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Date:______

I, ______, hereby submit this work as part of the requirements for the degree of: in:

It is entitled:

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Chair: ______

TAPHONOMY OF THE MOTHER’S DAY QUARRY: IMPLICATIONS FOR GREGARIOUS BEHAVIOR IN SAUROPOD DINOSAURS

A thesis submitted to the

Division of Research and Advanced Studies Of the University of Cincinnati

in partial fulfillment of the requirements of the degree of

MASTER OF SCIENCE

In the Department of Geology of the College of Arts and Sciences

2004

Timothy S. Myers

B.A. & B.S., Rice University, 2002

Committee Chair: Dr. Glenn W. Storrs

ABSTRACT

The Mother’s Day Quarry, located in the Upper Jurassic Morrison

Formation of south-central Montana, contains the remains of a number of

immature diplodocid sauropod dinosaurs. If the sauropod individuals were initially

part of a herd group, the site would be one of only a few thought to provide

skeletal evidence of gregarious behavior in sauropods. To date, none of these

sites has been taphonomically constrained in order to determine if they do, in fact, contain the remains of a herd; our only reliable data on sauropod herds

currently comes from the ichnological record, which often provides conflicting

information. Therefore, the Mother’s Day site has the potential to reveal

interesting details of sauropod behavior and clarify ambiguities in the trackway data.

This study determines the suitability of the Mother’s Day Quarry as a basis for behavioral interpretations. By reconstructing the taphonomic history of the assemblage, potential biases may be identified, and their implications explored.

Analysis of the site reveals that post-mortem biases are minimal, and the sauropods in the quarry likely represent the remnants of a mobile social group.

The absence of mature adult individuals in the assemblage appears to be real, and is probably the result of age segregation of some sauropod herd groups.

iii

ACKNOWLEDGEMENTS

I would like to start by thanking my committee chair, Dr. Glenn Storrs, for his valuable guidance and support throughout the course of this research. His careful editing and insightful comments have made this a much better document than it would otherwise be. I would also like to thank my other committee members, Dr. Carlton Brett and Dr. David Meyer, for the helpful suggestions and candid assessments that they provided in committee meetings and on the initial draft of this work.

Dr. Warren Huff patiently provided much needed assistance with the XRD portion of this study, for which I am most grateful.

I am also grateful for the dedication and stoicism of my field crews in

Montana. Dr. Jim Clark, Peter Falkingham, Dale Gnidovec, Angela Horner, Ben

Otto, Sam Perry, Ji-Yeon Shin, and Shizuko Watanabe all have my gratitude for working tirelessly in the hot sun to help collect data and fossils. I owe an equal debt to all the volunteers in the Museum Center Paleo Lab, who spent so many hours preparing Mother’s Day material, especially Charlotte Cox. Their help was indispensable.

I would like to thank Dr. Kristi Curry-Rogers for the use of her field notes from the 1995 and 1996 seasons, which proved invaluable for improving the MNI estimate for the assemblage. She and Dr. Ray Rogers have been a source of continual support and encouragement from the first summer I spent at the site.

I am especially grateful to Mike Papp, who often spent his entire weekends in the lab preparing Mother’s Day specimens. He was also a pleasure

to work with in the field, both for his skill at excavation and his delightful

company.

Finally, I must thank Bill Garcia for his constant support in the field and in

the office. He has lent his valuable proofreading skills on more than one occasion

and has been a source of knowledge, advice, and constant friendship throughout

this project.

Financial support for this research was provided by the Cincinnati

Museum Center, a summer research stipend from the University of Cincinnati

Department of Geology, and grants from Sigma Xi, the Jurassic Foundation, and the Theodore Roosevelt Memorial Fund of the American Museum of Natural

History.

TABLE OF CONTENTS

ABSTRACT ...... ii

ACKNOWLEDGEMENTS ...... iv

LIST OF FIGURES ...... 3

LIST OF TABLES...... 5

LIST OF EQUATIONS ...... 6

CHAPTER 1. Taphonomy of the Mother’s Day Quarry

INTRODUCTION...... 7

PREVIOUS WORK...... 9

REGIONAL GEOLOGY ...... 10

STRATIGRAPHY ...... 11

REGIONAL PALEOCLIMATIC SETTING...... 17

METHODOLOGY...... 19 Field Methods...... 19 Lab Methods...... 24

TAPHONOMY ...... 27

GEOLOGICAL EVIDENCE...... 27 Sedimentology...... 27

BIOLOGICAL EVIDENCE ...... 45 Element Location...... 45 Element Orientation...... 48 Voorhies Groups...... 54 Bone Modification ...... 56 Preservation Pattern...... 61 Taxonomic Diversity ...... 68 Number of Individuals...... 71 Age Profile...... 72

DISCUSSION...... 76

CONCLUSIONS...... 82

CHAPTER 2. Implications for Gregarious Behavior in Sauropod Dinosaurs

INTRODUCTION...... 85

DEFINITION OF A HERD ...... 86

ICHNOLOGICAL EVIDENCE...... 87

LIMITS OF THE ICHNOLOGICAL RECORD ...... 89

TAPHONOMIC CONSTRAINTS FOR SKELETAL EVIDENCE ...... 92

MOTHER’S DAY QUARRY...... 93

OTHER POSSIBLE HERD ASSEMBLAGES ...... 98

BEHAVIORAL INFERENCES ...... 99

CONCLUSIONS...... 102

REFERENCES...... 103

APPENDIX ...... 114

LIST OF FIGURES

Figure 1. Map of Montana ...... 8

Figure 2. Map of the Bighorn Basin...... 12

Figure 3. Extent of the Morrison Formation...... 13

Figure 4. Percent smectite in the Morrison section on the Colorado Plateau...... 16

Figure 5. Percent smectite in the Morrison section at the Mother’s Day site ...... 16

Figure 6. Lithologic logs for the Morrison section at the Mother’s Day site...... 18

Figure 7. Position of lateral grab samples and vertical test pits within quarry...... 23

Figure 8. Diffractograms for two lateral grab samples...... 28

Figure 9. Grain size plots of vertical grab samples from Test Pit I ...... 29

Figure 10. Grain size plots of vertical grab samples from Test Pit II ...... 30

Figure 11. Grain size plots of lateral grab samples ...... 31

Figure 12. Sedimentary features ...... 32

Figure 13. Distribution of pebbles and thoracic skeletal elements...... 34

Figure 14. Depth distribution of pebbles in bonebed...... 36

Figure 15. Plot of clast shape for Mother’s Day pebbles ...... 39

Figure 16. Plot of clast shape for Cedarosaurus gastroliths ...... 39

Figure 17. Plot of clast shape for Seismosaurus clasts...... 40

Figure 18. Plot of sphericity vs. clast size for Mother’s Day pebbles...... 41

Figure 19. Plot of sphericity vs. clast size for Cedarosaurus gastroliths...... 41

Figure 20. Comparison of clast shape distributions...... 42

Figure 21. Vertical distribution of items within the quarry ...... 46

Figure 22. Horizontal distribution of elements from 25 cm depth intervals...... 47

Figure 23. Vertical distribution of large and small elements ...... 49

Figure 24. Rose diagram of 2-D orientations of elongate elements...... 51

Figure 25. Stereonet plot of 3-D orientations of elongate elements...... 53

Figure 26. Ratios of Voorhies Group II to Group I ...... 58

Figure 27. Distribution of weathering stages in assemblage ...... 62

Figure 28. Bone clasts...... 62

Figure 29. Skin impressions ...... 64

Figure 30. Articulated elements...... 65

Figure 31. Articulated elements in assemblage ranked according to different disarticulation sequences ...... 67

Figure 32. Articulated elements in assemblage ranked according to differing disarticulation sequences (some pelvic joints excluded) ...... 67

Figure 33. Diagnostic elements...... 70

Figure 34. Vertebrae with unfused neurocentral sutures...... 74

Figure 35. Ratios of appendicular to axial skeletal elements...... 78

Figure 36. Ratios of anterior to posterior skeletal elements ...... 78

LIST OF TABLES

Table 1. Calculation of significance for petals of rose diagram ...... 50

Table 2. Calculation of expected Voorhies Group II / Group I ratio ...... 57

Table 3. Bone modification features...... 59

Table 4. Calculation of percent adult size for elements...... 75

LIST OF EQUATIONS

Equation 1. Dip correction formula...... 21

Equation 2. Formula for sphericity...... 25

Equation 3. Formula for calculating hydraulic equivalence...... 43

CHAPTER 1. Taphonomy of the Mother’s Day Quarry

INTRODUCTION

The Mother’s Day Quarry is a paucispecific fossil locality located in the Upper

Jurassic Morrison Formation of south-central Montana (Fig 1). The bonebed is located in the lower Salt Wash member of the Morrison and dates to approximately 150 Ma.

Species diversity at the site is relatively low for a large Morrison fossil accumulation. Six years of excavation have confirmed the presence of only two taxa: a theropod genus identified as ?Allosaurus and diplodocid sauropods determined to be Diplodocus sp.

The sauropod material comprises over 99% of the recovered, identifiable material, with only five elements – all tooth crowns – definitively attributable to theropods.

Interestingly, all the sauropod material retrieved from the quarry thus far belongs to juvenile or subadult individuals; no adult material has yet been found. Such an odd age profile and heavily skewed relative taxonomic abundance are rare in vertebrate fossil assemblages and must be the product of either unique taphonomic processes or unique characteristics of the living assemblage.

This study attempts to answer the following questions:

1) Does the Mother’s Day Quarry contain the remnants of a sauropod herd?

2) If so, is the taphonomic overprint at the site either minimal or removable?

If the answer to both these questions is yes, we can also ask the following

behavioral question:

3) Is there evidence of age segregation or a multispecies group at the site?

Figure 1. Map of Montana showing location of Carbon County and the Mother’s Day Quarry.

The Mother’s Day site is one of only a few known skeletal accumulations that

are thought to contain the remnants of a herd of sauropods, and none of these other

sites has yet been taphonomically analyzed to insure that the perceived patterns are not

artifacts of post-mortem processes. If the unique characteristics of the Mother’s Day

Quarry – the monospecific nature of the assemblage and the lack of adult individuals –

are not simply taphonomic artifacts, the site may yield the first well-constrained skeletal evidence of age segregation of sauropod herds, providing confirmation that age was one of the factors controlling sauropod herd composition. Once they have been thoroughly investigated, this locality and others with similar suites of characteristics have the potential to reveal intriguing details of gregarious behavior in sauropods and to confirm or elaborate on behaviors that were previously known only from the ichnological record.

PREVIOUS WORK

The Mother’s Day Quarry was discovered in 1994 by a volunteer at the Museum of the Rockies in Bozeman, MT (Horner & Dobbs, 1997). Subsequently, the Museum of the Rockies (MOR) launched excavation operations in the summer of 1995 and concluded their recovery operations after a second season of collecting in 1996. In their two years of excavation activity, MOR crews, under the direction of Kristi Curry-Rogers, removed and mapped a total of 502 identifiable elements (Curry-Rogers, unpublished data). The Cincinnati Museum Center (CMC) assumed excavation responsibilities beginning with an abbreviated exploratory season in 1999, and work has continued for the past four summer field seasons. To date, 779 identifiable elements have been

removed by Museum Center workers. Of this number, approximately 38% have been

prepared and curated in the CMC collections.

REGIONAL GEOLOGY

The Morrison sediments containing the Mother’s Day Quarry were deposited

during the Late Jurassic following the northward withdrawal of the Oxfordian seaway

from the Western Interior at the end of the upper Sundance transgressive/regressive

cycle (Peterson, 1988). As the Elko Orogeny drove uplifts to the west of the Colorado

Plateau in present-day western Utah and eastern Nevada, Morrison sediments were

shed eastward, forming a broad alluvial plain composed of predominantly fluvial and

lacustrine environments (Peterson, 1994). These sediments continued to accrete

throughout the Kimmeridgian and the early Tithonian, creating sections several hundred

meters thick in the southwest, thinning to around 25 m at the formation’s northeastern

limits (Foster, 2003). Based on 40Ar/39Ar dating done by Kowallis et al. (1998), Morrison

sedimentation is estimated to have spanned a total of 8 million years, beginning around

155 Ma and ceasing at roughly 148 Ma. The primarily Kimmeridgian/Tithonian age of

the Morrison implied by isotopic dates is corroborated by palynological,

magnetostratigraphic, and biostratigraphic evidence (Litwin et al., 1998; Schudack et al.,

1998; Steiner, 1998). Morrison accretion ended near the middle of the Tithonian. The remainder of the Jurassic, an interval of approximately 3-7 million years, was marked by non-deposition and significant amounts of erosion in some areas that resulted in a region-wide unconformity (Pipiringos, 1968). Following this hiatus, deposition resumed and continued throughout the Cretaceous until the beginning of the Laramide Orogeny

in the early Paleocene (Baars et al., 1988). This event was marked by uplift of the

Beartooth Mountains and the Absarokas to the west, the Pryor Mountains and the

Bighorn Mountains to the east, and the Owl Creek Range to the south (Baars et al.,

1988). These uplifts created the asymmetric feature known today as the Bighorn Basin, stretching from central Wyoming northward to southern Montana (Fig 2). Strata on the western margin of the basin are sharply folded, and even overturned in some areas. In contrast, strata on the eastern and southern edges dip more gently basinward. The northernmost extension of the basin opens into south-central Montana, and the

Mother’s Day Quarry is located on the northeastern rim, amidst strata dipping shallowly southwest from the Pryor Mountains.

STRATIGRAPHY

The Morrison Formation is a vast stratigraphic unit, stretching from Utah in the west to Oklahoma in the east, and from New Mexico north to Canada (Fig 3). The lower boundary of the Morrison is defined by the J-5 unconformity (Pipiringos, 1968). In

Montana, lower Morrison strata rest atop the Swift Formation, a shallow marine sandstone unit that is late Oxfordian in age (Peterson, 1988). At the upper end of the section, the disconformity that signals the end of Morrison deposition and divides the upper Morrison from Lower Cretaceous Cloverly (Kootenai) sediments is termed the K-1 unconformity (Pipiringos, 1968). In many areas of the northeastern Bighorn Basin, the upper margin of the K-1 unconformity is marked by a chert pebble conglomerate known as the Pryor Conglomerate (Moberly, 1960). This unit is thought to be Early Cretaceous in age and has been correlated to conglomerates in the nearby Powder River Basin, the

Figure 2. Map of Bighorn Basin showing Laramide uplifts and location of Mother’s Day Quarry. Modified from Baars et al. (1988).

Figure 3. Map showing the extent of the Morrison Formation (outcrop and subsurface) relative to location of Mother’s Day Quarry. Modified from Turner & Peterson (1999).

Ephraim Conglomerate in the Gannet Group farther west, and the lower portion of the

Kootenai Formation in southwestern Montana (Swierc & Johnson, 1997).

In some areas of the Colorado Plateau, the Morrison can be divided into as many as nine distinct members (Peterson, 1994); however, in Montana and portions of South

Dakota, it is divisible into only two: the lower Salt Wash Member and the upper Brushy

Basin Member (Turner & Peterson, 1999). The Salt Wash is generally characterized by a high frequency of sandstone beds and conglomerates, suggesting significant fluvial influence and a proximal location relative to regional clastic sources (Kowallis et al.,

1998). In contrast, the Brushy Basin is dominated by thick mudstones indicative of lacustrine conditions and increasing fine-grained volcaniclastic input (Bilbey, 1998;

Kowallis et al., 1998). Traditionally, the Morrison has been subdivided into Upper and

Lower sections based on the presence of a clay mineral transition located within the

Brushy Basin Member (Turner & Peterson, 1999). This transitional horizon marks the boundary between units of the Lower Morrison that contain detrital clay minerals such as illite and its derivatives, and rocks in the upper part of the formation that consist primarily of authigenic clays, such as montmorillonite and smectite, that were produced by the argillation of volcanic ash (Keller, 1962; Turner & Fishman, 1991). Since the

Morrison clay change represents a more or less isochronous horizon, it has been used as a marker bed in the correlation of different Morrison sections throughout the Western

Interior (Owen et al, 1989; Turner & Peterson, 1999).

First identified in the Colorado Plateau area (Turner & Fishman, 1991), the clay change horizon diminishes in intensity to the north and east of the plateau to the point that it is no longer detectable in Montana, South Dakota, or northeastern Wyoming

(Turner & Peterson, 1999). Schudack et al. (1998) attributed this progressive weakening of the clay transition signal to reduced levels of smectite content in the upper Brushy

Basin that resulted from decreasing volcaniclastic input with increasing distance from the magmatic arc located to the west. Since the clay transition is not present in southern

Montana, it is extremely difficult to correlate the stratigraphic position of the Mother’s

Day Quarry with other Morrison sections of known age and stratigraphic location. The absence of the clay change at the Mother’s Day locale was confirmed by recent investigations connected with this study. X-ray diffraction analysis of the mudstone layers in the Morrison section adjacent to the quarry revealed no smectite-rich clays similar to those reported from localities on the Colorado Plateau by Owen et al. (1989)

(Figs 4-5).

Biostratigraphic indicators are the other method typically used to determine stratigraphic position within the Morrison Formation. Schudack et al. (1998) defined stratigraphic zones within the Morrison based on the presence of different assemblages of ostracodes and charophytes. Although ostracodes have been observed in the stratum directly overlying the Mother’s Day Quarry, this location was not sampled, and no ostracodes were present in samples taken from stratigraphically equivalent beds. No biostratigraphically useful fossils were observed in or recovered from other portions of the local section, making identification of Schudack’s biozones in the area of the quarry impossible. However, based on regional lithologic correlations done by researchers at the USGS, the Mother’s Day site is projected to fall within the Salt Wash Member of the

Morrison (Peterson, pers. comm.), suggesting an age close to 150 Ma for the quarry.

The uppermost portion of the Brushy Basin appears to be missing near the site

15 Meters Above Section Base

0 0 102030405060708090100 Percent Smectite in Illite/Smectite

Figure 4. Graph showing percent smectite in mixed clay mineral mudstones in the Morrison section at the Mother’s Day site. The clay transition is not present at this locale.

Figure 5. Graph showing percent smectite in mixed clay mineral mudstones in the Brushy Basin Member at a locality on the eastern Colorado Plateau (Owen et al., 1989). The clay transition is clearly evident in this area of the Morrison Formation.

(Peterson, pers. comm.), eroded away during the deposition of the Pryor Conglomerate that sits atop the Mother’s Day section (Fig 6).

REGIONAL PALEOCLIMATIC SETTING

The paleoclimate of the Morrison Formation, long a subject of controversy, has been historically interpreted as both a mesic and arid environment (see references in

Dodson et al., 1980). The paleoenvironmental indicators used to validate these conclusions often conflict with one another. Much of the macrofloral remains in the

Morrison consists of ferns, cycadophytes, and other plant types that are normally indicative of wet, humid conditions (Ash & Tidwell, 1998); and the presence of aquatic vertebrates and invertebrates indicates that some of the water sources in the formation were perennial (Dodson et al., 1980; Demko & Parrish, 1998). However, geological indicators such as eolian deposits and evaporites suggest that the Morrison environment experienced at least intermittent periods of aridity (Peterson, 1994).

Paleogeography also supports the case for predominately arid climatic, for the downwind position of the Western Interior relative to the magmatic arc located to the southwest placed much of the Morrison depositional area within a large rain shadow, and the mid-latitude position (30°-35°N) of this portion of the continent in the Late

Jurassic would only magnify the already arid conditions (Peterson, 1994). What were initially seen as mutually exclusive environmental interpretations have now been incorporated within a seasonal model (Dodson et al., 1980). Accordingly, the Morrison is thought to have formed under conditions that were generally semi-arid, with periodic

Figure 6. Lithologic logs of Morrison section at the Mother’s Day site. Note location of Pryor Conglomerate at top of section and contact with Swift Formation at bottom of right two columns.

increases in precipitation that may have occurred seasonally (Demko & Parrish, 1998).

The presence of carbonate nodules along many of the horizons in the formation supports the seasonal model, for such sedimentary features are typically associated with soil formation in episodically moist environments (Demko & Parrish, 1998).

METHODOLOGY

This study consists of two fundamental parts: one based in the field and the other concentrated in the lab. The field component of the investigation focused on excavation and collection of material at the Mother’s Day Quarry, recording observations of in situ elements, and sampling of quarry sediments and nearby strata. The lab portion encompassed preparation of the elements collected in the field, recording of element characteristics, and analysis of sediment samples. Analytical and observational specifics for each portion of the study are detailed below.

Field Methods

The position of each bone in the quarry was mapped prior to its removal. Bearing and distance of the center of each bone from a datum spike were measured using a

Brunton pocket transit and tape measure. Depth below datum, measured with a line level and tape measure, was also recorded, with the highest point on the bone used for reference. If an element qualified as elongate (length to width ratio of 2:1 or greater), trend and plunge were also recorded. Trend was always measured toward the distal end of an element when proximal-distal orientation could be determined, and the plunge was given a corresponding positive or negative value. For example, an element

plunging distally would be assigned a negative plunge value, whereas an element

plunging proximally would have a positive plunge value in the distal trend direction.

Vertebral trends were recorded using the anterior end of the centrum when possible.

These measurement standards allow for precise reconstruction of the 3-dimensional

orientation of each element within the bonebed.

Before these data could be used for assessing orientation and depth distribution

in the quarry, the values had to be corrected for the dip of the bonebed. No planar

bedding surfaces appropriate for measuring orientation were available in the quarry, so

measurements were taken on adjacent strata, and these values were averaged to

produce an estimate of the orientation of the bonebed. Based on these measurements,

the bonebed is inferred to dip at an angle of 19° along a bearing of 255° – a shallow dip

to the southwest. To correct the bone orientation data for the dip of the quarry strata,

the trend and plunge of each elongate element were entered into Cauldron 2 (Stuart,

2001), a stereonet program, and rotated 19° around the strike of the bed (345°).

However, while orientation information was recorded for all four years of CMC excavation, entries for the first two years (2000-2001) include only trend data. MOR field

notes from 1995 and 1996 did not record either trend or plunge information. Without

knowing the plunge of the elements collected in 2000-2001, their recorded trends could

not be corrected using the stereonet rotation. Therefore, the dip correction was applied

to only the data collected in 2002 and 2003.

When the corrected 2002-2003 data were compared with the uncorrected data

from those same seasons, the mean lineation orientation for the corrected data did not move from the 95% confidence interval for the uncorrected data set. So, the rotation

technique used to correct for the dip of the bonebed was assumed to have a negligible effect on the orientation of elements, and both corrected (2002-2003) and uncorrected

(2000-2001) data were included in the rose diagram of elongate elements. The lower hemisphere stereonet projection used to assess imbrication within the quarry was constructed using only the corrected 2002-2003 data set since both trend and plunge information were necessary for this diagram.

The dip of the quarry beds affected depth calculations as well as orientation measurements, so all depth-below-datum measurements required correction for the dip of the local bedding so that they would reflect actual depth below the upper contact of the bonebed unit (Eq 1).

δ – corrected true depth q – dip of quarry bed (19°) δ = (cos q)⋅(d − D ⋅[tan q ⋅cos(t − b)]) t – trend of quarry dip (255°) b – bearing of element from datum d – depth of element below datum D – distance of element from datum Equation 1. Dip correction formula for depth-below-datum values associated with elements.

The applied correction factors produced some true depth values that indicated positions above the upper contact of the bonebed, suggesting that the average of the local dips does not reflect the precise dip in the quarry, thereby introducing error into the calculated depths-below-surface. However, any errors resulting from inaccuracies in the dip measurements should be the same for each calculated true depth value, generating a data set that does not accurately represent the depths of elements below the upper contact of the bonebed, but is still internally consistent and faithfully reproduces the relative depth distribution of all items. Other factors also could be responsible for the observed inaccuracies. The functions used to correct the depth values are extremely

sensitive to variations in measurement. Changes in the measured bearing as small as

1° can produce errors as large as 7 cm in the calculated true depth-below-surface.

Errors of 5 cm in measured depth-below-datum result in changes of ± 5 cm in calculated depth; however, the depth calculations are less sensitive to error in distance-from- datum, with changes as great a 10 cm having no substantial impact. Assuming error in all measured aspects of element position generated estimated total errors of ± 12 cm, the corrected depth values calculated for the elements in the quarry are only reliable when used in relatively low resolution analyses with vertical depth intervals on the order of 25 cm thick.

In addition to data collection related to the bone material in the quarry, a number of sediment grab samples were taken from the quarry and the surrounding strata. The samples were intended to reveal any grain size variation in the bonebed deposit that might be indicative of the depositional environment at the quarry. Each sample was removed from the quarry floor, and its position was mapped relative to the datum (Fig

7). Great care was taken to sample only in situ matrix rather than displaced rock, and an effort was made to achieve a wide lateral distribution of samples so that every part of the quarry was represented. Any areas that appeared to have unique coloration or texture were also sampled. The samples taken from the layers surrounding the quarry stratum were collected for x-ray diffraction analysis in order to confirm the absence of the Morrison clay change in the area.

Figure 7. Map showing the position of lateral grab samples and vertical test pits within the quarry.

Lab Methods The lab component of this study involved analyses of both the sediment grab samples and the skeletal elements collected from the quarry. All sediment samples were first disaggregated using dilute hydrogen peroxide for the mudstones and concentrated hydrochloric acid for the sandstones. Following disaggregation, samples were dried and used in either x-ray diffraction or grain size analysis.

X-ray diffraction analysis of two powdered bulk samples from the quarry, performed with a Siemens D500 diffractometer, was used to ascertain the mineralogical composition of the quarry sediments. Samples from mudstones in the local stratigraphy were used to make clay mounts in an attempt to isolate the Morrison clay transition near the site. The difference in peak area between heated and glycolated samples, calculated using MacDiff (Petschick, 2000), allowed a determination of the relative abundance of smectite in the mixed illite-smectite clays. The grab samples collected from the quarry floor and the vertical test pits nearby were analyzed with a Beckman

Coulter particle size analyzer in order to reveal any horizontal or vertical transitions in grain size. Each sample was sonicated before analysis to break down any pieces of matrix that were not fully disaggregated by the acid and peroxide treatments. Three analytical runs were performed on every sample, and the average of these analyses was used to produce the grain size curves.

The gravel-sized clasts found within the bonebed were also carefully analyzed in order to determine whether they were gastroliths derived from the sauropod carcasses or were simply part of a fluvial lag that was entrained in a transport event along with the skeletal material in the quarry. The pebbles were described using the technique outlined in Sanders et al. (2001). The three principal axes of each clast were measured using

calipers. Masses were measured using a digital analytical balance, and volumes were estimated using the density of quartz (2.65 g/cm3) – since most of the clasts are composed of quartzite or chert – and dividing this density value by the mass. Once a volume was obtained, the sphericity of the pebbles could be quantified with the formula provided in Sanders et al. (2001) (Eq 2).

 d  6V  1  3 sphericity =   where d1 =  d2  π

Equation 2. Formula for sphericity from Sanders et al. (2001). Variable d1 is the diameter of a sphere with the same volume as the clast in the calculation, and d2 is the length of the major axis of the clast.

Using this equation, sphericity is defined as the ratio of the diameter of a perfect sphere with the same volume as the clast being measured to the diameter of the major axis of that clast. Roundness values for each of the clasts were determined by visual comparison with the roundness chart provided in Krumbein (1941). As visual characterization of roundness is a subjective measure, this variable was not emphasized in comparison of the Mother’s Day clasts with known gastroliths. Instead, more easily quantifiable measures, such as sphericity, were the focus of the evaluation.

Lab work also involved careful observation and documentation of prepared skeletal elements. Each element was examined for evidence of bone modification features such as bite marks, weathering cracks, abrasion, and breakage. Weathering damage for each element was categorized using the stages developed by

Behrensmeyer (1978). However, some difficulty arose in trying to assess the weathering stage for elements with significant, external, post-mineralization damage caused by recent erosion, excavation, preparation, or some combination thereof. Although great care was taken in both excavation and preparation of the bones, many are extremely

friable and susceptible to crumbling. Post-mineralization damage not only obscured subtle surface modification features, the affected surfaces were also difficult to differentiate from areas damaged prior to diagenesis. Assessment of breakage patterns involved less ambiguity. Bone breakage was classified as parallel, oblique or transverse to the direction of the bone fiber in order to determine whether the break occurred before or after fossilization. Parallel or oblique breaks were attributed to damage that occurred prior to fossilization, and transverse breaks were associated with damage that followed remineralization of the bone during diagenesis (Myers et al., 1980).

Sedimentary matrix adhering to broken surfaces was interpreted as evidence that breakage occurred prior to burial, whereas jagged, but clean, breaks were assumed to have been caused by recent erosive processes or excavation.

Trample marks were included in the observational suite of characteristics, but their identification in the Mother’s Day assemblage is considered unlikely. Trample marks, small subparallel scratches on the surface of a bone, are generated by contact with coarse-grained sedimentary particles as an element is forced downward into the substrate by trampling activity (Behrensmeyer et al., 1986; Fiorillo, 1989). Experiments have shown that substrates consisting of relatively coarse sand-sized particles are necessary to create this type of modification feature; very fine-grained particles lack the abrasive power to produce trample marks (Fiorillo, 1989), so in situ trampling at the

Mother’s Day Quarry should not have produced any discernable surficial modification features. Even if the Mother’s Day bones, prior to being transported, had rested on a coarse-grained substrate that was suitable for the creation of trample marks prior to transport, the generally poor external condition of many of the elements as a result of

both erosional and excavational damage precludes the identification of subtle surface features on most of the prepared specimens.

TAPHONOMY

Before we can draw any conclusions about sauropod gregarious behavior based on data from the Mother’s Day Quarry, we must insure that the patterns observed in the death assemblage accurately represent the living assemblage from which it was derived and are not simply the result of the post-mortem processes responsible for the formation of the site. The most effective way to assess potential bias is to construct a detailed taphonomic history for the site. First the geological evidence for the processes of accumulation at the site is assessed; a discussion of the biological evidence related to site formation follows.

Geological Evidence

Sedimentology

The bonebed matrix is a light brown mixture of very fine-grained sand, silt, and clay; the unit is composed primarily of quartz grains, but also includes lesser amounts of feldspar and calcium carbonate (Fig 8). The bed is approximately 2.5–3.0 m thick and has no primary sedimentary structures. Grain size analysis reveals no horizontal or vertical variation within the quarry, with the exception of scattered patches of sand that contain less silt and clay than the surrounding sediments (Figs 9-11). Although the matrix itself is relatively homogeneous, clay rip-up clasts, carbonate nodules, and gravel are common inclusions (Fig 12). The gravel clasts, measured across their greatest

Figure 8. Diffractograms for two lateral grab samples from quarry. Calcium carbonate removed during HCl sample processing. A) Diffraction pattern for sample QLS-7. B) Diffraction pattern for sample QLS-11. See figure 7 for sample positions.

Figure 9. Grain size plots of vertical grab samples from Test Pit I. Samples consist primarily of very fine-grained sand and silt, but are also include a small amount of clay. No vertical grain size variation is evident.

Figure 10. Grain size plots of vertical grab samples from Test Pit II. Samples are composed of the same mixture of very fine-grained sand, silt, and clay observed in Test Pit I, and show the same lack of vertical variation in size.

Figure 11. Grain size plots of lateral grab samples taken from exposed areas in quarry. Samples have the same composition of very fine-grained sand, silt, and clay observed in vertical pits. No systematic lateral variations in grain size are apparent. Blue dotted line represents sample taken from patch of cleaner sand in quarry.

Figure 12. Sedimentary features in the Mother’s Day Quarry. A) Carbonate nodules. B) Clay rip-up clasts. C) Pebbles in situ with carbonized plant material. D) Pebbles removed from the quarry and washed for analysis.

diameter, range in size from 0.9 cm to 5 cm, with an average of 1.9 cm. The dominant pebble lithologies are gray microcrystalline quartzite and gray chert, materials probably derived from uplifted Precambrian basement rocks located to the west of the site. A single source is postulated for both materials since they co-occur in some of the clasts.

These pebbles are scattered randomly throughout the bonebed unit, with no indications of clustering or positive association with the skeletal elements.

Storrs et al. (2002) suggested that the gravel may represent gastroliths ingested by the sauropods whose remains fill the quarry. Taphonomic scenarios for the presence of gastroliths within the quarry differ depending on whether the Mother’s Day accumulation is autochthonous or allochthonous. If the gravel pieces in the quarry are gastroliths that were contained within the thoracic cavities of sauropod carcasses in an autochthonous assemblage, we might expect to see positive spatial associations of these clasts with thoracic skeletal elements; however, clustering of thoracic elements and pebbles is not apparent in the bonebed (Fig 13). Both the bones and the pebbles are scattered throughout the quarry; the only concentrations visible on the quarry maps are the result of increased sampling density. It is possible, given the semi-disarticulated state of the bones in the quarry, that gastrolith clusters could have been dispersed by either bioturbation or fluvial reworking, effectively obliterating any pattern of positive spatial association; but this interpretation does not conform to the taphonomic evidence revealed by the bones, which show no convincing indications of in situ trampling or reworking.

Alternatively, the pebbles could be part of an allochthonous assemblage, transported to the site within bloated, buoyant carcasses and released as the floating

Figure 13. Map of the horizontal distribution of pebbles relative to thoracic skeletal elements (dorsal vertebrae, thoracic ribs, and gastralia). No clumping or positive associations are apparent between the two.

carcasses decayed. In order for gastroliths to be transported within the carcasses to the site of final burial and dispersed as the bodies ruptured and disintegrated, the remains must have been fairly fresh at the time of transport, for carcasses have been shown to release their gastroliths fairly rapidly under warm environmental conditions (Wings,

2003). Hence, most of the disarticulation observed in the assemblage would have had to occur in the general vicinity of the quarry rather than in another location prior to transport. However, the sedimentology of the quarry and the taphonomy of the bones contained therein suggest that deposition of the bonebed was rapid. Also, since bioturbation and fluvial reworking appear to have had no major influence on the assemblage, gastroliths released from floating carcasses should be distributed in a relatively small vertical interval in the quarry, but a plot of the depth distribution for the

Mother’s Day clasts shows that they are spread throughout the quarry unit over a vertical interval of several meters (Fig 14).

If the Mother’s Day pebbles are to be classified as gastroliths, only one plausible taphonomic explanation for their presence in the quarry remains: they must have been released from the decaying carcasses at an initial location of death and accumulation, and both gastroliths and skeletal remains must have been carried to the quarry site in the same transport event. A taphonomic perspective of the gravel clasts in the bonebed cannot be considered conclusive, but a comparison of the Mother’s Day clasts with a collection of gastroliths found in situ within the skeletal remains of a sauropod may shed some light on their origin. Unfortunately, no single physical parameter or surface characteristic has been shown to definitively distinguish gastroliths from stream pebbles

(Lucas, 2000; Wings, pers. comm.). Some researchers assert that gastroliths may be

Vertical Distribution of Pebbles

0-24 0.0

25-49 1.1

50-74 0.0

75-99 0.0

100-124 0.5

125-149 4.3

150-174 17.2

175-199 46.2

Depth Below Stratum Top (cm) Stratum Below Depth 200-224 23.7

225-249 7.0

250-274 0.0

275-299 0.0

0 5 10 15 20 25 30 35 40 45 50 Percentage of Pebbles Collected

Figure 14. Plot of depth distribution for pebbles in bonebed.

differentiated from naturally abraded pebbles based on either surface reflectance values

(Johnston et al., 1990; Manley, 1993; Johnston et al., 1994) or abrasion patterns

(Chatelain, 1991; Chatelain, 1993), but these claims have been met with some skepticism (Lucas, 2000), leaving identification of gastroliths in an ambiguous state. The only reliable indicator of biologic affinity for clasts purported to be gastroliths remains clear association with the thoracic cavity of a carcass and parallel absence from the surrounding sedimentary matrix (Lucas, 2000). Nonetheless, a suite of similarities shared by both the clasts from the quarry and a known gastrolith assemblage would be at least suggestive of a biologic origin, whereas significant dissimilarity would imply a probable fluvial source for the pebbles.

Gastrolith assemblages have been reported from several sauropod specimens, as summarized in Christiansen (1996), but the most thoroughly quantified of these occurrences is the set of gastroliths associated with a Cedarosaurus weiskopfae skeleton from the Lower Cretaceous of Utah, excavated by the Denver Museum of

Natural History. Sanders et al. (2001) reported a number of the physical characteristics of the assemblage, including trends based on the mass, volume, roundness, sphericity, and reflectivity of each of the 115 individual clasts. A comparison of these data with the

Mother’s Day pebbles, using many of these same parameters, should reveal if the two assemblages are more similar or dissimilar, and thereby suggest a biotic or abiotic origin for the clasts.

Approximately 240 clasts associated with sauropod skeletal material have also been reported from the Seismosaurus type locality in New Mexico (Gillette, 1994A).

Gillette (1994B) interpreted these pebbles as gastroliths based on their position relative

to the thoracic cavity and their high degree of surface polish, but Lucas (2000) questioned this conclusion, suggesting instead that the pebbles were merely the remnant of a fluvial lag. Although the origins of the Seismosaurus clasts remain unresolved, inclusion of the sphericity values published in Lucas’ 2000 paper may facilitate comparison of the Mother’s Day and Cedarosaurus assemblages.

A quadrant plot of clast shape distribution for the pebbles from the quarry reveals a similar distribution to that reported for both the Cedarosaurus and Seismosaurus assemblages (Figs 15-17). A plot of sphericity versus clast size for the Mother’s Day pebbles also shows striking similarity to the corresponding plot for the in situ

Cedarosaurus gastroliths (Figs 18 & 19). All three assemblages consist primarily of spherical components, while few of the clasts from either set are ellipsoidal (Fig 20).

However, the Mother’s Day collection has a slightly higher number of spherical clasts, and fewer disc-shaped constituents than either of the other two assemblages. Although the similarities in clast shape between the three collections suggest that the Mother’s

Day pebbles may, in fact, be gastroliths, the evidence is far from definitive. If the

Seismosaurus pebbles are gastroliths, as argued by Gillette, the slight deviation of the

Mother’s Day shape distribution from the other assemblages may indicate an alternate fluvial origin. However, if the Seismosaurus clasts are simply part of a fluvial lag, as suggested by Lucas, the close correspondence of their shape distribution with that of the Cedarosaurus gastroliths would indicate that stream pebbles cannot be distinguished from true gastroliths by shape alone. Since the Cedarosaurus assemblage represents only a single data point, we cannot be sure whether the physical characteristics of its clasts are truly diagnostic indicators that are shared among other

Figure 15. Plot of clast shape for Mother’s Day pebble assemblage.

Figure 16. Plot of clast shape for Cedarosaurus gastrolith assemblage. (Sanders et al., 2001)

Figure 17. Plot of clast shape for Seismosaurus assemblage. Constructed from sphericity values in Lucas (2001).

Figure 18. Plot of sphericity vs. clast size (volume) for Mother’s Day pebble assemblage.

Figure 19. Plot of sphericity vs. clast size for Cedarosaurus gastrolith assemblage (Sanders et al., 2001)

45 44 45 43

34 35 33 32

20 19

Percent of Assemblage of Percent 16 15 12 11 10 7

5 4

0 Oblate Spheroid Spheroid Prolate Spheroid Ellipsoid

Cedarosaurus Mother's Day Quarry Seismosaurus

Figure 20. Comparison of shape distribution for Cedarosaurus gastroliths (Sanders et al, 2001) and Seismosaurus clasts (Lucas, 2000) with Mother’s Day pebble assemblage.

gastrolith assemblages and cannot be replicated by abiotic fluvial processes. Without further investigation, it is not possible to definitively determine if the Mother’s Day pebbles are gastroliths or merely gravel derived from a channel lag. The next step in the analysis of the depositional processes responsible for creation of the quarry assemblage is an investigation of possible agents of transport.

The presence of the clay rip-ups and gravel clasts in the bonebed unit provides a stark contrast to the relatively fine-grained matrix that dominates the quarry; but many of the larger skeletal elements, some with lengths in excess of 1 m, seem to represent an entirely different transport mode. Using the equation developed by Behrensmeyer

(1975) based on observed settling velocities of bones (Eq 3), the hydraulic equivalence was calculated for CMC VP7747, a left femur, and one of the larger elements recovered from the quarry, with a total length of 112 cm.

db = nominal diameter of the bone = 3 dq = (ρb −1)⋅db / 1.65 1.91× bone volume

ρb = bone density

Equation 3. Formula for calculating hydraulic quartz grain equivalent (dq) to a bone of known volume and density. (Behrensmeyer, 1975)

Bone volume was determined by modeling the femur as a series of cylinders of varying lengths and diameters, based on circumferences and lengths measured on the specimen. Bone density was assumed to be 1.47 g/cm3, using a 2:1 ratio of the densities of cancellous (1.21 g/cm3) and compact (1.65 g/cm3) bone as calculated by

Richmond and Morris (1998) from the values given in Behrensmeyer (1975). Applying the formulas provided above in Equation 3, the femur has a nominal diameter (db) of 29 cm, the diameter associated with a spherical clast with equal density and an equivalent

settling velocity; the quartz grain equivalent (dq) is calculated to be 9 cm. Since femora are not spherical elements, the observed settling velocity of CMC VP7747 would be dependent on its orientation. Consequently, the estimated quartz grain equivalence value will contain a certain amount of error, ±25% according to Behrensmeyer.

Incorporating this range of error, the quartz equivalent of the femur is between 7 cm and

11 cm. Nonetheless, it is apparent that the primary sedimentary matrix of the quarry, and even the gravel-sized inclusions, are hydraulically inequivalent to the large skeletal elements in the bonebed, and it is unlikely that they were deposited together by a simple fluvial current. Unique, alternative forms of deposition are required to explain the observed juxtaposition of inequivalent grain sizes.

A catastrophic, cohesive debris flow is the best explanation for the transport and deposition of the carcasses entombed at the Mother’s Day site. Floods typically generate a fining upwards sequence as their current velocity wanes, their competency decreases, and suspended fines are deposited by receding waters (Pettijohn et al.,

1987); yet, the Mother’s Day bonebed is vertically invariant in grain size. The presence of the scattered patches of relatively clean sand is also difficult to explain in the context of a flood model, for turbulent floodwaters should fully homogenize sediment layers as they are deposited. These patches are more easily explained as the remnant of a cohesive plug of sediment in a debris flow (Stow et al., 1996). The high competence and internal deformation associated with such flows also fits well with the unsorted nature of the deposit and the lack of any sedimentary structure throughout.

Biological Evidence

Element Location

The calculated true depths of all elements recovered from the bonebed were divided into six 50 cm deep intervals to assess the vertical distribution of bones within the quarry. The vast majority of the bones (85%) fall into the 150-224 cm interval (Fig

21). It is difficult to determine whether this pattern reflects a concentrated layer of skeletal remains within the quarry stratum or whether it is simply an artifact of the location and intensity of sampling. If the apparent depth distribution is related to sampling density, elements from the densely populated 74 cm thick interval extending from 150-224 cm below surface should lie entirely within the areas of the quarry from which the most elements have been removed. When the horizontal distribution of elements is mapped (Fig 22), the elements from the most productive vertical intervals spread laterally throughout quarry and do not simply clump in a few areas where sampling density is the highest. This distribution implies that the presence of the productive layer within the bonebed is not simply an artifact of sampling.

Elements were also divided into roughly-determined “large” and “small” size categories based on the type of packaging used in the field in order to determine if there is any evidence of sorting or grading of elements by size. In general, specimens encased within plaster were characterized as “large,” and elements collected in plastic film cans or wrapped in tin foil were designated as “small.” Exceptions were made when multiple elements that are generally known to be small in size (e.g. distal caudal vertebrae) were grouped into a single jacket. A plot of the percentage of occurrences for each of these two categories reveals a similar vertical distribution in each size range

Vertical Distribution of All Items

0-24 0.3

25-49 0.6

50-74 0.0

75-99 0.8

100-124 2.1

125-149 8.3

150-174 21.3

175-199 43.3

Depth Below Stratum Top (cm) Stratum Below Depth 200-224 20.5

225-249 2.6

250-274 0.0

275-299 0.0

0 5 10 15 20 25 30 35 40 45 50 Percentage of Items

Figure 21. Bar graph showing vertical distribution of items within the quarry by percent of total assemblage.

Figure 22. Map of horizontal distribution of groups of elements from 25 cm vertical intervals of depth.

(Fig 23), indicating that there are no vertical trends in element size within the bonebed.

This lack of size sorting is independent of sampling density because, even if the overall pattern of vertical distribution for all the elements is skewed by uneven sampling density, this bias should produce internally consistent variations relative to the size distribution.

Element Orientation

In order to assess the orientation of the bones within the quarry, all elements were characterized according to their general shape. The orientations of all elongate elements, defined as those with a length-to-width ratio of 2:1 or greater, were plotted on a rose diagram in 10° intervals to determine the existence of any preferred orientation that could be the result of fluid flow. Using the statistical method suggested by Everitt

(1992) and previously employed by Kreutzer (1988), adjusted residuals were calculated for each petal of the rose diagram and used to evaluate the significance of the rays

(Table 1). Only three symmetrical petals on the diagram differ significantly from random.

Two petals, oriented at 175°/355° and 65°/245°, contain fewer elements than expected, and one petal, at 125°/305°, has more elements than can be accounted for by chance alone (Fig 24). These statistically significant petals indicate a flow direction oriented parallel to the 125°/305° axis, although the polarity of the flow along this axis remains unresolved. The overall pattern in the diagram approximates the bimodal distribution typically associated with assemblages that have been either deposited or reoriented by a unidirectional current (Morris et al., 1996). However, the orientation pattern of the

Mother’s Day elements shows a large amount of dispersion, suggesting either short transport distance, weak flow velocities, or a turbulent flow regime (Morris et al., 1996).

Vertical Distribution of Large and Small Items

0.0 0-24 0.4

0.9 25-49 0.6

0.0 50-74 0.0

0.9 75-99 0.8

0.9 100-124 2.1

7.1 125-149 9.1

25.9 150-174 19.2

48.2 175-199 42.9

16.1 Depth Below Stratum Top (cm) Stratum Below Depth 200-224 21.6

0.0 225-249 3.3

0.0 250-274 0.0

0.0 275-299 0.0

0 102030405060 Percentage of Item Categories Large Items Small Items

Figure 23. Bar graph showing vertical distribution of large and small elements within the quarry by percent of total assemblage.

Table 1. Calculation of significance for petals of rose diagram determined by comparison of adjusted residuals with a table of standard normal distribution. Shaded rows indicate significant values. Mean of residuals is 0.01 and standard deviation is 0.

Adjusted Probability of Group Expected Value Observed Value Residual Significance 1 20.94 19 -0.44 0.3300 2 20.94 24 0.69 0.2451 3 20.94 18 -0.66 0.2546 4 20.94 25 0.92 0.1814 5 20.94 18 -0.66 0.2546 6 20.94 15 -1.32 0.0951 7 20.94 13 -1.77 0.0384 8 20.94 15 -1.32 0.0934 9 20.94 26 1.15 0.1251 10 20.94 18 -0.66 0.2546 11 20.94 21 0.01 0.4602 12 20.94 26 1.15 0.4404 13 20.94 34 2.99 0.0014 14 20.94 27 1.37 0.0853 15 20.94 25 0.92 0.1788 16 20.94 17 -0.88 0.1894 17 20.94 24 0.69 0.2451 18 20.94 12 -1.99 0.0233

Figure 24. Rose diagram showing 2-D orientation of elongate elements. White petals denote statistically significant values.

A subset of the elongate elements – including only those associated with both trend and plunge data – was plotted as a set of lineations on a lower hemisphere, equal area stereonet projection. This method is typically used to reveal any imbrication of elements within an assemblage (Fiorillo, 1991C). As bones imbricate against one another, they will become oriented so that they plunge in an upstream direction.

Consequently, a stereonet diagram of imbricated elements will show a cluster of points in the quadrant opposite that of the direction of flow. The diagram generated from the

Mother’s Day elements shows no single cluster of points; instead, the points are concentrated near the edges of the diagram, indicating that most of the bones have subhorizontal orientations (Fig 25). Interestingly, 18% of the identifiable elements for which orientation data exist have high-angle orientations of at least 30° to horizontal.

Large percentages of bones in unstable, high angle orientations have been attributed to trampling, which pushes bones into the substrate and rotates them on end (Hill &

Walker, 1972). However, examining Voorhies’ 1969 data from the Verdigree Quarry,

Fiorillo (1989) noted that high angle orientations could be produced in fluvial channel settings as a result of imbrication, and the percentages of high angle elements in these fluvial assemblages could be greater than those purported to result from trampling.

Since there is no evidence of in situ trampling at the Mother’s Day site, and no evidence of imbrication of elements within the assemblage, another mechanism must be responsible for the high-angle bone orientations, in this case, a cohesive debris flow.

The high degree of internal deformation and homogenization that occurs in a debris flow can generate the unstable orientations that characterize the quarry assemblage (Stow et al., 1996), and the percentage of mud (silt and clay) in the bonebed is greater than

Figure 25. Stereonet plot showing 3-D orientations of elongate elements from 2002-2003 field seasons. Clustering around periphery of diagram indicates predominance of horizontal elements and lack of imbrication in assemblage. Note high-angle elements plotting nearer to the center of the diagram.

the 10% minimum typically required to sustain cohesive flow (Anderson, 2000). If a high velocity fluvial current were responsible for either deposition or reworking of the bones, one would expect a stronger preferred orientation of elongate elements, even under turbulent flow conditions (Morris et al., 1996), and preservation of unstable clast orientations would be unlikely in a waning flow regime.

Voorhies Groups

Another method of assessing fluvial influence in a fossil assemblage is that developed by Voorhies (1969). Using modern mammalian bones in flume experiments,

Voorhies divided skeletal elements into three different categories based on their relative transport potentials. Group I consists of those bones that are most easily winnowed from an assemblage, Group II contains elements that are less easily transported, and

Group III is composed of elements with low transport potentials that typically form the lag component in a fluvial setting (Voorhies, 1969). However, as recognized by Fiorillo

(1991C), some portions of the Voorhies classification scheme are not appropriate for use in analyses of assemblages dominated by dinosaurian taxa since the Voorhies element groupings are based upon observations of mammalian bones. For example,

Voorhies Group III is comprised of the skull and mandible; however, in sauropods, these elements are frequently dissociated from their postcrania due to differing transport potentials (McIntosh & Berman, 1975). Fiorillo (1991C) suggested removing Group III from the analysis and calculating the ratio of Group II to Group I elements in the assemblage. The observed ratio can then be compared to the ratio of Voorhies Group elements found in a standard skeleton (Fiorillo, 1991C). Deviations from this expected

ratio will represent the degree of fluvial influence on the accumulation. Unfortunately, elimination of traditional Group III elements from consideration does not resolve all of the problems related to the application of Voorhies groups to sauropod bones.

The three intrinsic factors that exert the greatest influence over the hydrodynamic behavior of a bone are – in order of decreasing importance – density, shape, and size

(Behrensmeyer, 1975). Many sauropod elements differ from their mammalian homologues in all of these aspects. For example, sauropod cervical vertebrae possess elaborate neural spines that substantially alter their profile and would affect the interaction of that element type with a fluid current. The pneumatic cavities within the centra would also dramatically decrease the density of the cervical vertebrae (Wedel,

2003), reducing the flow velocities necessary to mobilize those elements. As a result of the unique densities, sizes, and external morphologies that characterize these and many other types of sauropod skeletal elements, the transport potentials predicted by

Voorhies may not be reflective of the true hydrodynamic behavior of the bones at the

Mother’s Day Quarry. Additionally, some sauropod elements such as sternal plates and gastralia have no analogues in the mammalian skeletons that served as the experimental basis for Voorhies’ categories. Without conducting independent flume experiments using models of the elements in question, it is impossible to accurately estimate the hydrodynamic behavior of sauropod bones. Rather than speculate about the transport potential of each individual element, I have elected to make only minimal changes to the traditional versions of Voorhies Groups I and II. Gastralia, ossified ventral ribs that have no mammalian analogues, have been added to Group I because

they are so similar to thoracic ribs in terms of morphology that the two types of elements should be characterized by similar transport potentials (Table 2).

When calculated, the Voorhies ratio for the Mother’s Day assemblage does not differ greatly from the ratio expected under ideal conditions with no winnowing (Fig 26).

This weak Voorhies signal is not entirely unexpected, given the proposed method of transport by debris flow. Debris flows are generally short-lived events and have a high competency due to their increased fluid density resulting from large amounts of entrained sediment (Collinson, 1996). In contrast, fluvial currents are typically sustained over a longer duration and are characterized by lower Reynolds numbers. The small deviation of the observed Mother’s Day Voorhies ratio from the expected ratio is also indicative of a short transport distance from the original point of death (Turnbull &

Martill, 1988).

Bone Modification

For the purpose of this study, bone modification is defined as any type of alteration that occurs between the time of death and ultimate discovery, including processes that operate before or after fossilization. Bone modification features such as bite marks, weathering cracks, breakage, and abrasion will accumulate in an assemblage as exposure time, and consequently time-averaging, increase (Fiorillo,

1988). Conversely, an accumulation of carcasses that is buried shortly after death will show little or no evidence of modification of skeletal elements. Of the bones prepared from the Mother’s Day assemblage (262), 71% show some evidence of bone modification (Table 3).

Table 2. Values used to calculate expected Voorhies Group II / Group I ratio for sauropod material at the Mother’s Day Quarry. Voorhies’ original groupings are preserved except for the addition of gastralia to Group I. Elements that fall into more than one group (e.g. phalanges) are listed in both. Element tallies are derived from McIntosh (1990A).

Element Element Voorhies Group I Voorhies Group II Counts Counts Cervical Vertebrae 15 Metapodia 20

Dorsal Vertebrae 10 Pubes (Pelvis) 2 Sacral Vertebrae 5 Ischia (Pelvis) 2 (Sacrum) Caudal Vertebrae ~80 Ilia (Pelvis) 2

Thoracic Ribs 20 Radii 2

Gastralia 18 Ulnae 2

Phalanges 48 Tibiae 2 Scapulocoracoid 2 Femora 2 (Scapula) Total 198 Humeri 2

Fibulae 2

Phalanges 48

Scapulocoracoid (Scapula) 2

Total 88

Expected Voorhies 0.44 GII:GI Ratio

Voorhies Group II / Group I Ratio

1.00 0.90 0.80 0.70 0.52 0.60 0.44 0.50 0.40 0.30 0.20 0.10 0.00 Expected Observed

Figure 26. Graph of ratios of Voorhies Group II to Group I showing only a small bias towards Group II elements. Small deviation from expected value is probably due to short transport time and distance.

Table 3. Bone modification features observed on prepared sauropod elements. Bone clasts not included. BONE MODIFICATION FEATURES Pre-Fossilization Post-Fossilization Bite Weathering Abrasion Breakage Breakage Marks Cracks Number of 19 99 24 2 6 Elements Percent of Applicable 7% 38% 10% 1% 3% Elements

The most frequently occurring modification feature is transverse breakage of elements due to post-fossilization damage. Post-fossilization breakage accounts for

84% of broken elements (38% of total sample), whereas only 16% of observed breaks occurred prior to fossilization (7% of total sample). Many bones in the quarry are also crushed and distorted, likely as a result of local tectonism or lithostatic compaction of the fine-grained matrix in the bonebed. Some specimens are even cut by small faults, although this type of deformation is less common. One of the most frequently observed pre-fossilization bone modification in the assemblage is fresh breakage, identified by oblique or infilled breakage surfaces. Typically, this type of modification is attributed to scavenging activity or trampling (Behrensmeyer et al., 1989; Hill, 1989; Myers et al.,

1980). Given the paucity of bite marks in the assemblage, scavengers are an unlikely agent of the observed breakage. Additionally, Mesozoic carnivores are not thought to have gnawed or broken bone for the nutritional value of its marrow as many modern mammalian scavengers such as hyenas do (Chure et al., 1998). Bite marks on the bones of Mesozoic taxa are instead the result of accidental contact during feeding on soft tissues (Fiorillo, 1991B; Chure et al., 1998). Consequently, even extensive scavenging of an assemblage should not produce significant breakage. Transport is also an unlikely mechanism of breakage since fresh bone will abrade during transport, but will not typically break (Behrensmeyer et al., 1989); however, trampling could have been the responsible mechanism.

Preservation Pattern

The Mother’s Day assemblage consists of two primary taphonomic modes. The sauropod elements that are strewn throughout the quarry comprise the dominant constituent of this bimodal pattern. These elements are well preserved, with little evidence of external modification such as weathering cracks (Fig 27), suggesting that they were exposed for only a brief period and were transported only a short distance from the site of initial accumulation. The bones belonging to the second mode are highly abraded and broken beyond the point of recognition (Fig 28). Most have been reduced to small, rounded clasts that represent the endmember opposite the sauropod elements on a spectrum of preservation condition. Since no bones in the intermediate stages of modification have been recovered from the quarry, the two taphonomic endmembers present are inferred to be temporally distinct from one another. The highly modified bone clasts are also assumed to have traveled a greater distance from their initial source and had a longer transit time than the relatively unmodified sauropod material.

Since bone clasts are often associated with channel lags produced in fluvial environments (Behrensmeyer, 1988), the bone pebbles in the bonebed may have been entrained at the same time as the chert gravel scattered throughout the quarry, assuming that the pebbles are fluvially derived.

In addition to the bones in the Mother’s Day Quarry, small fragments of plant debris are also present, scattered throughout the bonebed unit. The debris consists primarily of carbonized leafy material, with an occasional elongate stems; but the specimens are so fragmentary that few, if any, are identifiable (Tidwell, pers. comm.).

Weathering Stages in Assemblage

120 100 80 60 40 20 Percent of Weathered Elements Percent 0 Stage 0 Stage 1 Stage 2 Stage 3 Stage 4 Stage 5

Figure 27. Graph showing distribution of weathering stages observed on prepared sauropod elements from the Mother’s Day assemblage. Bone clasts not included. Stages from Behrensmeyer (1978).

Figure 28. Bone clasts removed from the Mother’s Day Quarry.

The longest specimen collected is not more than 10 cm in length, and the average fragment is only a few centimeters wide, so the effects of the plant material on any flow regime associated with transport of the skeletal assemblage would be negligible, unlike large logs that can concentrate and imbricate skeletal elements (e.g. Zeigler, 2003).

Relatively small, delicate skeletal elements such as distal chevrons and vertebral processes are common in the Mother’s Day sample. Such bones are more susceptible to destruction by long periods of exposure or transport (Shipman, 1981), so their presence in the assemblage in significant amounts corroborates the evidence from other quarters for a relatively short exposure interval and transport time. Additionally, the presence of soft tissue remnants, in the form of skin impressions (Fig 29), indicates that the carcasses were not completely decomposed at the time of burial. Although fragments of skin may remain in the vicinity of a carcass long after other soft tissues have been stripped away by decay (Coe, 1978), the association of skin and bones in this context is a strong indicator of a short transport distance. Given longer periods and distances of transport, delicate fragments of skin would be either destroyed or separated from their associated skeletal remains.

Other evidence for the presence of soft tissue at the time of deposition is the semi-articulated state of some of the skeletal remains. Series of articulated vertebrae are common in the assemblage, as are articulated foot bones and pelvic elements (Fig

30). Disarticulation patterns are controlled by a number of factors, including scavenging, environmental conditions, and joint anatomy (Lyman, 2001). Many of the articulated elements in the Mother’s Day Quarry, such as caudal vertebrae, are associated with significant amounts of connective tissue and tendons in life

Figure 29. A-B) Skin impressions collected from the Mother’s Day Quarry (A. CMC VP8075, B. CMC VP8078).

Figure 30. Articulated elements from the Mother’s Day Quarry. A) Articulated left metatarsals I and II. B) Articulated ischia associated with a sacrum. C) Series of articulated proximal caudal vertebrae. D) Series of articulated distal caudal vertebrae. E) Semi-articulated left pes (CMC VP8080). Photo courtesy of M. Papp (2001).

(Rothschild & Berman, 1991; Dodson, 1990). Drying of such tendons in arid conditions can prevent bones from fully disarticulating for some time after death (Hillman &

Hillman, 1977; Hill, 1980; Weigelt, 1989), but following rehydration, previously desiccated carcasses will disarticulate rapidly (Brand et al., 2003). Articulated elements were tabulated from the field notes for the four years of excavation from 2000 to 2003, and the percentage of surviving articulations (PSA) was calculated for each joint observed in the assemblage. The PSA statistic, described in Lyman (2001), is calculated by dividing the observed frequency of articulation for a joint by the number of occurrences of that joint in a complete skeleton. This value is then divided by the minimum number of individuals calculated for the assemblage, and the result is multiplied by 100 in order to obtain a percentage. Once the PSA value was calculated for every joint in the assemblage, the values were ordered based on the different disarticulation sequences reported by Hill & Behrensmeyer (1984) for topi, wildebeest, domestic cow, Burchell’s zebra, and Grant’s gazelle.

A graph of the five ordered sequences of PSA values reveals that many of the articulated bones in the quarry are characterized by relatively late disarticulation in the sequences observed for the five modern taxa (Fig 31). Although some of the articulated pelvic joints in the assemblage have no analogues reported in the mammal sequences

(e.g. articulated ischia), removal of these joints from the analysis does not significantly alter the observed pattern of late stage disarticulation (Fig 32). Since the disarticulation profile for the Mother’s Day assemblage is centered over the late-stage half of the disarticulation spectrum, short exposure time prior to burial was likely the critical component controlling the disarticulation sequence, though desiccation due to aridity

30 20 PSA Value 10

0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 Disarticulation Rank

Topi Seq. Wildebeest Seq. Cow Seq. Zebra Seq. Gazelle Seq.

Figure 31. Graph of PSA values for articulated elements from Mother’s Day assemblage ranked according to differing disarticulation sequences from Hill & Behrensmeyer (1984). Each color shows the PSA values ranked according to a different taxonomic sequence. Extra pelvic joints are included and plotted at the high end of the disarticulation spectrum

5 PSA Value

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Disarticulation Rank

Topi Seq. Wildebeest Seq. Cow Seq. Zebra Seq. Gazelle Seq.

Figure 32. Graph of articulated elements from Mother’s Day assemblage ranked according to differing disarticulation sequences from Hill & Behrensmeyer (1984). Extra pelvic joints are not included, so distribution of values is more even, yet emphasis on late-stage disarticulation still remains apparent.

was also likely a factor in the formation of the original, pre-transport death assemblage.

Despite the fact that disarticulation sequences are somewhat taxon-specific, certain observations appear to hold true for a wide range of higher taxonomic groups

(Hill & Behrensmeyer, 1984). One of these is the characteristic sequence of limb disarticulation in arid versus aqueous environments. In dry terrestrial conditions, limb disarticulation begins proximally and proceeds distally; the opposite trend occurs in aqueous environments, with disarticulation beginning distally and proceeding in a proximal direction (Hill, 1979; Brand et al., 2003). The predominance of articulated distal limb elements in the Mother’s Day assemblage relative to articulated proximal elements suggests that initial disarticulation of the carcasses took place in a non-aqueous environment.

Taxonomic Diversity

Perhaps the simplest and most forceful evidence of a relatively rapid, rather than long-term attritional, mortality for the sauropod individuals in the Mother’s Day Quarry is the monospecific nature of the primary component of the accumulation. Though the bone clasts scattered throughout the bonebed may be derived from a variety of taxa, they represent a separate taphonomic mode from the sauropod material, and the two types are probably not temporally associated. The five theropod teeth identified in the quarry may either be related to the sauropod material in a scavenging context, or they may be reworked from other deposits. It is difficult to tell from the condition of the teeth how long they have been exposed or whether they have been reworked because tooth enamel is highly resistant to abrasion and surficial damage (Argast et al., 1987),

modification features that are typically indicative of exposure or transport. Also, many of the tooth specimens sustained significant damage during excavation, making characterization of surface condition and taxonomic affinity exceedingly difficult.

Regardless, the tooth specimens consist of only the crown portion and are therefore assumed to have been shed from a living animal rather than derived from a carcass.

Excluding the theropod teeth and unidentifiable clast material, the sauropod bones appear to comprise a distinct monospecific subset within the larger paucispecific accumulation. All the sauropod material recovered thus far is attributable to gracile diplodocids, based primarily on the observation of slender limb proportions, distinctive cervical vertebral morphology, forked chevrons, biconvex distal caudal vertebrae, and bifurcate neural spines on posterior cervical and anterior dorsal vertebrae (McIntosh,

1990B; Wilson, 2002). The cervical vertebrae – possessing elongate centra with cervical ribs extending to the posterior flange, but not beyond that margin – suggest that the Mother’s Day sauropod material is referable to Diplodocus, for the centra are not as elongate as those characteristic of Barosaurus specimens (Hatcher, 1901; Lull, 1919)

(Fig 33). Monospecificity is typical of accumulations with minimal amounts of time- averaging, since taxonomic richness will accrue with an increasing degree of time- averaging resulting from either reworking or extended intervals of exposure. Thus, the low taxonomic diversity observed at the Mother’s Day site is a function of a mass mortality coupled with little to no time-averaging.

Figure 33. Diagnostic Mother’s Day elements. A) Posterior cervical vertebra with bifurcate neural spine. B) Forked chevron (CMC VP7753) typical of diplodocid sauropods. C) Biconvex distal caudal vertebra (CMC VP7930) typical of diplodocids. D) Cervical vertebra (CMC VP8064) with slender cervical rib not extending past posterior margin of centrum. E) Diplodocus cervical vertebra 10 from Hatcher (1901).

Number of Individuals

In order to estimate the number of individuals represented by the bones in the quarry, I calculated the minimum number of individuals (MNI) based on the most common element type. Paired appendicular elements were used so that siding (e.g. right or left) and size comparison of the elements could be utilized to further differentiate between individuals, thereby increasing counting accuracy. The MNI counting method was chosen instead of methods such as number of identified specimens per taxon

(NISP) or minimum number of elements per taxon (MNE) because the skeletal remains in the quarry are loosely associated and have undergone minimal amounts of post- mortem mixing. NISP and MNE produce accurate counts for assemblages that are composed of either totally dissociated or highly fragmented specimens, respectively

(Badgley, 1986). Consequently, these methods will produce inflated counts if applied to an assemblage with whole, associated elements. Since the lateral extent of the bonebed is as yet unknown, and a significant portion of the assemblage remains unexcavated, the MNI count will doubtless underestimate the true number of sauropod individuals contained in the quarry (Grayson, 1978); however, the error associated with the MNI counting method should be less than that connected with either NISP or MNE.

Metatarsal I proved to be the most common element in the excavated fraction of the Mother’s Day assemblage, with a total of 15 specimens. Since not all of these metatarsals are currently prepared, the total is based on identifications in the MOR and

CMC field notes rather than observation of curated specimens. Consequently, much information about the size and side of the specimens is not available. Based on the data present in the field notes and the eight specimens that have been prepared, five of the

metatarsals are from the left side, five are from the right, and side was not recorded for the remaining five specimens. So, the minimum number of individuals represented in the portion of the assemblage that has been excavated thus far is eight (assuming that

14 of the 15 specimens were matching pairs).

Age Profile

The age of the individuals in a death assemblage also has the potential to yield valuable information regarding the mode of mortality, specifically whether it is catastrophic or attritional. Attritional assemblages are typically enriched in both juveniles and mature individuals of advanced age because these two age groups are characterized by significantly higher mortality rates than other age classes in a stable population (Lyman, 2001). Such an attritional accumulation will be relatively depauperate in advanced subadult and fully mature individuals, generating a bimodal mortality profile. In contrast, a catastrophic mortality should preserve a cross-section of a population, creating a death assemblage with a unimodal, positively skewed mortality distribution (Lyman, 2001).

Age determination in sauropod dinosaurs is difficult, as no morphologically based ontogenetic studies are available. Currently, the only detailed work on sauropod ontogeny is a histological study on Apatosaurus (Curry, 1999). Unfortunately, the most common element in the Mother’s Day assemblage, metatarsal I, is not useful for age determination. The young age of the individuals within the quarry was determined based on two general factors: relatively small size of the skeletal elements and lack of fusion of neurocentral sutures on many of the cervical, dorsal, and caudal vertebrae (Brochu,

1996) (Fig 34). In order to verify the immature status of the material, limb elements were measured and compared to measurements of adult Diplodocus specimens in the

Carnegie Museum (CM) and the Yale Peabody Museum (YPM) (Hatcher, 1901;

McIntosh & Carpenter, 1998). The length of each Mother’s Day specimen was compared to that of its homologous CM and YPM specimens, and a length percentage was calculated and compared to the size categories proposed by Curry (1999) (Table

6). According to Curry’s histological categories, juveniles are up to 73% of adult size, and subadults are up to 91% the size of a fully-grown adult. The elements selected for comparison were a femur (CMC VP7747), the largest element recovered from the quarry thus far, and a humerus (CMC VP7746) that is fairly typical of limb element sizes at the site. Even compared with the smallest of the adult femora (CM-94), CMC VP7747 is only 76% of that specimen’s total length. The humerus is estimated to be 74% of the smallest comparative adult specimen (CM-94). Since the Diplodocus specimens from the Carnegie and the Peabody did not include humeri, humeral lengths were estimated using femoral lengths and the humero-femoral ratio provided in McIntosh (1990). Both

Mother’s Day limb elements, even the large femur (CMC VP7747) fall within the lower portion of the subadult size range of Curry (1999) and the upper portion of the juvenile range. These findings, based on some of the larger elements from the quarry, verify that no adult material has been recovered from the Mother’s Day site thus far.

Since the Mother’s Day Quarry contains both juveniles and subadults, but contains no fully-adult individuals, it does not conform to either of the ideal attritional or catastrophic mortality scenarios. The absence of adults precludes the possibility that the assemblage is attritional, but this pattern also means that the accumulation cannot be

Figure 34. Vertebrae with unfused neurocentral sutures. Neural arches separated from centra post-mortem. A) Dorsal vertebra (CMC VP7913). B) Medial caudal vertebra (CMC VP7929).

Table 4. Calculation of percent adult size for largest forelimb and hindlimb elements recovered from quarry. Asterisks indicate humeral length values estimated from femoral lengths using the humero-femoral ratio for Diplodocus (0.65) given in McIntosh (1990). Age class size ranges from Curry (1999).

Percent Age Class Specimen Taxon Element Length (cm) Length Size Range Subadult CMC VP7747 Diplodocus sp. Left femur 1120 - (73%

an accurate reflection of sauropod population structure. The unique age profile at the site is either the result of biasing by the taphonomic processes responsible for the formation of the site, or the mortality event sampled only a subset of a sauropod population.

DISCUSSION

The totality of the geological and biological taphonomic evidence provides a relatively clear picture of the post-mortem processes responsible for the formation of the

Mother’s Day Quarry; however, the agent of mortality for an accumulation can often be difficult to discern, even in the most completely documented assemblages. A number of different agents have been suggested as the cause of vertebrate mass mortalities identified in the fossil record, including miring (Hungerbühler, 1998), drowning (Ryan et al., 2001), drought (Rogers, 1990), and volcanism (Voorhies, 1978).

Miring has generally been inferred as the agent of mortality when articulated skeletal sections are found in life position within a fine-grained matrix (Gallup, 1989;

Sander, 1992; Wells & Tedford, 1995; Hungerbühler, 1998). Appendicular and posterior skeletal elements dominate miring assemblages because these are the portions of the body that become submerged in the mire and are consequently protected from the effects of scavenging, bioturbation and other agents of reworking and destruction

(Abler, 1985; Hungerbühler, 1998). Miring was originally proposed as the formative process for the Mother’s Day site, based largely on the presence of articulated pes units and the purported absence of axial skeletal material in the deposit (Horner & Dobbs,

1997). Subsequent excavation has revealed no significant bias towards either

appendicular or posterior elements. In fact, the observed appendicular-to-axial ratio corresponds almost exactly to the expected ratio from a standard Diplodocus skeleton

(Fig 35); and the anterior-to-posterior skeletal ratio indicates a slight enrichment in anterior elements (Fig 36), a trend in opposition to the expected direction of bias associated with mires (Hungerbühler, 1998). Since the articulated pes units in the assemblage are simply one example of a number of articulated series of elements – including caudal vertebrae, cervical vertebrae, and limb bones – they cannot be considered a diagnostic indication of miring in this context. If miring is not a valid explanation for the formation of the Mother’s Day accumulation, alternative explanations for the cause of death of the Mother’s Day sauropods must be explored.

Modern ungulates have been observed to perish in significant numbers as their herds cross swollen rivers along seasonal migration routes (McHugh, 1972; Schaller,

1972). As a result of swift currents or close packing within the herd, some individuals are trampled or swept away, and ultimately drown. The carcasses of animals killed in such situations have a tendency to collect in channel meanders (Aslan &

Behrensmeyer, 1996), where the aqueous environment inhibits decay and high carcass density mitigates the effects of scavengers. Such a scenario is compatible with most of the taphonomic observations related to the biological component of the quarry, with the exception of the apparent proximal-to-distal limb disarticulation sequence. If decay and decomposition had, in fact, occurred in an aqueous setting, disarticulation of the limbs should have proceeded in the opposite direction – distal to proximal – leaving few articulated pes units relative to propodials and epipodials. The high-angle orientation of

Appendicular / Axial Skeletal Element Ratio

1.00 0.90 0.80 0.59 0.70 0.51 0.60 0.50 0.40 0.30 0.20 0.10 0.00 Expected Observed

Figure 35. Graph of ratios of appendicular to axial skeletal elements. Only a small bias towards appendicular elements is detectable. Miring should produce a strong bias towards appendicular elements.

Anterior / Posterior Skeletal Element Ratio

1.00 0.90 0.80 0.70 0.60 0.44 0.50 0.40 0.21 0.30 0.20 0.10 0.00 Expected Observed

Figure 36. Graph of ratios of anterior to posterior skeletal elements. Bias towards anterior elements is opposite that expected for a miring site.

many of the elongate bones is not difficult to reconcile with deposition by high-velocity currents, for imbrication of elements one upon another could create those orientations, but the lack of an imbrication signal in the stereonet diagram suggests that fluvial currents are not responsible for the high-angle elements. Furthermore, if a flood were responsible for the transport and deposition of the skeletal elements at the Mother’s Day site, we would also expect the quarry sediments to differ significantly from the observed lithology. As current velocities wane and floodwaters recede, flow competency decreases and suspended sediments begin to settle, producing a sequence showing at least a nominal fining-upward trend in grain size (Pettijohn et al., 1987). Notably, no vertical or lateral grain size variations are present within the bonebed. Instead, clay-, silt-, and sand-size particles are mixed throughout the unit, producing a poorly sorted bed.

A small number of mass mortalities, both fossil and modern, have been attributed to volcanic factors such as eruptions, poisonous gases, or ash clouds (Voorhies, 1978;

Lyman, 1989). Although such an event cannot be dismissed out-of-hand as an agent of mortality whose traces were erased by subsequent transport, volcanism is an unlikely cause of death for the animals of the Mother’s Day site. Although increased volcanism in the Western Cordillera is cited as the cause of increased levels of smectite in the upper portion of the Brushy Basin member where the clay transition exists, the source of this ash input is far from the Montana region of Morrison deposition, as indicated by the absence of the clay change in this area. Also, if a volcanic event had affected the region containing the Mother’s Day Quarry, the taxonomic diversity of the assemblage would likely be much higher. A mortality event with a regional magnitude, such as a

volcanic eruption, would kill a broad range of taxa, and transport processes would concentrate carcasses in depositional sinks, further increasing diversity counts (e.g.

Zeigler, 2003). If the aforementioned scenarios can be rejected with a reasonable degree of certainty, what were the agents of mortality and transport responsible for the formation of the assemblage?

The taphonomic data, both biological and geological, indicate that the sauropods preserved at the Mother’s Day site were probably victims of a drought and that their carcasses were transported from the original location of death by a debris flow. The interpretation of a mass mortality with little time-averaging is supported primarily by the incomplete disarticulation and generally well-preserved condition of the sauropod remains. The partially disarticulated state of the bones in the quarry indicates that the carcasses must have been sequestered for some period of time prior to their deposition and final burial at the site. However, the low taxonomic diversity of the assemblage lessens the likelihood of significant amounts of time-averaging, since diversity will accrue with increasing periods of exposure. Both the paucity of pre-fossilization bone modification features in the assemblage and the evidence that soft tissue was still present at the time of deposition – skin impressions and articulated series of bones – diminish the possibility of an exposure interval exceeding one year. Based on disarticulation and decay rates observed in modern animals, exposure time was probably on the order of months (Coe, 1978).

Though the cause of death for the Mother’s Day sauropods is difficult to determine definitively, the observed proximal-to-distal disarticulation sequence of limb elements in the assemblage is indicative of arid terrestrial conditions at the time of

death (Hill, 1979; Brand et al., 2003) that would be compatible with a drought mortality.

The articulated vertebral elements recovered from the site were characterized by strong tendon attachments in life (Dodson, 1990; Rothschild & Berman, 1991), and drying of tendons has been shown to delay skeletal disarticulation (Hillman & Hillman, 1977; Hill,

1980; Weigelt, 1989). Although the presence of these articulated elements suggests that desiccation was a factor in the preservation of the semi-articulated carcasses, short exposure time appears to have been the primary controlling factor, for the overall disarticulation pattern of the assemblage indicates that the carcasses were buried before late stage disarticulation could fully progress. Since the Morrison Formation is thought to have been a dry, somewhat seasonal environment (Dodson et al., 1980;

Demko & Parrish, 1998), the interpretation of a mortality site influenced by arid conditions is compatible with the regional climatic model. Furthermore, the presence of carbonate nodules in the bonebed suggests that the local conditions near the quarry site were also seasonally arid. The lack of evidence against a drought mortality, in conjunction with the rejection of all other plausible modes of death for the assemblage, suggests that a severe drought is the best explanation for the death of the Mother’s Day sauropods. However, the carcasses were apparently not preserved in situ at the initial location of death and accumulation.

Given the unstable, high-angle orientations of many of the skeletal elements, the preferred orientation of elongate elements, and the association of the bones with clay rip-ups, the Mother’s Day assemblage is best interpreted as an allochthonous accumulation. Remobilization of an autochthonous assemblage could theoretically produce similar flow indicators, but a reworking event likely would have destroyed all

soft tissue traces. Furthermore, the broad vertical distribution of the pebbles and clay clastsin the bonebed cannot be readily explained in terms of a reworking current.

Although fluvial flow is one of the most common abiotic transport mechanisms for terrestrial vertebrate remains (Behrensmeyer, 1988), certain characteristics of the

Mother’s Day deposit are more typical of debris flows than water currents.

The poor sorting of the quarry sediments indicates that the flow must have rapidly frozen in place before settling could produce a graded bed. This freezing mechanism is effectively impossible to generate in the context of a flood, but debris flows are known to exhibit this unique characteristic because of their plastic flow behavior (Stow et al., 1996). The patches of cleaner sand scattered throughout the bonebed may represent the fragmentary remains of a larger sediment plug that initially formed at the center of the flow. The fact that the plug did not completely disintegrate and mix with the rest of the quarry sediments indicates that the flow must not have traveled far from its source. Although the relatively flat environments of the Morrison may not seem like an appropriate setting, debris flows may be triggered by rainfall alone and do not require steep gradients to facilitate movement (Pettijohn et al., 1987). Debris flows are also capable of freezing in place once internal shear stresses fall below a critical threshold, preserving unstable clast orientations such as those observed in the

Mother’s Day accumulation (Stow et al., 1996).

CONCLUSIONS

The Mother’s Day Quarry contains a unique fossil assemblage, composed almost entirely of juvenile and subadult diplodocid sauropod material. A thorough taphonomic

evaluation of the site indicates that the sauropod remains represent the remnant of a herd group. All the elements recovered thus far that are useful for diagnosing taxonomic affinity at the genus level are attributable to Diplodocus, but the vast majority of the recovered elements are identifiable only as diplodocid sauropod.

The assemblage is allochthonous, and the effects of time-averaging produced minimal taphonomic distortion in this context, although they were a factor in site formation. The preferred orientation and unstable high-angle plunges of the elongate elements in the quarry attest to the influence of a high-energy flow regime. Relatively delicate skin impressions preserved throughout the quarry – often not in direct association with bones – preclude in situ reworking of the deposit following initial deposition and burial. Such soft tissue traces could only be preserved by the brief transport and rapid burial of relatively fresh carcasses derived from an initial site of death and accumulation.

The sedimentology of the bonebed unit also supports the idea of a high-energy flow. The quarry stratum is poorly sorted, containing clay-, silt-, and sand-sized particles, as well as gravel-sized clasts and clay rip-ups. The lack of sorting or grading within the bed – in conjunction with the occurrence of discontinuous patches of clean, relatively well-sorted sand that are interpreted here as the remnants of a rigid sediment plug – suggests that a catastrophic, cohesive debris flow was the agent of transport and burial at the site.

The debris flow model of deposition fits well with the weak Voorhies signal and lack of abrasion observed in the assemblage, thought to reflect short transport time and distance. Post-mortem transport can produce significant taphonomic bias due to

temporal and ecological mixing, so a transport agent, such as a debris flow, that is characterized by minimal distance and duration of movement will be associated with a relatively low degree of time-averaging, given a brief exposure interval prior to transport.

The paucity of pre-mineralization bone modification in the Mother’s Day assemblage and indications that soft tissue was still present at the time of burial suggest that exposure time at the location of initial accumulation was relatively short.

The sequence of limb disarticulation observed at the site is consistent with arid subaerial conditions during the decomposition phase, suggesting the sauropods in the quarry may have succumbed to starvation or malnutrition related to severe drought conditions. Although drought mortalities cannot be classified as strictly catastrophic due to the extended duration of their effects, drought assemblages should represent less than a year of carcass accumulation (Badgley, 1982). The practically monospecific composition of the Mother’s Day Quarry argues against significant amounts of post- mortem mixing during the carcass accumulation phase.

Since the taphonomic biases at the site appear to be minimal, the Mother’s Day accumulation should provide a faithful representation of the social group from which it was derived. Therefore, the assemblage presents a unique opportunity to study the taphonomically constrained remains of a sauropod herd group and has the potential to contribute substantially to our understanding of gregarious behavior in sauropod dinosaurs.

CHAPTER 2. Implications for Gregarious Behavior in Sauropod Dinosaurs

INTRODUCTION

Gregarious behavior has been postulated for a number of dinosaur taxa,

including ceratopsids, ornithopods, theropods, and sauropods. This behavioral model is

supported by multiple examples of skeletal evidence in the Ceratopsia (Currie &

Dodson, 1984; Ryan et al., 2001), Ornithopoda (Hooker, 1987; Forster, 1990; Rogers,

1990), and Theropoda (Schwartz & Gillette, 1994; Currie, 1998; Kobayashi & Lu, 2003);

however, evidence of sauropod herding behavior comes primarily from the ichnological

record (Ostrom, 1985; Pittman & Gillette, 1989; Barnes & Lockley, 1994; Lockley et al.,

1994; Day et al., 2004), with only a few possible examples of skeletal evidence (Dong,

1990; Coria, 1994; Fiorillo & Montgomery, 2001), all of which lack the taphonomic

constraints necessary for behavioral interpretations. While trace fossils can tell us a

great deal about sauropod herds, and certainly give no reason to doubt the interpretation of sauropods as gregarious animals, in some respects, trackways cannot provide as much behavioral detail as a taphonomically constrained body fossil assemblage because of difficulties related to identification of trackmakers and determination of amounts of time-averaging.

Most purported sauropod herd trackways are found in large megatracksites that may extend laterally for several kilometers and often contain multiple track-bearing horizons at different stratigraphic levels. The passage of a herd is inferred from a subset of trackways that are oriented subparallel to one another (Ostrom, 1972). The smaller the deviations in orientation observed within the group of trackways, the stronger the evidence for the passage of a herd group. Small intertrackway spacing is also used as

an indication of gregarious behavior at tracksites (Lockley et al., 1986). Ichnological interpretations of herding behavior are not without their complications, though. Time- averaging in ichnological assemblages can often be more difficult to constrain than in skeletal assemblages, and tracks usually cannot be easily matched with lower-level taxonomic groups due to the effects of substrate variability on print shape and conservative pes morphology that may not reflect diagnostic differences between genera or species (Farlow, 2001; Day et al., 2004). Together, these issues can create uncertainty in the identification of herd trackways and the numbers and types of taxa present in a herd group. Skeletal evidence of sauropod herding has the potential to clarify some of these ambiguities.

DEFINITION OF A HERD

Scientific resources generally consider social groups or herds to be defined by group activity and voluntary interaction, since unsocial aggregates may occur involuntarily as a result of concentrating physical factors or voluntarily due to environmental patchiness and habitat preference (Ostrom, 1972; Morse, 1980). This interpretation of a herd could apply to a number of different social entities, including groups of individuals that assemble during mating seasons, but remain relatively stationary during that time period. In contrast, a strict interpretation of the term could theoretically define a herd as a social group that is specifically migratory in nature. For the purposes of this study, I have adopted a moderate definition under which a herd group is considered to be any mobile social aggregate that may or may not be comprised of a single taxon. This version of the term would exclude stationary mating

and nesting groups, but not presuppose any seasonal migratory habits, allowing that

movement may be minimal and confined to a specific geographic range or territory.

ICHNOLOGICAL EVIDENCE

Before discussing skeletal evidence for gregarious behavior in sauropods, it is

necessary to explore the extent of our knowledge of sauropod herds based on

tracksites alone. The best examples of sauropod herd trackways are found in the

Jurassic of Portugal, the United Kingdom, and the United States (Lockley et al., 1986;

Lockley et al., 1994; Day et al., 2004), and in the Cretaceous of Texas and Arkansas

(Ostrom, 1985; Pittman & Gillette, 1989). The ichnological data from these sites suggest

that herds typically numbered in the tens of individuals, judging from the numbers of

distinctly preserved trackways. However, tracksites offer conflicting evidence of herd composition, both in terms of the age of individuals and the number of taxa present in the herd group.

The one of the track-bearing horizons at Lagosteiros Bay in the Late Jurassic of

Portugal preserves a series of juvenile trackways, distinguished by their remarkably small track size, but the assemblage lacks any evidence for the contemporary passage of adult individuals (Lockley et al., 1994). The absence of adult tracks in this context implies that herds may have been segregated on the basis of age, with juveniles forming groups that were separate and distinct from those composed of fully-mature, adult animals. The Briar site in the Lower Cretaceous of Arkansas likewise records the passage of individuals belonging to a single age group, in this case, adult (Pittman &

Gillette, 1989). Conversely, the Davenport Ranch Site from the Lower Cretaceous of

Texas contains a jumble of sauropod trackways belonging to both juveniles and adults

(Ostrom, 1985). Bakker claimed that herd structure was apparent in the Glen Rose track assemblage (Bakker, 1968), suggesting that juveniles were concentrated near the center of the herd group and were surrounded by a protective ring of adults. However, this assertion has been met with some skepticism (Ostrom, 1985; Lockley et al., 1994), since visual inspection of sketches of the trackways in question reveals no apparent spatial patterns that would support Bakker’s statement.

The massive Purgatoire Valley tracksite, located in the Morrison Formation of

Colorado, records the passage of two groups of sauropods of different age classes

(Lockley et al., 1986). One set of trackways appears to have been made by adults, while the other group consists of smaller track sizes that are probably attributable to subadults (Lockley et al., 1986). Despite the assertion that these two age groups may have been traveling together (Lockley et al., 1986), there is no evidence to support this claim since the groups are separate and not oriented parallel to each other. The lack of juvenile tracks at the site is also notable, though Lockley et al. (1986) explained this absence in terms of rapid juvenile growth rates. The Purgatoire site is regarded here as evidence for age segregation among sauropod herds, similar to that recorded at the site in Portugal.

The Middle Jurassic Ardley tracksite in the United Kingdom is unique among these ichnological assemblages, for instead of containing a single sauropod track-type, two distinct ichnotaxa are present. The two track types are characterized by different gauges (Day et al., 2002; Day et al., 2004), the horizontal distance between the planes defined by the medial track margins (Lockley & Hunt, 1995). Trackway gauge in

sauropods has been attributed to morphological differences in posture related to

femoral morphology and the angle of articulation of the femoral head with the acetabulum (Wilson & Carrano, 1999). Based on the suite of morphological characteristics related to trackway gauge, wide-gauge tracks are thought to have been produced by titanosaurids and brachiosaurids, whereas narrow-gauge tracks are attributed to diplodocoid sauropods (Wilson & Carrano, 1999). Since the UK assemblage contains both wide- and narrow-gauge trackways, Day et al. (2002) inferred the presence of two different sauropod taxa in a single herd group, the only case of a multispecies (interspecific) herd group of sauropods reported thus far. An

interspecific herd may at first seem an unusual prospect; however, a multispecies

skeletal herd assemblage has been reported for ornithopod dinosaurs (Varricchio &

Horner, 1995), and interspecific groups have also been observed in modern birds,

ungulates, primates, and fish, according to Morse (1980) and references therein.

Currently, although the ichnological sources of evidence for sauropod gregarious

behavior agree on some aspects such as average group size, many other details, such

as age profile and taxonomic composition of herd groups, remain unresolved. A well-

preserved body fossil assemblage has the potential to clarify some of these ambiguous

behavioral aspects.

LIMITS OF THE ICHNOLOGICAL RECORD

Tracksites are assumed to be significantly averaged when trackways frequently

intersect and overprint one another or show signs of exposure such as erosion or

slumping around track margins (Day et al., 2004). However, even well-preserved, non-

intersecting and non-overlapping trackways located on a single bedding plane are not

guaranteed to have been created in a short time frame. Firm sedimentary substrates

may mitigate the effects of erosion; and, in some cases, subparallel trackways may be

the product of topographic constraints such as shorelines that limit the number of

possible directions of travel, thereby condensing trackways along certain routes

(Lockley & Hunt, 1995). By the same token, overlapping trackways could be created by

the contemporaneous passage of a group of animals, giving the impression of time-

averaging where none exists.

The time represented by the elements in a skeletal accumulation is easier to

assess since bone is less likely to be destroyed than sedimentary features such as

tracks; bone modification features may begin to accrue shortly after death, but continue

to develop throughout a period of several years before a bone is finally destroyed. Since

tracks are more susceptible to destruction, tracksites are characterized by smaller

maximum values of potential time-averaging, but are less likely to accumulate a

continuum of modification features that could be used to diagnose exposure time.

Although a lesser degree of averaging may seem like an obvious benefit of ichnological

data, caution is warranted, for even small amounts of time-averaging in tracksites may

significantly skew interpretations. For example, several solitary individuals traveling in a

common direction and passing the same point within hours of one another could easily produce a set of subparallel trackways that, although characterized by an extremely small amount of time-averaging, creates the false impression of a mobile social group.

Since tracksites are generally characterized by an accumulation of more than one trackway, it is not unreasonable to assume that these assemblages are almost never

perfect census accumulations, representing a single moment in time. Some subset of the preserved trackways may have been contemporaneous, but the difficulty arises in distinguishing these from the unrelated noise in the assemblage.

Although many skeletal accumulations have similar problems with time- averaging, distinct temporal subsets in an assemblage are often identifiable based on differing taphonomic modes, usually defined by the differential accumulation of bone modification features. Furthermore, catastrophic mass mortalities are not uncommon causes of body fossil assemblages, rendering the skeletal record especially appropriate for investigations of the dynamics of ephemeral social groups. Difficulty assessing time- averaging is not the only problem related to the use of trackway assemblages in studies of herd dynamics, however.

In many instances, the taxonomic affinities of trackmakers cannot be readily determined from ichnocoenoses since track appearance and clarity is a function of substrate characteristics and quality of preservation, respectively (Nadon, 2001 and references therein). Even when ichnotaxa are easily identified in an assemblage, it is important to note that these designations do not necessarily correspond to recognized, biologically-based, taxonomic classifications. Aspects of preservation and classification can complicate identification of the taxa responsible for creating an ichnocoenose, thereby reducing the resolution of tracksites with respect to questions of herd composition. Additionally, ambiguities in ichnological taxonomic identification make it difficult to determine if conflicting behavioral data are the result of variation within or between taxa. While body fossil evidence avoids many of these pitfalls related to trackmaker identification, a site must still fall within specific taphonomic parameters

before it can be determined if the assemblage contains the remains of a herd and provides a suitable basis for interpretations of gregarious behavior.

TAPHONOMIC CONSTRAINTS FOR SKELETAL EVIDENCE

In order to provide useful information about herd behavior, a skeletal assemblage must meet certain taphonomic criteria; otherwise, bias generated by post-mortem processes may obscure any behavioral signals originating from the initial biocoenose.

Low taxonomic diversity is the primary indicator that an assemblage may contain the remnants of herd group. However, certain taphonomic processes may generate a similar diversity signal when the remains of the individuals at the site were not, in fact, sampled from a herd. Taxon-specific agents of mortality, such as miring of large animals, may produce monospecific or paucispecific skeletal accumulations. A single locale may thus amass a number of non-contemporaneous individuals over a period of time, creating a time-averaged assemblage that has no bearing on herd dynamics (e.g.

Agenbroad, 1984; Bilbey, 1999). Selective agents of mortality may also preferentially sample a subsection of individuals in a group (Lyman, 2001), providing a biased view of herd composition. Consequently, a deposit containing only juvenile individuals may simply record the aftermath of a mortality in which survival was directly tied to physical robustness and ability to withstand environmental stress, for in such situations young individuals are at a distinct disadvantage relative to fully-mature animals. To avoid misinterpretations related to these taphonomic biases, one must construct a detailed taphonomic history of a potential site before proceeding with any behavioral interpretation.

MOTHER’S DAY QUARRY

The Mother’s Day assemblage has been identified most recently as the remnant

of a drought mortality, entrained in a debris flow and transported a short distance from a

point of initial accumulation to the final site of burial. The quarry has a strikingly low

diversity, with only two taxa confirmed present. However, the relative abundance of

elements from these taxa is so uneven – sauropod material comprises 98% of the

recovered bones – that the quarry is effectively monospecific. Of this sauropod material,

all identified elements belong to either juvenile or subadult individuals; none are

attributable to fully-adult individuals. Although the site meets the initial criterion for a

potential herd assemblage in that it is paucispecific, it must be determined whether this diversity pattern and the unusual age profile of the deposit are taphonomic artifacts or are accurate reflections of the original biologic assemblage. Possible biasing factors that are relevant to this study include a selective agent of mortality, an attritional mode of death, and transport processes that are either age-specific or taxon-specific.

A selective agent of mortality can produce much of the possible bias in an assemblage, so it is imperative that the influence of the mode of death on the thanatocoenose be carefully examined. The Mother’s Day accumulation has been most recently interpreted as a drought assemblage. Severe droughts are unique causes of mortality, for they consist of a single event spread over a period of several months.

Evanoff and Carpenter (1998) have termed this type of mortality a non-catastrophic event. Since Rogers (1990) first identified drought assemblages in the Two Medicine

Formation, droughts have been invoked with increasing frequency as the agents of mortality responsible for the formation of Mesozoic vertebrate accumulations (Varricchio

& Horner, 1995; Evanoff & Carpenter, 1998; Richmond & Morris, 1998). Observations of

drought-affected communities of large African mammals reveal that death occurs as a

result of food shortage due to declines in plant productivity and nutritional value;

surprisingly, water stress does not seem to be the controlling factor for herbivore

mortality (Carpenter, 1987). Theoretically, droughts should induce higher rates of

mortality among the weaker individuals of a population, especially young individuals that

are more vulnerable to environmental stress, and modern studies of drought responses

in populations of large herbivores confirm that juvenile mortality rates do exceed those

of adults (Corfield, 1973; Barnes, 1982; Oba, 2001). However, since adults comprise a

larger percent of the total population, they will represent a majority, or at least a large

portion, of the death assemblage relative to juveniles (Hillman & Hillman, 1977).

Consequently, depending on the severity and duration of the drought period, the

resultant thanatocoenoses should not accurately reflect age ratios associated with the

affected biological group. However, all age groups present in the population should appear in the assemblage, even if they are represented by relatively few individuals.

Age profiles of fossil drought assemblages are consistent with these conclusions, for

they typically contain animals from a broad spectrum of age classes and do not exhibit

an overabundance of juveniles as would be expected if the effects of drought conditions

were nearly exclusive to young animals (Rogers, 1990; Varricchio & Horner, 1995;

Evanoff & Carpenter, 1998).

The effect of drought mortality on taxonomic diversity must also be addressed

before droughts may be categorized as selective or indiscriminate causes of death. This

potential bias could have profound implications for interpretations of herd dynamics

based on skeletal assemblages. If drought mortalities affect a certain taxon disproportionately more than other taxa that share its geographic range, then droughts may generate monospecific and paucispecific accumulations that represent only a small fraction of the taxonomic diversity in that region. Since herds are themselves small subsets of regional populations and typically consist of similar taxa, this sort of taxonomic bias is not of great concern in the context of this study. However, if droughts are taxon-specific killing agents that operate over a period of several months, the resulting low-diversity death assemblages cannot necessarily be assumed to be catastrophic in nature. Since droughts may last for months (Hillman & Hillman, 1977), they have the potential to create assemblages with relatively low, but not negligible, amounts of time-averaging. An extended duration of mortality, in conjunction with a taxon-specific cause of death, may easily generate fossil assemblages that appear to be the result of a catastrophic mass mortality, but are, in fact, the product of attritional processes. Once again, we must turn to accounts of modern droughts to assess the potential impact of drought on paleocommunities. Although modern droughts negatively affect herbivores to a much greater extent than they affect carnivores, periods of severe aridity are characterized by roughly equivalent mortality rates of different herbivorous taxa within a single trophic group (Hillman & Hillman, 1977). Grazers typically exhibit much higher mortality rates than browsers because their forage is more susceptible to short-term fluctuations in climate, whereas food resources utilized by browsing animals are stable, but more dispersed, supporting only relatively small populations (Hillman &

Hillman, 1977). All herbivorous dinosaurian taxa must have been browsers, since grasses were not present in the Mesozoic. Studies of tooth abrasion, dental

morphology, and jaw mechanics suggest only minor subdivisions were present within

this niche for the sauropods (Fiorillo, 1991A; Calvo, 1994). Although a trophic bias could

produce inaccuracies in relative abundances of some dinosaur taxa in a drought

assemblage, a drought mortality should affect all sauropod taxa equally, and there were

many such taxa in the Morrison environment; therefore, trophic selectivity cannot be

invoked to explain the monospecific nature of the site.

The mode of death (catastrophic or attritional) also has the potential to

significantly bias a fossil assemblage so that it no longer resembles the population from

which it was originally derived. Herd groups are ephemeral entities in terms of both time

and space, so a fossil assemblage that accumulates through attritional processes

cannot possibly represent such an aggregate of gregarious individuals. Since the

Mother’s Day assemblage is interpreted as a drought mortality, the total interval of

carcass accumulation associated with the assemblage was likely months in length. In

the case of the Mother’s Day site, there are no indications that this small amount of

time-averaging had a significant impact on the composition of the death assemblage.

The general lack of pre-fossilization bone modification observed on the Mother’s Day

specimens corroborates this assertion of minimal time-averaging, for subaerial

weathering in arid conditions can cause extensive surficial damage after less than one year of exposure (Coe, 1978).

The final source of potential bias to consider for interpretation of the Mother’s

Day Quarry is post-mortem transport. In many cases, significant taphonomic bias may

be introduced into fossil assemblages as they are transported prior to their final burial.

Fluvial currents have been shown to mix disparate skeletal elements from different

environments and segregate elements based on size and density differences that may be either taxon-specific or age specific (Behrensmeyer, 1975; Behrensmeyer, 1982;

Aslan & Behrensmeyer, 1996). Even if the initial death accumulation was an accurate reflection of the herd group from which it was sampled, a transport mechanism could sufficiently alter the assemblage so that it was no longer representative of the original biological entity. For example, juvenile bones, characterized by lower density and smaller size than adult elements, may be preferentially transported or winnowed from a broader assemblage, leaving a deposit composed of a single age group. The degree of transport bias for flow processes is controlled primarily by the distance and duration of transport. The bones at the Mother’s Day site were apparently entrained in a debris flow and transported a short distance from the site of initial accumulation to the location of ultimate burial. Given the short transport distance and duration hypothesized for this deposit, no transport biases are expected to be present. Observations of the assemblage are consistent with this conclusion, for there are no size-dependent trends in the vertical distribution of elements that would suggest a flow-induced taphonomic bias toward large or small bones.

In summary, the Mother’s Day assemblage has not been subjected to significant taphonomic biasing as a result of either selective mortality, attritional mode of death, or differential transport mechanisms. Because the deposit apparently has not been significantly altered by taphonomic processes, the skeletal material contained therein should be an accurate reflection of the composition of the biological group from which the assemblage was derived.

OTHER POSSIBLE HERD ASSEMBLAGES

Before continuing with a behavioral interpretation based on the Mother’s Day

data, let us briefly review the other fossil assemblages that have been identified as herd

group mortalities. Currently, only a small number of sites are known to contain

significant numbers of sauropod individuals in a monospecific or paucispecific context.

One site, located in the Upper Jurassic Lower Shishugou Formation of northwestern

China reportedly preserves the remains of 19 juvenile Bellusaurus sui individuals, with

no indication of the presence of adults (Dong, 1990; Dong, 1992). Although this

evidence is certainly suggestive of an age segregated herd group, the unusual

preservation patterns could be attributable to taphonomic factors. Time-averaging may

also be a factor in the assemblage, for the bonebed contains the remains of theropods,

ornithopods, and fish, in addition to the sauropod material (Dong, 1990), indicating a

significant amount of post-mortem mixing. A thorough investigation of the taphonomic history of the site could determine whether the 19 sauropod individuals do, in fact, represent the remnants of a mobile social group, but no such study has yet been published.

Another sauropod herd assemblage has been reported from the Middle Jurassic of Argentina (Coria, 1994). Unlike the Chinese site, this locality preserves the remains of both juveniles and adults, identified as the cetiosaurid Patagosaurus fariasi. The assemblage consists of five specimens in total: two adults and three juveniles. This co- occurrence of mature and immature individuals has been interpreted as evidence for extended parental care in sauropods (Coria, 1994). Coria (1994) reported that the remains were disarticulated, with little cranial material present. Although the taphonomic

investigation of the site was cursory, Coria noted that there was no significant variation in weathering stages between specimens, suggesting little time-averaging in the assemblage.

Multiple sauropod individuals belonging to a single taxon have also been recovered from the Late Cretaceous Javalina Formation of Big Bend National Park in

Texas (Fiorillo & Montgomery, 2001). This site contains the remains of at least three juvenile Alamosaurus sanjuanensis (Main & Montgomery, 2000; Fiorillo & Montgomery,

2001) The site preserves a diversity of taxa (Main & Montgomery, 2000), indicating that post-mortem mixing contributed to the formation of the assemblage. There is also evidence that the accumulation endured significant bioturbation following deposition in a shallow lacustrine setting (Fiorillo & Montgomery, 2001). This assemblage has not yet been suggested to represent a fossilized herd remnant; and, given the potential for time-averaging implied by the taxonomic richness and evidence of trampling, caution is advised in making any such interpretation.

Skeletal evidence of sauropod herds hold great potential, but before sites are used in behavioral interpretations, they must undergo a rigorous taphonomic analysis.

The evidence presented by Coria, like that related by Dong about the Bellusaurus accumulation, is certainly suggestive of a herd assemblage, but without a more thorough taphonomic investigation of these sites, this possibility cannot be confirmed.

BEHAVIORAL INFERENCES

The skeletal remains contained within the Mother’s Day Quarry are unique for two primary reasons. First, an overwhelming majority of the material is derived from a

single taxon, and second, no adult individuals are present. Projecting these characteristics of the fossil assemblage back to the original biological assemblage, we can infer that the initial herd group was monospecific and consisted solely of juvenile and subadult individuals. Skeletal evidence of age segregation of social groups has been noted for ornithopod dinosaurs (Forster, 1990), but the sauropod track record has provided conflicting data on this behavioral aspect (Ostrom, 1972; Lockley et al., 1994).

The body fossil evidence from the Mother’s Day site confirms that social groups of diplodocid sauropods were divided on the basis of age, at least in some instances.

However, there are two alternative behavioral interpretations that may be proposed for the assemblage. First, the animals that formed the fossil assemblage may have simply aggregated in one spot as a result of environmental patchiness and not been part of a true herd group. Second, the sauropods in the quarry may be derived from a stationary social group and therefore do not fit the definition of a herd provided earlier in this paper. If environmental patchiness was the primary factor controlling the taxonomic composition of the accumulation, the site would probably contain multiple taxa. The high diversity of many large Morrison localities (see Foster, 2003) suggests that the taxa within a single trophic group did not maintain strict spatial segregation with one another. Consequently, if the location of mortality represented a preferred habitat for browsing herbivores, more than one taxon of sauropod might be expected to be present in the resultant fossil assemblage.

The question of whether or not the group of animals in the living assemblage was a mobile group is somewhat less complicated, although again only circumstantial, negative evidence may be brought to bear. The site clearly does not represent a mating

100

or nesting site because of the absence of adults. More difficult to discount are the

assertions that some fossil accumulations that consist predominately of juvenile

dinosaurs are simply the products of fast juvenile growth rates rather than age

segregation of herds (Varricchio & Horner, 1995). Although this is certainly a viable

hypothesis, significant size differences between some of the juvenile and subadult

individuals within the Mother’s Day Quarry suggest that the assemblage may not

represent a single cohort, so a gregarious social group segregated on the basis of age

remains the best interpretation of the available data.

Judging from patterns of gregarious behavior observed in modern herbivores,

though, it is probable that Mesozoic herd characteristics were highly variable.

Accordingly, it would be impossible to capture all behavioral permutations in a single

assemblage of either tracks or skeletal remains. The composition and size of modern

herd groups vary between taxa and often change seasonally, depending on the timing

of reproductive cycles or the availability of forage (Western & Lindsay, 1984; Bender &

Haufler, 1999). Some animals, such as elephants, form herds composed of adult

females and immature individuals, whereas adult males are solitary (Katugaha et al.,

1999). Elk exhibit similar patterns of partitioning based on sex and age, except that mature males may form herd groups rather than remain solitary. The most striking aspect of gregarious behavior observed in elk is the extreme variation in group size and composition within a single geographically-defined population. Although certain aspects

of elk social behavior follow a seasonal schedule, at any given point in the year some

herds are segregated, others are mixed, and some individuals remain solitary (Bender &

Haufler, 1999). If this much variation in herd behavior is observed in a single modern

101

population irrespective of predictable seasonal cycles, we must be cautious indeed as

we attempt to generalize characteristics of gregarious behavior in extinct higher-level taxonomic groups. Assuming that interpretations of age segregation based on the

Mother’s Day evidence are correct, and that adult material is not present in the quarry and simply remains unexcavated at this time, the Mother’s Day Quarry confirms a unique aspect of sauropod behavior that has few other supporting examples in the fossil record.

CONCLUSIONS

The Mother’s Day Quarry apparently contains the remains of a herd of diplodocid sauropod dinosaurs that succumbed to a drought-related mass mortality. The herd group from which the death assemblage was derived consisted of only juvenile and

subadult individuals. Although sauropods likely exhibited a wide variety of gregarious

behaviors, the Mother’s Day data reveal evidence of clear age partitioning of herd

groups. Age segregation of sauropod herds has been postulated previously on the basis

of trackway evidence, but the Mother’s Day assemblage provides the first well-

constrained example of body fossil corroboration of this type of behavior.

Investigations of other monospecific or paucispecific sites that may contain herd

groups could eventually allow us to delineate behavioral trends and diagnose the extent of behavioral variation within certain groups of herbivorous dinosaurs. Developing a

detailed taphonomic history for each site is a necessary first step in this process, for

without an understanding of the processes responsible for site formation, biases that

have the potential to affect behavioral interpretations cannot be adequately identified.

102

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APPENDIX

Includes a quarry map of all elements removed during CMC excavations (2000-

2003) and individual quarry maps for each year on which the elements are labeled with their MDS field numbers.

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Figure A-5. Quarry map of elements removed during 2003 field season.

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