.
Season of death is then calculated by the terminal band coloration. Bone and antler pedicel analysis are the least used means for estimating seasonality and, therefore, should be used to supplement more reliable methods (Leigh 1998:25; Monks 1981:198).
Otoliths, scales, operculae, and vertebrae are incremental structures in fish that are used to assess seasonality. While growth is seasonal, it is influenced by
20 environmental conditions, age, and nutrition. These variables may lead to abnormal growth fluctuations, where errors in seasonality estimates may arise (Pike-Tay 1991:30).
Fish elements are also subject to breakage and there is difficulty in interpreting growth increments (Leigh 1998:25).
Mollusks also have incremental growth lines that can be evaluated both internally and externally for seasonality studies. During analysis, a band within the growth demonstrates the annual cessation of growth. As with fish, various environmental factors can impact the growth cycle and careful attention must be paid to distinguish between annual cessation in growth oppose to a temporary disturbance line. Methods for evaluating annual growth in mollusks include an external examination of shell growth as well as cross-sections and acetate peels.
Mollusks are widely distributed in both marine and freshwater ecosystems and have been exploited both prehistorically and historically. A shell growth increment measurement method for freshwater mussels is the focus of this study and in the following pages I will explore and explain the history and current use of these methods in more depth.
Shell Growth Increment Measurement Research
Shell annuli provide pertinent information for age and growth research in freshwater mussels. The annuli, or growth rings, are examined both through macroscopic analysis of the external surface of the shell and microscopically through internal sectioning. The rings signify the cessation of growth during the winter months and are used as annual markers. Growth rings in freshwater mussels function as an important
21 research tool in archaeology (Chatters 2007, 1997, 1986; Chatters, Campbell et al. 1995;
Claassen 1986; Eugster 1990), ecology (Arter 1989; Coon et al. 1977; Ziuganov et al.
2000), climate change studies (Chatters, Butler et al. 1995), ecotoxicology (Dunca et al.
2005), malacology (Bauer 1992; Grier 1922; Hastie et al. 2000a, 2000b), and resource management (Chamberlain 1930).
Initial studies examining the correlation between the rings on freshwater mussel shells and annual growth patterns began in the middle of the nineteenth century.
By the early twentieth century, researchers began to ascertain the significance of the rings in relation to annual growth (Chamberlain 1930:714-715). There are four primary techniques for measurement and quantification of shell growth increments: analysis of external growth rings, thick-sectioning, acetate peels, and thin-sectioning (Chatters
1997:373; Chatters 2007:70).
Macroscopic Analysis of Shell Growth Increments The earlier studies of shell growth increment measurements on freshwater mussels developed in response to the shell button industry in the early part of the twentieth century (Coker et al. 1921; Chamberlain 1930). These earlier attempts at estimating age involved the analysis of growth bands on the external shell; this is known as the growth ring or annual ring method (Neves and Moyer 1988). The growth ring method measures the distance between the annually laid external annuli by transmitting a light through the shell to better illuminate the annuli. The terminal ring is measured and then compared to the average lengths of the previous annuli (Brousseau 1984: 233).
In the analysis of the natural history, life history, and structure of freshwater mussels, Coker et al. (1921) describe the shell structure and formation of the growth
22 lines. The shell is made up of four layers. The outermost layer is the periostracum (see
Figure 1) and is typically light to dark brown or black (Nadeau et al. 2005:34). This protein-enriched layer protects the shell from dissolution and displays annual growth
Hinge
Nacre
Umbo Periostracum
Mantle
Figure 1. Inner and outer structure of the left valve of a Gonidea angulata specimen. Photograph by Deanna M. Commons (not to scale).
bands (Bauer 2001:4). Following the periostracum is the thicker prismatic layer. It is comprised of “prisms of calcium carbonate set vertically to the surface” (Coker et al.
1921:129). Beneath the prismatic layer is the nacreous layer. This layer lines the inner portion of the bivalve and is composed of thin laminae that are positioned parallel to the
23 inner shell surface. The final layer is the hypostracum. This thin layer consists of laminae that intersect the laminae of the nacre (Coker et al. 1921:130). See Figure 1 for a visual of the outer and inner structure of a mussel shell.
The shell grows in thickness by the successive layering of laminae. The laminae are extremely thin where it takes thousands of layers to constitute an inch of shell. The periostracum, prismatic, and nacreous layers all contribute to the shell growth in length and breadth near the margin of the mantle. During arrested growth, the shell mantle actually withdraws into the shell. As growth resumes, the new periostracum and prismatic layers do not align with the older layers but rather overlap. The new layers begin growth behind the end of the old layers. When examining the outer shell through the growth ring method, the overlapping layers create a dark band called the growth ring
(Coker et al. 1921:131-132).
To gain a better understanding of growth rates among freshwater mussels,
Grier (1922) analyzed over twelve different mussel species from Lake Erie. This analysis used the growth ring method and shell length measurements while acknowledging that environmental conditions could produce false annuli and hamper results. Results demonstrated that mussels grew quicker in the first few years of life and growth is variable within and among species. Grier (1922) also noted that Lake dwelling species grow slower than river species.
Chamberlain (1930) conducted research in response to the heavy exploitation and depletion of freshwater mussels for the pearl button industry to better understand age and growth patterns in order to serve economic and conservation efforts. Using the growth ring method along with length and weight measurements, over 1,000 mussel
24 specimen of four different species were examined: Lake Pepin mucket (Lampsilis siliquoidea pepinensis), yellow sand shell (Lampsilis anodontoides), buckhorn
(Tritogonia verrucosa), and Pope’s purple (Unio popei). Results demonstrated a positive linear growth until the sixth year of life, when all the mussels demonstrated a decline in growth rate with less shell deposited annually. Chamberlain (1930:737) noted that the growth ring method was reliable and practical and the “major annual ring was rather easily differentiated from the other narrower interruption rings.”
Similar to previous research, Negus (1966) conducted a study on five unionid mussels from the River Thames in Reading, England, to gain a better understanding of growth and production patterns. The study examined weight and length measurements along with growth increment measurements obtained through the growth ring method.
Negus (1966) was able to conclude that growth rings are annually laid down during the winter months, the time of arrested growth. The annual rings “were used to interpret the growth history of each specimen” (Negus 1966:531) and warmer temperatures were observed to impact growth patterns. It was also concluded that growth patterns are species-specific and environmental factors influence each unionid species separately.
Coon et al. (1977) examined twenty-three species of unionid mussels from the
Upper Mississippi River focusing on growth and population change over time. The growth ring method and shell length and weight measurements were performed along with a reanalysis of data collected in the 1930s. Results demonstrated a sharp decline in mussel diversity and a change in frequency of the current populations. The data obtained from the growth ring method and length measurements show how variable growth is between species.
25
Downing et al. (1992), interested in the validity of external annuli as annual growth markers, marked 128 specimens of Anodonta grandis grandis and Lampsilis radiate siliquoidea and collected them between two to five years later. After analysis, results indicated that fourteen did not demonstrate annulus formation and forty had incomplete external annuli. One shell did not deposit any annuli in five years while one deposited four in only two years. The researchers concluded that external annuli are not a reliable means for estimating age and more studies should be conducted to test if annuli are formed annually.
Several variables inhibit effective use of the growth ring method. Erosion of the outer surface of the shell, the indistinctness among the bands on dark-colored valves, and the lack of definition among annuli near the margin of older mussels all hamper the use of the growth ring method. Growth lines are more defined in lentic species of freshwater mussels oppose to river dwelling specimens (Neves and Moyer 1988:179).
The presence of false annuli is one of the more debated variables inhibiting the growth ring method. False annuli result from an additional cessation of growth often due to some environmental variable or human interaction. Coon et al. (1977:280) and Neves and
Moyer (1988:184) experienced difficulty differentiating false from true annuli utilizing the growth ring method, yet Negus (1966:516) and Chamberlain (1930) could easily distinguish the false from the annual ring. Nicholson (1980:20) discovered that the results from external examination of growth lines do not necessarily correlate with the information provided by sectioning the shell and lack accuracy.
The early and middle part of the twentieth century brought great strides in research involving freshwater mussels, particularly through use of the growth ring
26 method. This method was commonly used in conjunction with weight and length measurement data. By the 1970s, alternative approaches to shell growth increment measurement involving microscopic analyses were being used and tested to combat the inaccuracies and ambiguities of the growth ring method.
Microscopic Analysis of Shell Growth Increments
Thin-sectioning, thick-sectioning, and acetate peels are more invasive shell growth increment measurement methods than the growth ring method. With these techniques, the shell is actually sliced, typically from the umbo to the ventral margin.
Depending on the method, further sectioning may take place or an application of acetone to better etch the surface. The cut surface is then observed under a microscope. These methods have become a popular means for assessing shell growth as true and false annuli are better distinguished, providing for a more accurate analysis of shell growth.
Thick-Sectioning. The thick-sectioning method is a relatively simple process that includes one basic step. The shell is cut into two halves. If the shell is in a poor state of preservation, it may be embedded in epoxy to prevent fracturing during sectioning.
However, the epoxy may hinder the readability of the shell and often poorly preserved shell is difficult to read anyways (Claassen 1998:155). The growth lines are then analyzed on the cut surface to determine if the shell was in a period of fast or slow growth. Fast growth represents the time of year of annual growth while slow growth is indicative of the temporary cessation period. “The percentage of fast growth individuals in a sample is compared to a control curve for the species under investigation to generate a seasonality estimate” (Chatters 1997:373-374).
27
Minimal information is provided on this method and few researchers use it.
Chatters (1997:374) insisted there are several limitations to this method. These limitations include the ambiguities and inaccuracies that are produced as well as the destructive nature to the shell.
Acetate Peels. The process of applying acetate peels to shell begins with embedding the bivalves in epoxy to reduce the occurrence of splintering or fracturing while sectioning the specimen. Once the epoxy hardens, the shell is sectioned using a low-speed and or diamond rock saw. The sectioning surface should bisect the growth lines at a right angle and on the plane of maximum growth. Next, the cut surface is ground and polished. Then, the sections are etched with a weak hydrochloric solution, and immediately washed and dried. Finally, acetone is applied to the cut surface along with a piece of sheet acetate. Once the acetone evaporates, the peel is removed from the shell and applied to a glass slide to then be examined through a microscope (Kennish et al. 1980:597-600).
Acetate peels have eliminated some of the inconsistency and inaccuracies of the shell ring method. Compared to thin-sectioning, which is presented next, it is a relatively easy process that only requires a single section and is not time consuming.
Moyer (1984:54), with his evaluation of several shell growth increment measurement techniques, indicated that the acetate peel process took only 20 to 30 minutes per shell, although this was not including the two to three hours it took for the acetone to dry.
The acetate peel method does suffer from some limitations. The method does not consistently provide reliable, quality peels for analysis (Moyer 1984:54). Moyer also
28 found only 70% of the prepared specimen were actually readable. This method is also destructive to the shell.
Thin-Sectioning. The thin-sectioning method has become the preferred technique for measuring shell annuli due to the consistency and accuracy of the process
(Clark 1980:603; Moyer 1984: 130); Neves and Moyer 1988:186). While the method may vary among scientists, the principal steps and processes are the same. Claassen’s
(1998:158) thin-sectioning technique requires the bivalve to be sliced with a low-speed saw from the umbo to ventral margin. The cut surface is then polished with an electric grinder. After polishing, epoxy is applied to the prepared surface which is then firmly placed on a glass slide. The valve attached to the slide is then attached to a hot waffle chuck by wax. The valve is then sectioned again with a low-speed saw to about a tenth of a millimeter thick. The slide and waffle chuck are heated again to dislodge the slide. The thin section is then grounded and polished for transparency.
As mentioned above, the thin-sectioning technique is preferred as a more effective procedure. This method is more consistent and accurate than acetate peels.
Furthermore, false annuli are more clearly distinguished from true annuli. Moyer
(1984:130-133) “frequently could trace growth checks from the umbo to the shell margin” which embellished the continuity of internal and external growth increments and distinguished true from false annuli.
Although poised as the preferred technique, thin-sectioning endures several limitations. The primary problem with this method is the amount of time it takes to prepare a single specimen. Neves and Moyer (1988:183) indicated that it took from a half hour to a full hour to conduct the technique on one shell, which is not including the
29 overnight period to let the epoxy harden. Furthermore, large shells sometimes require several cuts in order to fit on the slides and small, thin shells have a tendency to break
(Claassen 1998:155-158; Moyer 1984:133; Neves and Moyer 1988:183-186). The procedure can be costly as it requires specialized and expensive equipment. As with the previous two mentioned methods, thin-sectioning is destructive to the shell.
Problems with Shell Growth Increment Measurements
While the four main shell growth increment measurement methods have been shown to exhibit limitations, they are regularly used in archaeological analysis. As previously mentioned, results from the methods are often ambiguous and inaccurate.
Although thin-sectioning may provide for more consistent and accurate results, it is a lengthy and expensive procedure. Furthermore, archaeological shell is typically fragmented. As a result, the ventral margin is normally missing and the terminal growth cannot be assessed. Additionally, when cut sections are etched, portions of the growth increments may be removed (Chatters 1997:374). The microscopic methods are also destructive to the archaeological specimen. One method has been developed that circumvents some of these problems: Chatters’ increment measurement technique.
Chatters’ Increment Measurement Technique
While numerous shell growth increment measurement techniques have been developed and explored throughout the archaeological sciences, they demonstrate varying degrees of success. The technique developed by Dr. James C. Chatters is one of the most effective and least destructive means to analyze freshwater mussel shell growth on archaeological specimens
30
This method involves the measurement of growth lines on the resilial tuberosity. The resilial tuberosity is a raised, flat-topped ridge situated on the shell hinge.
The resilium is the inner portion of the hinge ligament and is attached to the resilial tuberosity. It is a flexible, organic structure that records the incremental growth of the bivalve. This growth record continues into the shell via the resilial tuberosity (Chatters
2007:70).
The first step in measuring the growth increments is to scrape any excess organic matter from the resilial tuberosity with a razor blade or sharp implement to expose the annual growth bands. Next, to clearly accentuate the bands, a weak hydrochloric acid solution should be swabbed along the tuberosity. Then, the growth lines are examined under a microscope. Figure 2 illustrates how the annual growth increments appear under magnification. The resilial tuberosity is placed facing upwards with the shell propped up with clay on a Unislide moveable stage. The moveable stage, positioned underneath the microscope, is hooked up to a magnetic encoder that tracks the movement of the stage to the thousandth of a millimeters (See Figure 3). A cross-hair piece is fitted into one side of the microscope. The shell is then lined up, so the terminal end and the cross-hair are intersecting. Next, the Unislide moveable stage is scrolled so the crosshairs lines up with the next line of growth. The width is read on the machine and recorded. The machine is zeroed-out after each growth band is measured (Chatters 1987,
2007; Eugster 1990).
In order to determine the season of death for a mussel, the growth index is calculated. This is the percentage of new growth. To configure the growth index, the terminal growth is divided by the last full year of growth and the resulting number
31
01 mm
Terminal year of growth
Last full year of growth
Figure 2. Annual growth lines at the terminal end of the resilial tuberosity of a Gonidea angulata specimen. Photograph taken through a binocular microscope at magnification of 40x. Photograph by Deanna M. Commons.
multiplied by 100 (Eugster 1990:87). The growth index can then be compared to the annual growth curve of a population within the region of study.
In 1985, Chatters developed a growth curve for Margaritifera falcata by taking monthly mussel samples from the Yakima River in Washington. From March to
November, ten to thirty mussels were collected from the same bed. The mussels were then analyzed with the method described above. The growth curve was developed by calculating the mean percentage of new growth for each month.
Between the beginning of April and the middle of May, new growth appears on the mussels. The mussels complete the growth cycle by early November, displaying rapid growth from June to September. From December through March, the mussels are in a period of arrested growth development. Chatters also had collected a sample of
Margaritifera falcata from the Pit River in November of 1985. The growth index corresponded to the Yakima River mussels growth pattern (Chatters 1987).
32
3
1
2
Figure 3. Workstation for applying Chatters’ increment measurement technique. (1) ACU-RITE III magnetic encoder; (2) Unislide moveable stage by Velmex; (3) Binocular microscope. Photograph by Deanna M. Commons.
While Chatters technique for measuring shell growth increments is often referenced with Margaritifera falcata (Western pearl mussel), it has also been successfully employed on Lampsilis siliquoidea (fatmucket mussel), Uniomerus tetralasmus (pondhorn mussel), Quadrula quadrula (mapleleaf mussel), Ligumia recta
(black sandshell mussel), Lampsilis cardium (plain pocketbook mussel), Amblema plicata
(threeridge), Anodonta beringiana (Yukon floater), Gonidea angulata (Rocky Mountain ridged mussel), and Anodonta californiensis (California floater) (Chatters 1986, 1997,
2007; Chatters et al. 1995; Eugster 1990).
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Summary
The four principal shell growth increment measurement methods, direct methods for estimating seasonality with freshwater mussels, have faltered when assessing archaeological mussels. Chatters’ increment measurement technique accommodates for archaeological mussel specimen where only the hinge feature of the shell is necessary for analysis, which is coincidentally best preserved under taphonomic processes. While
CIMT provides a synopsis for site seasonality, an investigation into the natural and cultural seasonal patterns of a specific location will provide more insight when estimating seasonality from archaeological freshwater mussels.
CHAPTER III
TWO FRESHWATER MUSSELS AND
SEASONALITY OF NORTHERN
CALIFORNIA
Shell increment analysis can be a useful technique for determining seasonality at archaeological sites. One particular method, described in Chapter II, is Chatters’ increment measurement technique. This method not only circumvents the problems with archaeological mussel specimens, but is also applicable to several freshwater mussel species of northern California.
In this chapter, I begin with a description of two freshwater mussel species found locally in northern California and involved in this study. I provide information regarding reproduction, shell morphology, distribution, and habitat. Next, I discuss both the physical and cultural environments of one particular lowland region the freshwater mussels inhabit. This includes the local climate, vegetation, flora and fauna, as well as the territory and subsistence culture of the Yana and Nomlaki Indians. Lastly, I provide information regarding two archaeological rockshelters within this region that contain freshwater mussels in the artifact assemblages. This includes location, excavated depths, cultural components, and deposition rates.
34 35
Freshwater Mussels in Northern California
Freshwater mussels are used to assess seasonality through the analysis of the annual shell growth increments. The initial seasonality studies that used Chatters’ increment measurement technique were first employed on Margaritifera falcata (Western pearl mussel). Later studies demonstrated the effectiveness of CIMT on other freshwater mussel species including Gonidea angulata (Rocky Mountain ridged mussel). Figure 4 outlines the taxonomic classification of these two mussels from northern California.
The Phylum Mollusca is divided into six classes; the one we are concerned with is Pelecypoda or Bivalvia (Eugster 1990:18). The Class Bivalvia is comprised of several orders that are divided into marine and freshwater mussel categories. The Order we are concerned with is Unionoida and is characterized by large freshwater mussels.
The Unionoida Order contains two superfamilies: Etheriodea and Unionoidea (Bauer
2001a:3). In this study we are concerned with Unionoidea forms.
Phylum Mollusca Class Bivalvia Order Unionoida Superfamily Unionoidea Families Margaritiferidae Margaritifera falcata Unionidae Gonidea angulata
Figure 4. Taxonomic classification of two Northern California freshwater mussels: Margaritifera falcata and Gonidea angulata.
The superfamily Unionoidea is comprised of three families: Margaritiferidae;
Unionidae; and Hyriidae. This superfamily is characterized by a unique parasitic stage on
36 a host fish (Bogan 1993:599). Glochidia, or larva, are hatched from the mussel’s eggs that are attached to the gills lining the foot (Wachler et al. 2001:93). Once released, the glochidia must locate a host fish for survival as they are incapable of moving on their own. Typical host fish include salmonid fish species. For example, Margaritifera falcata exploit Chinook and Coho salmon, native cutthroat and rainbow trout, sockeye salmon, speckled dace, Lahontan redside, Tahoe sucker, and nonnative brook and brown trout
(Howard and Cuffey 2006:678; Nedeau et al. 2005:36). Common attachment sites on the host fish include the gills and fins. Once attached, a cyst forms on the fish where the larva resides, metamorphoses, and later releases to continue survival in a sandy substrate
(Wachtler et al. 2001:102).
Margaritifera falcata (Western Pearl Mussel)
Margaritifera falcata is a member of the Margaritiferidae family. It is typified by an elongated, rhomboid shell, displaying muscle scars at both the anterior and posterior ends on the inner surface (Smith 2001:34). (Refer to Figure 5 for an illustration of Margaritifera falcata with feature identification). The periostracum, or outer shell layer, is typically light to dark brown or black (Nedeau et al. 2005:34). This protein- enriched layer protects the shell from dissolution and displays annual growth bands
(Bauer 2001a:4). The nacre lines the inner valve surface and varies from a deep purple to a bluish purple within the adult population while also exhibiting salmon pink or even white coloring (Smith 2001:40; Nedeau et al. 2005:35). The mussel contains two pseudocardinal teeth in the left valve, with a reduction of the anterior cusp. The right valve only has one pseudocardinal tooth that positions itself in between the two left teeth when closed. Lateral teeth develop in the juvenile stage but are absent in the adults
37
Umbo Dorsal
Posterior
Anterior
Ventral
Resilial Tuberosity
Posterior Muscle Scar Anterior Muscle Scar
Figure 5. Illustration of Margaritifera falcata right valve with shell features identification. Drawn by Deanna M. Commons (not to scale).
38
(Smith 2001:43). While average life duration ranges from 40 to 70 years, the mussel has been documented to surpass the 100 year mark (Hastie et al. 2000b:40; Nedeau et al.
2005:34; Vannote and Minshall 1982:4103).
Margaritiferidae species are distributed throughout Europe, North America, and Asia. Margaritifera falcata have been documented to inhabit drainages throughout the Pacific Northwest from central California north to southern Alaska. To the east the mussel resides in Idaho, Utah, and parts of the Rocky Mountains, within water concourses west of the Continental Divide (Nedeau et al. 2005:36; Smith 2001:44).
Recent research has uncovered populations in the headwaters of the Missouri River, where migration is most likely attributed to host fish movement (Nedeau et al. 2005:36).
Locally, Margaritifera falcata have been reported in the Sacramento River, South Fork
Eel River, Navarro River, Pit River, Upper Kern River, Clear Lake, and throughout the
Central Valley (Gustafon et al. 1997; Howard and Cuffey 2006; Taylor 1981). Howard and Cuffey (2006) noted a depleted population of Margaritifera falcata in the Navarro
River since the 1970s that is attributed to the reduction in the host fish population and an increase in sediments. This situation may be the case with many of the other habitats of the freshwater mussel.
Margaritiferidae spend most of their life thriving from the substrate of a creek’s bed. The mussels reside in cold, clean bodies of continuously flowing water with sand and gravel substrates (Bjork 1962:19Chatters, Butler et al. 1995:491; Nedeau et al.
2005:36). Margaritifera requires a swift and continuous flow of cold water, yet water velocity cannot exceed the mussel’s ability to stay attached to the substrate. Furthermore, slow velocity may cause silt build-up and turbidity, reducing feeding time and oxygen
39 availability (Eugster 1990:21; Hastie et al. 2000b:60). The water source often contains boulders that provide a sheltered habitat, and sand in which the mussel may burrow. The boulders also prevent bed scour from floods that can eliminate the mussel population
(Eugster 1990:21-22; Hastie et al. 2000b:59; Vannote and Minshall 1982:4103). Bed depth where Margaritifera reside ranges from a half to two meters (Eugster 1990:21;
Vannote and Minshall 1982:4104).
Gonidea angulata (Rocky Mountain Ridged Mussel)
Gonidea angulata, also known as the Rocky Mountain ridged or Western ridged mussel, belongs to the Unionidae family. It contains a trapezoidal shaped shell that has a distinct ridge that runs from the umbo diagonally to the ventral margin (See
Figure 6). Like Margaritifera falcata, Gonidea angulata contains a darker colored periostracum but often a whitish nacre layer. Both the left and right valves contain one small pseudocardinal tooth although occasionally the tooth is missing in the left valve.
The mussel has no lateral teeth (Nedeau et al. 2005:38-39).
Gonidea angulata mussels are distributed from southern California to southern British Columbia, and east to northern Nevada and southern Idaho. Within
California, Gonidea angulata has been documented to inhabit the Klamath River, Lost
River, Pit River, Sacramento River, and the lower Eel River, as well as Clear Lake. The mussel is most likely extinct now from many water sources within the Central Valley and southern California as a result of polluted streams from advanced agricultural activities as well as change in stream flow and a reduction in the host fish population (Taylor
1981:142-143).
40
Umbo Ridge
Resilial Tuberosity
Figure 6. Illustration of Gonidea angulata left valve with shell features identification. Drawn by Deanna M. Commons (not to scale).
41
Gonidea and Margaritifera prefer similar habitats and have been found to inhabit the same rivers and streams. While Gonidea can tolerate fine sediments more than
Margaritifera, they will not survive in habitats with unstable or soft substrate. They are also less likely to be found in lakes or reservoirs. The fish host for Gonidea angulata is unknown, although suggested to be salmon and trout species like the host fish for
Margaritifera falcata (Nedeau et al. 2005:40).
Environment
The two freshwater mussels have been observed to inhabit the many drainages and water concourses emptying into, as well as including, the Sacramento River in the lower foothills of the Cascade Range. This area has a typical Mediterranean climate patterning that includes hot, dry summers and cool, wet winters. It is not unusual for temperatures to surpass 100 degrees Fahrenheit in the summers. Rainfall typically averages between 15 and 40 inches, the majority accumulating in the winter (Hamusek
1996:7).
The vegetation in this region is characteristic of mixed blue oak and gray pine woodland with scattered open grassland. The overstory consists primarily of blue oak, gray pine, live oak, and buckeye trees. The understory includes poison oak, toyon, yerba santa, various grasses, and a collection of geophytes (Hamusek 1996:8; Ritter and Tyree
1999:5).
There is a diverse collection of faunal species that inhabit this area. The common mammalian species include deer, jack rabbit, coyote, mountain lion, raccoon, ground squirrels and gophers. Salmon, trout, and suckers are common fish located in the
42 creeks while quail, waterfowl, hawks, and local songbirds are regular birds in the area
(Hamusek 1996:8).
Cultural Setting
One of the geographic regions that incorporate the Sacramento River and the surrounding lowlands to the east is close to an ethnographic boundary line between two tribes of contested relations: River Nomlaki and Yana. Much of the information surrounding the Nomlaki and Yana reside in ethnographic accounts describing the more recent events, residences, and practices of the native groups. Prehistoric boundaries were not stable lines of divisions as contested relations between neighboring groups may have constantly altered those lines through time.
The Nomlaki Indians are linguistically linked to the Wintun, speaking a slightly varied dialect and located just south of Wintu territory. The Wintu, Nomlaki, and
Patwin all comprise the Wintun group. The Nomlaki are divided into two geographical groups: Hill Nomlaki and River Nomlaki. The territory of the Nomlaki extends from
Cottonwood Creek in the north, Stony Creek in the south, to the lower foothills and area surrounding the Sacramento River to the east, and to the west their land stretches to the summit of the Coast Range (Goldschmidt 1978:341; Likins 2003:2-3). Goldschmidt
(1978:341) shows the territory of the River Nomlaki occupying the surrounding lowlands to the east and west of the Sacramento River, extending north to where Cottonwood
Creek empties into the Sacramento River and south to Toomes Creek. The River Nomlaki
Indians developed village sites along the Sacramento River and on the higher plains
(Kroeber 1970:354; Likins 2003:3).
43
The territory of the Yana ranged roughly from the Pit River in the north, Rock
Creek in the south, the eastern edge of the Sacramento Valley to the west, and up the drainages reaching Lassen Peak in the east (Johnson 1978:361; Kroeber 1970:337-338).
Johnson (1978), in his description of the Yana, acknowledges that various interpretations exist on their exact territory. One of these contested boundaries includes the western flank where “the Yana may have had villages or at least fishing camps on the Sacramento
River” (Johnson 1978:361).
The River Nomlaki and Yana Indians both exploited the resources that the land had to offer. Acorns were the primary food resource for the Yana. They were harvested in the early fall and, in good years, steadily supplied the Yana until the next harvest. Deer was another important economic commodity for the Yana and the primary hunted game animal. Other important game included rabbit and quail. Fish were another important food source for the Yana who used spears, harpoons, nets, traps, and plant poisons to get the fish. Additional resources that were exploited include roots, tubers, bulbs, and small rodents (Johnson 1978:364-365). The important foods of the River
Nomlaki included deer, elk, rabbit, birds, fish, acorns, and grass seeds (Goldschmidt
1978:347, Johnson 1978:364-365). The men were responsible for hunting and used the bow and arrow, nets, snares, slings, and traps. The women were responsible for the gathering of vegetal products that included at least eight varieties of acorns, clovers, seeds, tubers, pine nuts, mushrooms, and wild fruits and berries (Goldschmidt 1978:347).
Both the Yana and River Nomlaki Indians inhabited environments that would support freshwater mussel populations. Ironically, the ethnographic documents do not mention the presence of freshwater mussels in either of the indigenous group’s diet.
44
While the ethnographic material does not discuss the use of freshwater mussels, the archaeological data provides alternative information. The excavation of two archaeological sites, Sentinel Bluff and Paynes Creek Rockshelters, in Northern
California within the contested boundary of the River Nomlaki and Yana has uncovered archaeological freshwater mussel specimens.
Sentinel Bluff and Paynes Creek Rockshelters
While none of the ethnographic material mention freshwater mussels in either of the prehistoric populations’ subsistence economy, freshwater mussels were uncovered at two archaeological rockshelter sites. Both Sentinel Bluff and Paynes Creek
Rockshelters are situated on tributaries of the Sacramento River. They reside north of the town of Red Bluff within the vicinity of Bend, California, Tehama County (see Figure 7).
The rockshelters reside on land maintained by the Bureau of Land Management (BLM).
According to White (2005), numerous archaeological rockshelters inhabit the concourses of the Sacramento River, Paynes Creek, and Turtle Creek. The rock canyons are comprised of Pleistocene Tuscan Formation plateau volcanics and Modesto
Formation alluvium forming strata of alternating hard and soft layers. The hard layers form shelves that project out of the canyon walls and act as the roof of a rockshelter. The occupied rockshelters have deep, broad roofs that protect the inhabitants from falling debris.
Sentinel Bluff Rockshelter lays roughly one mile east of Paynes Creek
Rockshelter near the mouth of Turtle Creek (refer to Figure 7). In 1999, Sentinel Bluff
Rockshelter was excavated by students attending CSU Chico under the direction of Dr.
45
Figure 7. Archaeological site location map of Sentinel Bluff and Paynes Creek rockshelters.
Source: Adapted from United States Geological Survey Red Bluff 1:100,000 topographic quadrangle map and U.S. Census Bureau 2000 data.
Greg White. There were four units excavated, two reaching a maximum depth of 410 centimeters (White 2005). The shell material from these two main test pits, N0W4 and
N0W2,1 were analyzed and seasonality assessed with the results presented in Chapter V.
1 N0W4 and N0W2, two of the four excavated units from Sentinel Bluff Rockshelter, will be referred to as Unit 1 and Unit 2, respectively.
46
Sentinel Bluff Rockshelter has three distinct cultural layers: Component I;
Component II; and Component III (White 2005). Component I is characterized by a rich, organic loam and ashy deposits relatively void of rocks. Within the excavated units, this layer was present up to a depth of 120 centimeters. Artifacts recovered from this component include small Gunther-barbed and corner-notched points, manos, metates, core tools, clam disk beads, and various faunal material from both small and large game displaying evidence of modification. Several radiocarbon samples were taken from
Sentinel Bluff Rockshelter that suggests a deposition rate of roughly 515.5 years per meter of accumulation. Component I dates from the present to roughly 700 years BP
(White 2005).
Component I also rests on roughly a meter thick layer of rocks and cobble debris, most likely a result of roof and wall collapse. While some artifacts were recovered, White (2005) indicates that this layer may represent minimal prehistoric occupation.
Component II was found at a depth of 180 to 260 centimeters and characterized by a sandy, organic rich loam containing few boulders and cobbles.
Artifacts recovered from this component include Gunther-barbed points with a large triangular stem, pestles, and faunal material. This component dates between 950 to 1300 years BP (White 2005).
Component III consists of a sandy to silty loam soil containing considerably more boulders and rocks than the previous components. It was found at a depth of 260 to
420 centimeters. The relatively few artifacts recovered from this component include dart
47 size corner-notched projectile points, core tools, and faunal material. Component III dates from 1350 to 1800 years BP (White 2005).
Paynes Creek Rockshelter is located near the mouth of Paynes Creek on the west bank and has a deposition rate of roughly 1700 years per meter of accumulation
(White 2005) (See Figure 8). It was excavated in 2005 in collaboration with Shasta
College, CSU Chico, and BLM in response to the continuous looting of the site. The interest of this study focuses on the shell material from the Back Wall Complex that was excavated to a depth of 290 centimeters. There were no distinguishable cultural components in the Paynes Creek Rockshelter.
Paynes Creek Rockshelter
Figure 8. Approximate location of the Paynes Creek rockshelter. Photograph by Deanna M. Commons.
48
Summary
The two archaeological rockshelter sites reside within the ethnographic boundaries of the Yana and River Nomlaki Indians. No historical data mention the presence of freshwater mussels in either of the diets of the indigenous groups. In
Northern California, freshwater mussels were more apt to be harvested in the summer and autumn months when water levels were lower and mussels were more easily accessible.
A shell growth increment measurement technique performed on the mussels can estimate season of mussel death. Chatters’ increment measurement technique first must be evaluated as an accurate and reliable means for assessing seasonality before an application the on archaeological material.
CHAPTER IV
ASSESSMENT OF CHATTERS’
INCREMENT MEASUREMENT
TECHNIQUE
Freshwater mussels provide one way to estimate seasonality through the analysis of the annually deposited shell growth increments. The growth ring method, thick- sectioning, thin-sectioning, and acetate peels are all techniques that measure these increments, but that carry multiple limitations and ambiguous results, in particular when applied to archaeological specimens.
In the 1980s, Dr. James C. Chatters developed an alternative shell growth increment measurement method that not only accommodated for the limitations of the previous methods, but was designed specifically for archaeological freshwater mussels.
Because this potentially valuable technique has not yet been extensively tested, nor widely published or adopted, I decided to evaluate the accuracy and reliability of
Chatters’ increment measurement technique to determine if it was a plausible means of accessing seasonality, and if so, apply this method to an archaeological collection of freshwater mussels.
In this chapter, I present the methods I used to assess CIMT, as well as the ensuing results. There were three steps in this process. I begin with a discussion of the first step, collecting a control sample of Margaritifera falcata specimens, and the
49 50 subsequent modifications to the initial approach. I proceed to discuss the second step, the application of CIMT to the control sample. Third, I conduct a statistical analysis using independent t-tests to compare the results of the control sample to Chatters results of the respective collection dates (Chatters 1987:15). Lastly, I conclude with a summary of the results and a brief discussion on the implication of the outcome of the independent t-tests.
Step 1: Obtaining a Control Sample of Margaritifera falcata and Collection Plan
To evaluate the accuracy of Chatters’ increment measurement technique and the ability to duplicate the results, a control sample was required. I had planned to include
80 specimens of Margaritifera falcata in this sample. Twenty mussels were to be collected in the middle of June, July, August and September, respectively. The mussels were to be gathered in the summer months when water levels were lower, allowing for greater accessibility into the rivers or creeks.
In the summer of 2009, several attempts were made to find Margaritifera falcata specimens in the water concourses of northern California. The Pit River was the initial focus for a study area as Chatters had collected and observed Margaritifera falcata specimens there in 1985 (Chatters 1987, 1997). However, no Margaritifera mussels were found in the Pit River, or the alternative water concourses that were perused: the North
Fork of the Feather River, Deer Creek, Butte Creek, and Hat Creek. Ironically, Chatters has observed hundreds of Margaritifera falcata specimens in the Pit River and Hat Creek in the tailwaters of Lake Britton, and a few Gonidea angulata samples. On June 28, 2009,
51 in one of the earlier attempts to locate Margaritifera in the Pit River, I, however, was able to acquire six specimens of Gonidea angulata.
These Gonidea angulata specimens are one of the nearly 800 species of freshwater mussels that contain the resilial tuberosity feature on the shell hinge on which
Chatters’ increment measurement technique is based, as mentioned earlier. CIMT has been successfully performed on several other mussel species including Gonidea angulata. Both Margaritifera and Gonidea are larger freshwater mussels that have been documented to inhabit the same water concourses and demonstrate similar annual growth patterns. Further, mussel growth patterns in the Pacific Northwest has been tested and confirmed to be analogous to mussel growth throughout the temperate zone (Chatters
2007). Therefore, Gonidea angulata is an ideal substitute species for a control sample specimen
Since further attempts at acquiring Margaritifera falcata failed, I decided to pursue the Gonidea angulata specimens from the Pit River. Another trip to the Pit River took place on September 6, 2009, and thirteen mussels were collected. A total of nineteen mussels were collectively taken in the summer of 2009.
Step 2: Application of Chatters’ Increment Measurement Technique to the Control Sample
In preparation for analysis, the specimens were first boiled until the bivalves opened. The mussel was scraped out and the valves were sagittally sectioned through the ligament. I was able to perform the analysis on five mussels that were collected on June
28, 2009 and nine that were collected on September 6, 2009. During the boiling and
52 sectioning process, the terminal end of the resilial tuberosity was broken on three of the shells. One of the mussels had a deformity in the resilial tuberosity that inhibited analysis.
The final mussel was dead upon collection and omitted from the analysis.
The mussels were then brought to the Archaeological Laboratory at CSU
Chico. Next, the mussels were propped with clay on a Unislide moveable stage by
Velmex that was connected to a magnetic encoder by Accurite III and under a binocular microscope.
Several measurements should be made of the growth increments to ensure a more accurate and consistent reading and that the growth intervals, aside from the terminal growth, should be similar in lengths. Additionally, the measurements should be made on a level plane and the shell may have to be shifted to maintain that plane.
Depending on the size of the shell, an arc may be present in the resilial tuberosity that may require constant realignment to maintain the line of measurement.
The end of the resilial tuberosity displaying the final year of growth was lined-up with the cross-hair piece that was fitted into the eyepiece of the microscope.
Moving anteriorly along the hinge, the width of the terminal year of growth and the last full year of growth were recorded. Each growth increment was measured to a thousandth of a millimeter.
Chatters’ initial study of the growth increments he compared the final year of growth to the average of the three and five previous years (Chatters 1987). As there is considerable variation between the first few years of growth and later years, the final growth should be compared only to the penultimate year. I measured each valve several times until my measurements were consistent.
53
The growth index was then calculated for each mussel. It is the percentage of new growth in the last yearly cycle represented in the terminal end. The growth index is calculated by having the terminal end divided by the last full year of growth and the resulting number multiplied by 100.
Next, the mean growth index was computed for both the June 28 and
September 6 collection date. These mean growth indices for the two collection dates were then compared to the growth curve of the Margaritifera falcata population from the
Yakima River as established in Chatters’ 1987 study.
Step 3: Statistical Comparison of Growth Indices
In order to evaluate whether the differences between the mean growth indices of Chatters’ study1 and my research of respective dates were statistically significant, independent t-tests were used. A p-value of .05 was used to test for statistical significance. If the results were not statistically significant then the null hypothesis would be accepted, that there is no difference between the mean growth indices of the control sample and Chatters’ study. Accepting the null would confirm the accuracy and reliability of CIMT. The null would be rejected if the results were statistically significant, implying the means come from random samples of different populations.
1 Chatters’ study refers to his 1987 publication entitled “Shell of Margaritifera margaritifera falcata as a source of Paleoenvironmental and Cultural Data.” Appendix F. It is this study that my data are compared to in subsequent discussions.
54
Results Summary
In the following pages, I present the results from the application and analysis of Chatters’ increment measurement technique to the control sample. I display a table with the measurement of the terminal end and last full year of growth for each control specimen and the calculated growth index. In addition, I provide a table comparing the mean growth indices and standard deviations from the June 28 and September 6 collection dates to those from Chatters’ study for the respective dates. Lastly, I describe the results of the independent t-tests comparing the differences between the mean growth indices of respective dates and the significance of these results to Chatters’ increment measurement technique.
Table 1 summarizes the season of death data from the application of Chatters’ increment measurement technique to the Gonidea angulata control sample. The bivalves were grouped by collection date. The terminal end and the penultimate year of growth were measured on each specimen to the nearest thousandth of a millimeter. The growth index was calculated for each bivalve and represents the percent of new growth compared to the penultimate year of growth.
Table 2 displays the close correlation between the mean growth indices of the control sample and Chatters’ study of the same dates. The indices not only closely align, but the control sample mean growth indices are well within two standard deviations of the indices from Chatters’ study of the respective dates. Within the table are the mean growth index and standard deviation for each set of control sample collection dates, as compared with the respective mean growth indices and standard deviations from
Chatters’ study. Chatters did not collect mussels on either of those dates. The mean
55
Table 1. Growth Index Data from Gonidea angulata Control Sample.
Last Full Year Collection Terminal of Growth Growth Date Specimen End (mm) (mm) Index
06.28.2009 1 0.043 0.318 13.5 06.28.2009 2 0.043 0.307 14 06.28.2009 3 0.112 0.682 16.4 06.28.2009 4 0.076 0.497 15.3 06.28.2009 5 0.081 0.347 23.6
09.06.2009 6 0.581 0.738 78.7 09.06.2009 7 0.872 1.221 71.4 09.06.2009 8 0.686 1.019 67.3 09.06.2009 9 0.783 1.025 76.4 09.06.2009 10 0.214 0.269 79.6 09.06.2009 11 0.519 0.689 75.3 09.06.2009 12 0.514 0.829 62 09.06.2009 13 0.646 0.961 67.2 09.06.2009 14 0.331 0.454 72.9
Table 2. Mean Growth Index and Standard Deviation for Control Sample and Chatters’ Study.
Mussel Collection Sample Mean Growth Index Standard Deviation
June 28 Control Sample 16.6 3.8 Chatters’ Study 16 5
September 6 Control Sample 72.3 5.6 Chatters’ Study 70 15
growth index for each of those dates were obtained from Chatters’ graph of the annual growth curve for Margaritifera falcata from 1985 (Chatters 1987:15).
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Independent t-tests were performed to determine whether the results are statistically significant. The results from the t-test comparing the means for the June 28 growth index (t = 0.25, df = 23, p > .05) and the means for the September 6 growth index
(t = .44, df = 27, p > .05) are not statistically significant. These results indicate that there is no significant difference between the control sample growth index means and the growth index means from Chatters’ study of the respective dates. In other words, the results from the t-tests further validated CIMT as an accurate and reliable means of assessing seasonality.
These results have confirmed Chatters’ increment measurement technique as a reliable and accurate means for estimating seasonality. In a general retrospect, these data have further confirmed all previous published work that has been conducted using CIMT
(Chatters 1986, 1987, 1997, 2007; Chatters, Butler et al. 1995; Chatters, Campbell et al.
1995; Eugster 1990). More specifically, with the confirmation of the accuracy of CIMT, I felt comfortable to proceed to the second portion of the thesis, which is an application of the technique to an archaeological collection evaluate the applicability to archaeological specimens and to estimate seasonality of mussel harvesting and site habitation.
Summary
A control sample of freshwater mussels was obtained to test whether Chatters’ increment measurement technique is an accurate and reliable means for assessing seasonality. CIMT was applied to the control sample and independent t-tests were performed to test the statistical significance of the results. The results showed no statistically significant difference between the mean growth indices of the control sample
57 and the mussels from Chatters’ study. With further evidence supporting the validity of
CIMT, the technique can now be successfully employed on archaeological freshwater mussels to assess site seasonality.
CHAPTER V
APPLICATION OF CHATTERS’
INCREMENT MEASUREMENT
TECHNIQUE TO TWO
ARCHAEOLOGICAL
COLLECTIONS OF
FRESHWATER
MUSSELS
In the previous chapter, Chatters’ increment measurement technique was performed on a control sample of freshwater mussels obtained from the Pit River in
Northern California in the summer of 2009. The results from the control sample were statistically analyzed and confirmed analogous to the results from Chatters’ 1987 study.
This further validated CIMT as a legitimate tool for measuring shell growth increments.
CIMT was originally developed for use on archaeological specimens. In this chapter, CIMT was applied to an archaeological collection of Margaritifera falcata from two rockshelter sites. The results were designed to determine the applicability of CIMT on archaeological freshwater mussels, as well as estimate the time of year that the prehistoric populations were harvesting the mollusks and correspondingly occupying the sites. It was conjectured that mussel collection occurred in the summer and fall months when water levels were lower, providing greater accessibility to the mollusks.
58 59
In this chapter, I present the methods and results of Chatters’ increment measurement technique to the archaeological freshwater mussels. I describe the process of preparing and applying CIMT to the archaeological material, which was divided into three steps.
The first step involved the preparation of the archaeological freshwater mussels for analysis, which included counting and separating the useable specimens. The next step was the application of CIMT to the archaeological samples using only the left valves. In the final step, the resulting data were compiled. Lastly, I conclude with a summary of the results and a brief discussion of the data.
Sample Selection and Approach
Step 1: Preparing the Archaeological Freshwater Mussels
Sentinel Bluff and Paynes Creek rockshelters, located east of Bend,
California, both contain Margaritifera falcata specimens. Excavated in 1999 and 2005 respectively, the archaeological shells have been sorted, catalogued and bagged by ten centimeter levels. The shells had been housed in curation boxes at the Pacific Legacy office on Morrow Drive in Chico, California. The shells were then brought to the archaeology laboratory at CSU Chico for analysis. The number of specimens to which
Chatters’ increment measurement technique could be applied was counted and separated into right and left valves categories for each level. An individual specimen was part of the count if it contained a complete or even incomplete resilial tuberosity as long as the terminal end was present. Only the terminal end of the feature was required as I was comparing the terminal growth to the previous year of growth. With each bag, I carefully emptied the contents and separated out the shells and shell fragments that contained the
60 resilial tuberosity. The shells were then divided into left and right valve piles and the individual specimens counted.
In the following pages, I present the valve and aggregated weight data for the excavated units from the two rockshelters.
Sentinel Bluff Rockshelter. Sentinel Bluff Rockshelter is located near the mouth of Turtle Creek (see Figure 7 in Chapter III), a tributary of the Sacramento River.
Four units were excavated at the site that were one meter square. Shell material from two of the excavated pits, Unit 1 and Unit 2, was chosen for analysis based on the greater quantity of archaeological specimens and a further excavated depth than the other two units1.
Unit 1 contained shell to an excavated depth of 400 centimeters and the majority of the shell was recovered up to 160 centimeters. A total of 51 right valves and
70 left valves contained a usable portion of the resilial tuberosity (see Table 3).
Unit 2 contained shell to an excavated depth of 410 centimeters and the majority of the shell was recovered up to 190 centimeters. A total of 55 right valves and
65 left valves contained a usable portion of the resilial tuberosity (see Table 4).
Paynes Creek Rockshelter. Paynes Creek rockshelter is located near the mouth of Paynes Creek, another tributary of the Sacramento River. There were a few units excavated from the site, the largest was the Back Wall Complex. The archaeological shell from this unit was chosen for analysis as it was the most productive unit in relation to
1 The results from Sentinel Bluff Rockshelter were to be divided by cultural component. Component I was recorded to a depth of 120 centimeters. From 120 to 180 centimeters there was a level of apparent roof collapse characterized by a thick layer of cobbles and rock debris. This layer was not part of a cultural component due to the lack of cultural material. Since all of the mussels that were analyzed were within Component I and the roof collapse level, with the exception of two mussels from Unit 2 that were from the 180 to 190 centimeter level, the results were examined collectively instead by cultural component.
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Table 3. Valve Count and Total Shell Weight Data for Unit 1 from Sentinel Bluff Rockshelter.
Total Shell Level Right Valves Left Valves Weight (g) 10-20 cm 5 9 112.3 20-30 cm 8 6 147.8 30-40 cm 21 24 293.1 40-50 cm 5 5 81.1 50-60 cm 3 7 53.6 60-70 cm 1 3 22.4 70-80 cm 1 1 25.0 80-90 cm 2 3 44.1 90-100 cm 1 2 20.9 100-110 cm 0 2 31.5 110-120 cm 2 0 28.9 120-130 cm 1 2 33.2 150-160 cm 0 2 12.5 160-170 cm 0 2 6.7 220-230 cm 1 2 15.3 Total 51 70 928.4
archaeological material. It also was the deepest unit; the last level was 290 centimeters although shell was recovered only up to 180 centimeters. A total of 48 right valves and
62 left valves contained a usable portion of the resilial tuberosity (see Table 5).
Step 2: Application of CIMT to the Archaeological Freshwater Mussels
Chatters’ increment measurement technique was applied to the archaeological shells, following the same procedure that was conducted on the control sample in Chapter
IV. It was applied only to the left valves in order to eliminate the chance of a mussel being measured twice. The left valves were also chosen as there was a greater number of left opposed to right valves in all three of the excavated units. The resilial tuberosity on each specimen was cleaned with a brush and occasionally lightly scraped with an X-
ACTO knife. Water was applied to the resilial tuberosity while under the microscope
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Table 4. Valve Count and Total Shell Weight Data for Unit 2 from Sentinel Bluff Rockshelter.
Total Shell Level Right Valves Left Valves Weight Surface N/A N/A N/A 0-10 cm 2 0 10.9 10-20 cm 3 6 131.7 20-30 cm 9 12 166.3 30-40 cm 4 12 86.3 40-50 cm 2 4 37.8 50-60 cm 4 3 50.5 60-70 cm 5 7 93.2 70-80 cm 2 4 37.3 80-90 cm 4 1 15.5 90-100 cm 4 3 51.0 100-110 cm 3 0 31.0 110-120 cm 1 2 27.3 120-130 cm 1 2 31.8 130-140 cm 2 3 43.6 140-150 cm 2 1 19.4 150-160 cm 1 0 45.9 160-170 cm 3 1 29.4 170-180 cm 2 1 34.5 180-190 cm 0 3 29.4 290-300 cm 1 0 2.8 Total 55 65 975.6
which better illuminated the growth lines. All shells from a level that were analyzed were marked numerically. In numerous instances, shells were examined but the growth lines were not visible. The growth index, percentage of new growth, was then determined for each specimen that had visible growth lines.
Step 3: Aggregation of the Results
Once CIMT was applied to all usable mussel specimens, the results were compiled for analysis. The count of usable specimens as well as the actual count of specimens analyzed were calculated and compared. This data provided information to
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Table 5. Valve Count and Total Shell Weight Data for the Back Wall Complex from Paynes Creek Rockshelter.
Total Shell Level Right Valves Left Valves Weight Surface 0 3 26.5 0-10 cm 2 4 52.1 10-20 cm 6 1 68.4 20-30 cm 4 1 25.8 30-40 cm 2 1 67.1 40-50 cm 3 1 34.8 50-60 cm 0 3 115.9 60-70 cm 2 8 144 70-80 cm 3 4 103.7 80-90 cm 2 1 129.5 90-100 cm 3 4 108.7 100-110 cm 2 6 139.9 120-130 cm 6 11 196.3 130-140 cm 6 6 132.7 140-150 cm 2 2 173.9 150-160 cm 4 3 80.9 170-180 cm 1 3 61.7 Total 48 62 1661.9
better understand the applicability of CIMT to archaeological freshwater mussels and some of the limitations.
For each mussel specimen examined, the growth index was calculated. The
Paynes Creek and Sentinel Bluff rockshelters data was tabulated and graphed to determine the time of year the prehistoric population was harvesting the freshwater mussels and correspondingly occupying the sites.
Results
In the following pages, I present the results from the application of CIMT to the archaeological specimens from the two rockshelters. I provide a table listing the number of left valves used as well as a table comparing the number of total valves to the ones used. Furthermore, I provide tables of the season of death data for each excavated
64 unit. In addition, I exhibit the late summer and fall clustering of the death data in graph form for each unit.
Valve Count Data
As mentioned above, while the right and left valves for each level of the excavated units were separated and counted, CIMT was only applied to the left valves to eliminate the chance of a mussel measured twice. Table 6 summarizes the number of usable left valves for each excavated unit from the two rockshelters. While all the left valves were examined, I was not able to execute CIMT on all the valves.
Table 6. Left Valves Used from the Excavated Units from Sentinel Bluff and Paynes Creek Rockshelters.
Unit 1 Unit 2 Back Wall Level (SBRS) (SBRS) (PCRS) Surface 0 0 1 0-10 cm 0 0 2 10-20 cm 3 3 1 20-30 cm 1 2 1 30-40 cm 8 4 1 40-50 cm 3 1 1 50-60 cm 3 1 1 60-70 cm 0 3 6 70-80 cm 0 2 2 80-90 cm 0 1 1 90-100 cm 0 0 3 100-110 cm 2 0 3 110-120 cm 0 1 0 120-130 cm 1 0 4 130-140 cm 0 2 1 150-160 cm 0 0 1 160-170 cm 1 1 0 170-180 cm 0 0 1 180-190 cm 0 2 0 Total 22 23 30
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Table 7 illustrates the total valve counts for each rockshelter for both the left and right valves and the number of analyzed left valves. The percent of left valves analyzed compared to the total number of usable left valves is also calculated in the table.
Table 7. Valve Count Totals and Percent Used from Sentinel Bluff and Paynes Creek Rockshelters.
Total Right Total Left Total Left Percent of Left Rockshelter Unit Valves Valves Valves Used Valves Used N0W4 (SBRS) 51 70 22 31.4 N0W2 (SBRS) 55 65 23 35.4 Back Wall Complex (PCRS) 48 62 30 48.4
While the data from the application of Chatters’ increment measurement technique to the archaeological freshwater mussels indicate that the method was successfully executed, several limitations are also present. Of the three excavated units from the two rockshelters, 31.4 to 48.4 percent of the total useable left valves from the three excavated could actually have CIMT applied, although all the useable left valves were examined. The reduced number of usable specimens from the total of left valves is the result of the inability to distinguish the annual growth lines on all the specimens.
Carbonic acid and abrasion by sand and gravel can erode and damage portions of the shell, including the resilial tuberosity. This may have been the case with the archaeological specimens from the two rockshelters.
Season of Death Data
Sentinel Bluff Rockshelter. Tables 8 and 9 summarize the season of death data from the application of Chatters’ increment measurement technique to the archaeological specimens from Unit 1 and Unit 2 from Sentinel Bluff Rockshelter. The specimens are labeled according to the unit from which they were excavated followed by a numerical
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Table 8. Month of Death Data for Archaeological Margaritifera falcata Specimens from Unit 1 of Sentinel Bluff Rockshelter.
Terminal End Last Full Year of Month of Specimen (mm) Growth (mm) Growth Index Death 10-20:1 2.416 2.648 91.2 Oct 10-20:2 3.313 3.331 99.5 Oct 10-20:3 2.548 2.635 96.7 Oct 20-30:1 2.156 2.472 87.2 Oct 30-40:1 1.657 2.332 71.1 Sept 30-40:2 2.400 2.376 101 Nov-Apr 30-40:3 2.000 2.359 84.8 Oct 30-40:4 1.651 2.066 79.9 Sept 30-40:5 2.872 3.437 83.6 Sept 30-40:6 2.596 3.024 85.8 Oct 30-40:7 1.859 2.377 78.2 Sept 30-40:8 4.176 4.174 100 Oct 40-50:1 1.889 1.855 101.8 Nov-Apr 40-50:2 1.998 4.231 47.2 Aug 40-50:3 1.560 1.789 87.2 Oct 50-60:1 1.567 2.071 75.7 Sept 50-60:2 0.805 2.234 36 July 50-60:3 2.051 2.344 87.5 Oct 100-110:1 2.360 2.731 86.4 Oct 100-110:2 3.342 3.790 88.1 Oct 120-130:1 2.659 2.690 98.8 Oct 160-170:1 2.243 2.913 77 Sept
order. The terminal end and the last full year of growth measurements are presented for each specimen and were measured to the nearest thousandth of a millimeter. The growth index was calculated and is presented for each specimen. The growth index represents the percent of new growth compared to the penultimate year of growth. Lastly, the month of death is also presented. The month of death was determined by comparing the growth indices to those specific points on Chatters’ mean growth index for Margaritifera falcata
(Chatters 1987:15).
Growth indices over 100 were classified as a season of death sometime between November and April. Chatters (1987) documented that mussel growth ceases in
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Table 9. Month of Death Data for Archaeological Margaritifera falcata Specimens from Unit 2 of Sentinel Bluff Rockshelter.
Terminal End Last Full Year of Month of Specimen (mm) Growth (mm) Growth Index Death 10-20:1 2.522 2.688 93.8 Oct 10-20:2 1.103 1.376 80.2 Sept 10-20:3 1.786 2.047 87.2 Oct 20-30:1 2.999 2.753 108.9 Nov-Apr 20-30:2 2.825 3.138 90 Oct 30-40:1 2.011 2.392 84.1 Sept 30-40:2 2.903 4.127 70.3 Sept 30-40:3 1.478 1.581 93.5 Oct 30-40:4 1.714 1.757 97.6 Oct 40-50:1 1.947 2.417 80.6 Sept 50-60:1 2.396 2.817 85.1 Oct 60-70:1 2.418 2.790 86.7 Oct 60-70:2 3.568 3.833 93.1 Oct 60-70:3 2.584 2.870 90 Oct 70-80:1 3.565 3.149 113.2 Nov-Apr 70-80:2 3.283 3.224 101.8 Nov-Apr 80-90:1 2.684 2.805 95.7 Oct 110-120:1 2.424 2.790 86.9 Oct 130-140:1 0.440 2.609 16.7 Jun 130-140:2 4.764 5.354 89 Oct 160-170:1 1.738 1.549 112.2 Nov-Apr 180-190:1 2.077 2.306 90.1 Oct 180-190:2 2.992 3.093 96.7 Oct
early November. The next cycle of annual growth does not begin again until April. A growth index of 100 or more does not necessarily indicate a November death but could actually signify a death between growth cycles.
Unit 1. A summary of the month of death data for Unit 1 from Sentinel Bluff
Rockshelter is presented in Figure 9. A total of 22 mussels were examined. The majority, twelve mussels, demonstrated a death month of October. September was the next highest death month and contained six of the specimens. Of the remaining four mussels, one mussels was recorded to have died in July and one in August, and two sometime between
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Figure 9. Sentinel Bluff rockshelter, unit 1: Growth indices clustered by month of death for Margaritifera falcata
November and April. Of the 22 mussels, eighteen died in September and October collectively.
The month of death was determined from the exact value of the growth indices when compared to the annual growth curve for Margaritifera falcata. When compared to the 95% confidence limits, or two standard deviations from the mean, a growth index of, for example, 70 could indicate a death month ranging from August through October. All of the death dates for the mussels could be off plus or minus a month. Therefore, interpretation of the season of death data demonstrate this archaeological site was inhabited and freshwater mussels were harvested in a time ranging from later summer to the fall months.
Unit 2. A summary of the month of death data for Unit 2 from Sentinel Bluff
Rockshelter is presented in Figure 10. A total of 23 mussels were analyzed. The majority
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Figure 10. Sentinel Bluff, unit 2: Growth indices clustered by month of death for Margaritifera falcata
were recorded as having died in the month of October. Four mussels died in both
September and four sometime with the November to April time interval. One mussel was recorded to have died in the month of June.
Seasonality Summary of Sentinel Bluff Rockshelter. As mentioned above, since the month of death was determined from the actual value of the growth indices, a one month range on the data should be assumed. As a result, the data from Unit 2 are analogous with those of Unit 1. Sentinel Bluff Rockshelter was inhabited and mussel harvesting took place primarily from September through November. This is not to say that the site was not inhabited during the other parts of the year. The seasonality information obtained from the archaeological freshwater mussels through the use of
CIMT indicates a definite time of occupation during the late summer and fall.
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Paynes Creek Rockshelter. Table 10 and Figure 11 summarize the month of death data from the Margaritifera falcata specimens from the Back Wall Complex of
Table 10. Month of Death Data for Archaeological Margaritifera falcata Specimens from the Back Wall Complex of Paynes Creek Rockshelter.
Terminal End Last Full Year of Month of Specimen (mm) Growth (mm) Growth Index Death Surface:1 2.392 2.539 94.2 Oct 0-10:1 3.343 3.638 91.9 Oct 0-10:2 3.085 3.277 94.1 Oct 10-20:1 2.576 3.766 68.4 Sept 20-30:1 3.044 3.183 95.6 Oct 30-40:1 2.468 2.530 97.5 Oct 40-50:1 2.954 3.178 93 Oct 50-60:1 1.937 2.876 67.4 Sept 60-70:1 2.307 2.163 106.7 Nov-Apr 60-70:2 1.097 1.386 79.1 Sept 60-70:3 1.951 2.626 74.3 Sept 60-70:4 2.722 3.397 80.1 Sept 60-70:5 2.399 2.977 80.1 Sept 60-70:6 3.707 2.203 168.3 Nov-Apr 70-80:1 3.610 3.660 98.6 Oct 70-80:2 2.059 2.656 77.5 Sept 80-90:1 2.058 2.368 86.9 Oct 90-100:1 1.767 2.415 73.2 Sept 90-100:2 2.439 4.071 59.9 Aug 90-100:3 1.594 1.903 83.8 Sept 100-110:1 1.627 1.964 82.8 Sept 100-110:2 2.456 2.753 89.2 Oct 100-110:3 2.511 3.337 75.2 Sept 120-130:1 2.503 2.869 87.2 Oct 120-130:2 2.341 2.554 91.7 Oct 120-130:3 2.245 3.163 71 Sept 120-130:4 2.216 2.466 89.9 Oct 130-140:1 2.078 2.602 79.9 Sept 150-160:1 2.807 2.939 95.5 Oct 170-180:1 1.699 1.921 88.4 Oct
Paynes Creek Rockshelter. CIMT was applied to 30 Margaritifera falcata specimens. The majority of the mussels had a death month of September and October.
The growth indices of thirteen mussels indicated a September death month while fourteen
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Figure 11. Paynes Creek rockshelter, back wall complex: growth indices clustered by month of death for Margaritifera falcata
mussels indicated an October death month. One mussel demonstrated an August death month and two died within the November to April interval.
As with the data from Sentinel Bluff Rockshelter, the same one month variation for the death month of the mussels from Paynes Creek Rockshelter was applied.
The seasonality information concludes that Paynes Creek Rockshelter was definitely inhabited and mussel procurement occurred during the late summer and fall months, primarily September through November, which is the same general finding as the
Sentinel Bluff Rockshelter data.
Summary
Chatters’ increment measurement technique was first applied to a collection of archaeological Margaritifera falcata samples from two rockshelters in northern
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California to determine the applicability of the method to archaeological specimens, and then to estimate seasonality of site occupation and mussel collection at the two rockshelters. Valve counts of the archaeological specimens were compared to the actual number of valves that CIMT could be applied to assess the performance and success rate of the method. While CIMT was successfully applied, less than fifty percent of the usable valves had visible growth lines to measure.
The data from Chatters’ increment measurement technique was tabulated for each excavated unit to determine the months of death for the mussel specimens. All units demonstrated resource procurement was limited to the late summer and fall almost exclusively. These results are consistent with my hypothesized account of the data,
Eugster’s (1990) study, and the estimated prehistoric mussel procurement patterns of northern California. In the winter and early spring months, northern California is consumed in heavy rains that saturate the many creeks, rivers, and water drainages. As water levels drop in the warmer, drier part of the year, accessibility into the habitat of the freshwater mussels is greatly increased and provides for more optimal mussel harvesting conditions.
In both excavated units at Sentinel Bluff Rockshelter, the greatest quantity of shell material, as well as the quantity of shell fragments containing a usable portion of the resilial tuberosity, were recovered from primarily the first cultural component, an excavated depth of 120 centimeters. This component dates to roughly 750 years B.P. and represents a late Holocene period occupation. The majority of the shell recovered from the Back Wall Complex of Paynes Creek Rockshelter was from the first 150 centimeters, dating roughly 2500 years B.P., and represents a Middle to Late period occupation. Susan
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Elizabeth Eugster, in her 1990 study, investigated the shell material from an archaeological site along the Sacramento River that dates to 350 years B.P., and represents the terminal phase of the prehistoric period. All three of these sites demonstrated a relatively consistent seasonality patterning of the late summer and fall months. In addition, Sentinel Bluff and Paynes Creek Rockshelters both demonstrate a
Late prehistoric period concentration of smaller and less nutritional resources, mussels, which is consistent with Broughton’s (1994) findings. He concluded that there was a period of resource intensification in the Sacramento Valley during the late Holocene as a result of population pressure. The archaeological evidence throughout many sites in the
Sacramento Valley demonstrates a greater concentration and a more intense exploitation on smaller resources of less nutritional value during this time.
CHAPTER VI
DISCUSSION
In Chapters IV and V, I demonstrated successful applications of Chatters’ increment measurement technique to a control sample of modern freshwater mussels and archaeological specimens. First, a sample of Gonidea angulata was collected from the Pit
River in northern California during the summer of 2009. CIMT was applied, and the results demonstrated a summer season of procurement, aligning with the collection dates.
The conclusive results from the application of CIMT to the control sample of modern mussels further established the accuracy and reliability of the technique. CIMT was then applied to a collection of archaeological freshwater mussels from two rockshelter sites in northern California to estimate season of mussel harvest and site habitation. In both instances, the results demonstrated a late summer and fall period of these activities.
In this chapter, I discuss the potential value of CIMT, limitations that were encountered while applying CMIT, and a discussion on seasonality of site occupancy in northern California. The first part of the chapter focuses on the value of CIMT and what this means for archaeology as a whole. This section is then followed by a discussion of the problem of growth line visibility while applying CIMT to the archaeological collection of freshwater mussels and the subsequent reduction in sample size. Lastly, I provide a discussion on the estimated seasonality of site occupancy obtained in Chapter
V, incorporating principles of human behavioral ecology.
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Potential of CIMT
The outcome of the application of Chatters’ increment measurement technique to both the control sample and archaeological specimens of freshwater mussels further established the accuracy and reliability of this method for estimating seasonality. CIMT is one of the more effective shell growth increment measurement methods for archaeological specimens. It is not only applicable to freshwater mussels of the Pacific
Northwest, but mussel species throughout the United States.
The growth-ring method, thin-sectioning, thick-sectioning, and acetate peels are four additional methods of measuring shell growth increments. The main problem with these four techniques is the inability to measure growth increments on archaeological specimens as a complete shell valve is required. Most archaeological freshwater mussels are fragmented and brittle due to post-depositional and taphonomic processes. As a result, seasonality estimates on archaeological specimens have been limited. For example, Casey (1986) was unable to perform a shell growth increment measurement method on a collection of archaeological freshwater mussels from
Kentucky as there was no method that accommodated for the fragmentary nature of the shells.
Unlike the previous methods discussed, CIMT only requires a small feature, called the resilial tuberosity, found on the shell hinge. The hinge is a denser element of the shell. This feature demonstrates greater preservation than the brittle outer shell as it can better withstand post-depositional forces. As a result, the hinge component is more apt to be intact in archaeological excavations.
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Furthermore, Chatters’ increment measurement technique is not species specific. Chatters has performed the method on at least ten species of freshwater mussels
(Chatters 1986, 1997, 2007; Chatters, Butler et al. 1995). As mentioned above, CMIT uses the resilial tuberosity, a feature that is attached to the resilium on the hinge ligament.
This feature is represented within bivalves of the superfamily Unionoidea that contain roughly 800 mussel species worldwide (Bogan 2008:142).
Aside from freshwater mussels native to the western part of the United States,
Chatters has successfully performed his technique on freshwater mussels from the
Midwest (Chatters 2007). In order to employ CIMT on additional Unionid mussel species throughout the United States, annual growth curves should be developed to establish growth indices. Once a growth curve is calculated, growth indices of archaeological specimens can be compared to the curve to assess season of death.
Chatters’ increment measurement technique is a potentially powerful tool for the field of archaeology. No method prior to this one was specifically designed for archaeological specimens, and seasonality estimates using mussels had been limited.
Furthermore, CIMT is not restricted to a particular geographic zone, but has the potential to be used worldwide.
Problems with CIMT
While CIMT is an accurate and reliable means to estimate seasonality, I encountered a few problems while executing the method. In this section, I will first discuss the considerable time, effort, and focus it takes to learn and accurately record and measure the growth lines. Following this I will discuss how growth line visibility is
77 commonly reduced by natural agents and too much scraping of the organic matter from the resilial tuberosity. Lastly, as a result of the inability to read and measure the growth lines, I will discuss the ensuing reduction in sample size.
As with any specialized technique, it takes time to perfect the application and execution of Chatters’ increment measurement technique. Aside from some knowledge in shell anatomy, the executioner needs to be able to identify and precisely measure the growth increments. It took considerable time and effort to replicate CIMT, and one of the more important factors in my learning the technique was meeting with James C. Chatters himself to have him demonstrate the proper application. Even after meeting and learning the technique from Chatters, I still had to constantly re-measure the growth increments on each shell to obtain the most accurate measurements.
There was a considerable difference in the total number of left valves and the number of left valves from which the growth lines could be read. Of the three excavated units from the two archaeological rockshelters, 31.4 to 48.4 percent of the left valves had distinguished growth lines and CIMT could be applied. Natural agents like carbonic acid and abrasion from sand and gravel can erode and damage the growth lines on the resilial tuberosity. I also discovered the visibility of growth lines can be damaged by overly zealous scraping.
Chatters (1987; 2007) suggests lightly scraping the resilial tuberosity to rid the surface of organic matter that obstruct the visibility of the growth lines. There is a definite change in consistency when scraping the resilial tuberosity. The texture of the shell is stone-like while the organic matter flakes off when dry or becomes mushy when
78 moistened. One could potentially scrape the growth lines off the resilial tuberosity, so careful attention should be paid with this step.
During the application of CIMT to the archaeological specimens, I found that even lightly scraping the organic matter from the resilial tuberosity can easily eliminate the growth lines. In fact, when the resilial tuberosity was viewed under the microscope prior to scraping the organic matter, the growth lines were more often visible than after scraping the resilial tuberosity. There were several cases where the growth lines were never visible, whether scraping occurred or not, which I attributed to the consequences of the natural agents on the mussels. Due to the ease that the growth lines can be damaged, I recommend examining the resilial tuberosity under the microscope first before any scraping is completed.
The sample size was greatly reduced due to the inability to read the growth lines on a substantial percent of the left valves. Small samples can produce more specific, yet less accurate and reliable seasonality estimates (Monks 1981:223). Monks (1981:224) mentions that it “is not the absolute size of the sample but the relative reliability of seasonality estimates derived from whatever sample is available.” Freshwater mussels produce consistent seasonality estimates as a result of the annual deposition of growth lines. The calculated growth index is compared to an annual growth curve and a season of mussel death is reliably determined, with some variation considered. Chapter IV demonstrated the accuracy and reliability of Chatters’ increment measurement technique.
While working with a reduced sample size of archaeological mussels, definite conclusions of season of mussel collection and site occupation were formulated. In addition, the results demonstrated a strict seasonality pattern of mussel collection as a late
79 summer and fall activity, which is consistent with a previous study of the area (Eugster
1990). Furthermore, a reduction in sample size commonly plagues archaeological data and is not unique to this study.
Determination of Season of Habitation
In Chapter V, I applied CIMT to a collection of archaeological freshwater mussels from the Sentinel Bluff and Paynes Creek Rockshelter sites in northern
California. The results demonstrated that mussel harvest, and site habitation, was almost exclusively limited to September through November, primarily the late summer and fall seasons. The rockshelters were not necessarily unoccupied the rest of the year. While the prehistoric procurement of freshwater mussels was limited to the late summer and fall, the indigenous groups could have still been inhabiting the rockshelters the rest of the year, concentrating on other food resources. As seasonality studies are used to assess season of activity and season of site occupancy, and consequently have limitations, these are two distinct phenomena.
Season of activity and site occupancy cannot necessarily be assumed the same. Monks’ (1981) warning against seasonality assumption of habitation is based largely on the case of movement of archaeological material through both cultural and natural agents. While the material was recovered from the archaeological sites, it may have originated from a separate locale and transported. Seasonality of an activity, for instance mussel death, is inherent in the data regardless of movement.
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Monks (1981) indicates the possibility exists that the mussels could have been procured at a separate locale and transported to the sites To evaluate this question, I examine some fundamental principles of human behavioral ecology.
Mollusks have been viewed as a low nutritional food resource and, as a result, of low economic priority (Claassen 1998:181). Debates have formed concerning the economic importance of mussels to Native peoples and the associated behavioral practices and selectionist techniques (Claassen 1986; Erlandson 1988; Glassow and
Wilcoxon 1988; Parmalee and Kippel 1974). Most of these issues have been concerned with marine mussels, which have been viewed by some as a higher-ranked staple in prehistoric subsistence economies based on marine resources (Braje and Erlandson
2009:270).
Marine mussels are typically found in closely clustered groups and, as a result, are easy to harvest (Braje and Erlandson 2009:271). For example, the marine mussel
Mytilus californianus (California mussel), forms sheet colonies that are characterized by a large quantity of mussels that attach to each other, not the initial rock that was originally anchoring the colony (Bettinger et al. 1997:896). The high quantity of marine mussels in a colony provide for a more cost effective resource procurement activity as search time is greatly reduced. Furthermore, the marine habitat remains relatively constant as the seasons change with little variation in optimal conditions for mussel harvest. While prey size and nutritional value are commonly viewed as a positive correlation, smaller resources found in high abundance may attain a higher return rate for energy spent than larger resources of that same area (Braje and Erlandson 2009:271).
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The freshwater mussel Margaritifera falcata resides in beds with varying population size. These mussels do not attached to each other but separately subsist on a sandy substrate. Compared to the marine mussel, collection of this mussel takes considerable more time and energy as they are not as tightly clustered. Furthermore, unlike a marine setting, the freshwater mussels reside in a dynamic habitat. Changes in creek and river flow and discharge change with the seasons. In the winter and spring months in northern California, heavy rains plague the region that raise the water levels in the local water concourses. By the beginning of summer and into fall, the rains cease and the water levels continuously drop until winter when the rains begin once again. As a result, optimal conditions for freshwater mussel harvest in northern California occur in the late summer and fall months when water levels are low and consequently, the mussels are more easily accessible. The results from the application of CIMT to the archaeological shell coincide with this trend.
Concentration and transportation of freshwater mussels within the area of the two rockshelters would have been limited due to the reduced nutritional value of the mussels. In addition, the two archaeological rockshelters reside adjacent to water concourses and within close proximity to the Sacramento River. Collection of mussels from these water sources would have imposed a relatively low travel cost as the rockshelters are near the area of mussel procurement. If the mussels were harvested from a water source of considerable distance from the rockshelters then a much higher travel cost would have resulted. Therefore, it would have been economically cost effective to harvest the mussels from the adjacent water sources, indicating a similar season of habitation. Even if the mussels had come from a distant source and taken to the
82 rockshelters, the month of death data would still be indicative of the season of site habitation as the mussel cannot survive out of water and would have been consumed close to the time of collection.
Monks (1981) warning against the assumption of seasonality of site habitation or occupancy is based largely on artifact displacement. As freshwater mussels are of low nutritional value when compared to other resources in the area, it would not have been of great economic value to collect and then transport the freshwater mussels. Although, even if the mussels had come from a distant source, the mussel death data would be indicative of season of site habitation as the mussels cannot survive out of water and would have been consumed almost immediately. Similarly, the local harvest of mussels would be indicative of a similar season of site habitation.
Summary
Chatters’ increment measurement technique can potentially be employed on freshwater mussels throughout the United States, which could benefit the field of seasonality estimates through the analysis of archaeological shell. Seasonality estimates that employed the other four shell growth increment measurement methods have been limited as a result of the fragmentary nature of the archaeological shell. CIMT accommodates for the incomplete state of archaeological specimens by only using a small portion of a feature of the shell hinge, a component of the shell that coincidentally preserves well.
While several problems arose through the process of applying Chatters’ shell growth increment measurement method on the archaeological collection of freshwater
83 mussels, the accuracy and reliability of the technique was demonstrated. The inability to distinguish growth lines as a combined result of arbitrary natural processes and human error did reduce the sample size from each excavated unit of the two rockshelters, but
CIMT could still be applied. The reliability of the method as well as the concise results, consistent with other local studies, overshadow the reduced sample of usable archaeological specimens.
Finally, the determination of season of site occupancy that was concluded in
Chapter V was examined under the consideration of human behavioral ecology principles. If the archaeological shell had been collected in a separate location and later transported to the rockshelter sites, the estimated season of site occupancy of the late summer and fall months remains a probable outcome. It would not have been cost effective for the prehistoric inhabitant to transport the nutritional lower valued freshwater mussels to the rockshelters, and instead, a local harvest from the adjacent streams is more probable. As a result, the season of site occupancy and mussel harvest are considered comparable.
CHAPTER VII
CONCLUSIONS
Throughout this thesis I have provided an overview of shell growth increment measurement methods and an assessment of one of those techniques that was developed by James C. Chatters in the 1980s. In the first part of the thesis, I evaluated Chatters’ increment measurement technique by applying it to a control sample of Gonidea angulata specimens from the Pit River in northern California. My results statistically confirmed and the accuracy and reliability of CIMT.
I then employed Chatters’ increment measurement technique to two collections of freshwater mussels from archaeological rockshelter sites in northern
California and confirmed the applicability of CIMT to archaeological specimens, despite a reduce sample size. Furthermore, I demonstrated the season of prehistoric mussel procurement and site habitation were almost exclusively limited to the late summer and fall months. While acknowledging the potential error in assuming the season of site occupancy to be the same as mussel harvest, a look into the principles and models of human behavioral ecology affirmed the estimated season of site habitation.
Prior to the development of CIMT, there were four main shell growth increment measurement methods that lacked the applicability to archaeological specimens. Designed specifically for use on archaeological freshwater mussels and
84 85 applicable to nearly 800 mussel species worldwide, CIMT has the ability to further propel seasonality estimates in the field of archaeology.
Further Research
This thesis evaluated the seasonality of mussel procurement and habitation at
Sentinel Bluff and Paynes Creek Rockshelters in northern California. A more in-depth analysis of the additional faunal material from these sites would be desireable to offer a more comprehensive assessment of the overall seasonality at the two archaeological sites.
The estimation of seasonality obtained from the freshwater mussels have provided an initial synopsis of site seasonality. The direct methods for estimating seasonality provide a more comprehensible understanding of seasonality but are not without limitations. A combined use of several of these methods could provide for more affirmative estimations of overall site seasonality.
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