Application of X-Ray Computed Tomography to Interpreting The

Home , Conasauga Formation, Concretion, Demosponge, Drumian, Evolution of brachiopods, Lithification

Application of X-ray Computed Tomography to Interpreting the Origin and Fossil Content of Siliceous Concretions from the Conasauga Formation (Cambrian) of Georgia and Alabama, USA

THESIS

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University

By Jessica Kastigar Graduate Program in Geological Sciences

The Ohio State University 2016

Master's Examination Committee: Loren E. Babcock (Advisor) Ann E. Cook MaryMegan Daly

Abstract

Siliceous concretions from the Conasauga Formation (Cambrian: Drumian to

Guzhangian) of Georgia and Alabama, USA, yield a diverse assemblage of body and trace fossils. X-ray computed tomographic (XCT) imaging has been added to standard techniques of analysis including light microscopy, to help determine the early development and fossil content of the concretions. Most of the siliceous concretions show features consistent with biofilm-mediated early diagenesis of siliceous sponges

(poriferans), primarily hexactinellids, but perhaps also including some demosponges.

Leucon-grade body forms have been identified from images of the networks of canals and the spongocoels within the skeletons, and stauract spicules preserved in the concretions are consistent with a hexactinellid origin for many of the concretions. Some concretions appear to have formed from remobilized biogenic silica and nucleated around other forms of decaying organic matter, including biofilms associated with burrowing traces, fecal matter, and partially decayed remains of other organisms such as hyoliths, trilobites, and cap-shaped fossils. Most concretions probably lithified quickly but some show evidence of a longer-term process of lithification involving multiple generations of early diagenesis.

XCT-scans of Holocene and post-Cambrian sponges show remarkable similarities to siliceous concretions from the Conasauga Formation, which supports the conclusion

ii that many of the Conasauga Formation concretions formed around sponge skeletons.

XCT-scans of siliceous, calcareous, and other concretions from various Paleozoic localities in North America, South America, and Europe show features similar to those exemplified in Conasauga Formation concretions, which implies similar microorganism- mediated early diagenesis for their origins.

iii

Acknowledgments

This manuscript could not have been completed without the assistance of others.

First and foremost, I want to thank my husband, James Knobbe, for his immense support throughout the last few years, including technical help with computer problems and

Photoshop, a willingness to listen about rocks, and incredible moral support and understanding. I also want to thank my advisor, Loren Babcock, for helping me figure out what the data were trying to tell me, constantly nudging me in the right direction, and the many long conversations about these curious little concretions. I also relied heavily on my family, my parents, John and Mary Kastigar, and my father-in-law, Tom Knobbe, for their confidence in me and their support. I would also like to thank my fellow graduate students, especially Mark Peter, and faculty, including Bill Ausich, who were a source of great help, advice, and encouragement. Finally, I am grateful for the help from my committee members. Ann Cook provided access to the XCT-scanner and instructed me in its use and the use of ImageJ. Meg Daly helped me better understand the biology of sponges.

Loren Babcock, Jerry Armstrong, Charles Ciampaglio, Ron Ray, and Marc

Behrendt collected the Conasauga Formation concretions used in this study. Samuel

Zamora provided the concretions from the Valtorres Formation, Spain. Loren Babcock provided the concretions from the Sicasica Formation, Bolivia, which were collected

iv under support from a National Geographic Society Research Grant. Loren Babcock and

Shanchi Peng collected the orsten-type concretion from the Huaqiao Formation, China.

Dale Gnidovec provided access to specimens in the Orton Geological Museum at The

Ohio State University. Access to Holocene sponge material was provided in part by Tom

Watters of the Museum of Biological Diversity at The Ohio State University.

Vita

2008...... A.S. Geology, Heartland Community

College

2011...... B.S. Paleontology, Northern Illinois

University

2016...... M.A. Invertebrate Paleontology, The Ohio

State University

Field of Study

Major Field: Geological Sciences

Table of Contents

Abstract ...... ii

Acknowledgments...... iv

Vita...... vi

List of Figures ...... ix

Introduction ...... 1

Concretions ...... 2

Conasauga Formation ...... 8

Sponge Morphology ...... 12

Materials and Methods ...... 15

Research ...... 18

Conasauga Concretions ...... 18

Other Concretions ...... 22

Comparison of Concretions with Holocene and Fossil Poriferans ...... 25

Figures ...... 29

Discussion ...... 40 vii

Density Differences in Conasauga Formation Concretions ...... 40

Trilobites ...... 41

Hyoliths ...... 42

Trace Fossils ...... 43

Poriferans in the Conasauga Formation ...... 46

Generation of Concretion Development ...... 49

Twinned Concretions ...... 51

Additional Organic Material Preserved in Concretions ...... 51

Valtorres Formation Concretions ...... 53

Figures ...... 55

References ...... 66

Appendix A: Conasauga Concretions ...... 74

Appendix B: Comparative Concretions ...... 78

Appendix C: Porifera ...... 80

viii

List of Figures

Figure 1. The Coosa River Valley, northwest Georgia, and adjacent part of eastern

Alabama, USA showing localities of siliceous concretions...... 17

Figure 2. Surface textures of siliceous concretions from the Conasauga Formation

(Cambrian), Coosa River Valley, Georgia...... 29

Figure 3. Twinned siliceous concretions from the Conasauga Formation...... 30

Figure 4. Porosity types identified using XCT-scanning in concretions from the

Conasauga Formation...... 31

Figure 5. Examples of multiple diagenetic generations imaged using XCT-scanning in

siliceous concretions from the Conasauga Formation...... 32

Figure 6. Low density, oval-shaped structures stretching across a siliceous concretion

from the Conasauga Formation, imaged using XCT...... 33

Figure 7. Types of internal density differences identified using XCT in siliceous

concretions from the Conasauga Formation ...... 34

Figure 8. Stauract spicules in siliceous concretions from the Conasauga Formation...... 35

Figure 9. Internal structures of concretions from varied localities imaged using XCT-

scanning...... 36

Figure 10. XCT-slices of Holocene poriferans...... 38

Figure 11. XCT-slices of fossil poriferans...... 39 ix

Figure 12. Trilobite spines in a siliceous concretion, specimen 65, from the Conasauga

Formation, imaged using XCT...... 55

Figure 13. Density-related outlines surrounding hyoliths in siliceous concretions,

specimens 36 and 78, from the Conasauga Formation, imaged using XCT...... 56

Figure 14. Assemblages of hyoliths in siliceous concretions from the Conasauga

Formation...... 57

Figure 15. Trace fossils preserved on external surfaces of siliceous concretions from the

Conasauga Formation...... 58

Figure 16. Trace fossils at centers of siliceous concretions, and perhaps forming their

nuclei, from the Conasauga Formation, imaged using XCT ...... 59

Figure 17. Growth on siliceous concretion related to presence of a trace fossil, specimen

62, from the Conasauga Formation ...... 60

Figure 18. Fossilized sponges preserved in siliceous concretions from the Conasauga

Formation, imaged using XCT...... 61

Figure 19. Inverted three-dimensional model of siliceous concretion, specimen 31,

showing multi-generational diagenetic banding, from the Conasauga Formation,

imaged using XCT...... 62

Figure 20. Concretion showing two diagenetic generations and trace fossils, from the

Conasauga Formation, imaged using XCT; specimen 50...... 63

Figure 21. Cap-shaped fossils preserved in siliceous concretions from the Conasauga

Formation...... 64

Figure 22. XCT-slices of siliceous concretions from the Valtorres Formation (Cambrian),

Spain, showing fossils...... 65

Introduction

Concretions, or nodules, are rounded mineral masses generally composed of common minerals such as calcite, quartz, pyrite, or siderite, and found principally in sedimentary strata. They have long attracted interest from geologists and paleontologists for the information they provide about sedimentation and compaction history, for their use in stratigraphic correlation, and as an important source of exceptional fossil preservation. The early development of concretions has long been speculative, and until recently, few experimental studies or in-situ observational studies had been conducted to determine the origin and early diagenesis of concretions. Most discussions were centered on inferences made from ancient concretions. Today, with an expanding understanding of the early diagenetic history of sedimentary concretions, and new, powerful techniques of analyzing ancient concretions, it is possible to merge information from Holocene and ancient concretions to enhance our insight into the origin and early history of these fascinating objects.

The purpose of this study is to determine the origin and early diagenetic history of silica (chert or chalcedony, SiO2) concretions from the Conasauga Formation (Cambrian:

Drumian-Guzhangian) of Georgia and Alabama, USA. These concretions have been documented for more than a century, and are recognized as a source of exceptional fossil

1 preservation (REFS). The use of X-ray computed tomographic (XCT) scanning has been added to more traditional methods of study, including light microscopy, to gain insight into the nature and development of these concretions. For comparative purposes, selected concretions composed of silica (quartz var. chalcedony), calcium carbonate (calcite), iron carbonate (siderite), and barium sulfate (barite), all of which were collected from other areas and strata, have likewise been examined. Silica in the concretions from the

Conasauga Formation is evidently derived from the siliceous (opaline) silica of hexactinellid sponges, which were common in low-latitude shelf seas of the Cambrian

(e.g., Rigby, 1978, 1986, 1987; Briggs et al, 1985; Robison et al., 2015); and comprise a substantial share of the fossils recovered from the Conasauga Formation of the southeastern United States. Siliceous sponges previously have been identified as the origin of some Conasauga Formation concretions, the so-called ‘star-cobbles’ of Walcott

(1896, 1898; see also Ciampaglio et al., 2005, 2006; Schwimmer and Montante, 2007), but this work indicates that a more diverse poriferan assemblage contributed silica that formed concretions, and suggests a more complicated taphonomic-diagenetic history leading to early mineralization in concretionary masses.

Concretions

Definitions of what constitute a concretion vary, but according to the “Glossary of

Geology” (Neuendorf et al., 2005), a concretion is “a hard, compact mass or aggregate of mineral matter, normally subspherical but commonly oblate, disk-shaped, or irregular with odd or fantastic outlines; formed by precipitation from aqueous solution about a 2 nucleus or center, such as a leaf, shell, bone, or fossil, in the pores of a sedimentary or fragmental volcanic rock, and usually of a composition widely different from that of the rock in which it is found and from which it is rather sharply separated. It represents a concretion of some minor constituent of the enclosing rock or of cementing material, such as silica (chert), calcite, dolomite, iron oxide, pyrite, or gypsum, and it ranges in size from a small pellet-like object to a great spheroidal body as much as 3 m in diameter.

Most concretions were formed during diagenesis, and many (especially in limestone and shale) shortly after sediment deposition.”

Concretions have been described from formations worldwide, Archean to

Holocene, and are present in various lithologies. This suggests that conditions necessary to form them have occurred repeatedly and in multiple types of environments over geologic time. Additionally, during certain time intervals including the Cambrian

(Babcock et al., 2015) and Devonian (Hellstrom and Babcock, 2000), there is evidence that concretionary facies were time-specific. A relationship to eustatic sea level history has also been inferred for some concretionary facies in the Cambrian sea level (Babcock et al., 2015).

The mineralogic composition of concretions varies, seemingly according to the abundance of dissolved ions present at the time of sediment deposition. Common constituents of concretions include calcium carbonate, calcium phosphate, siderite, pyrite, manganese, and silica (e.g., Walcott, 1896, 1898; Westergård, 1922; Revelle and Emery,

1951; Weeks, 1967; Berner, 1968; Martinsson, 1974; Nitecki, 1979; Bremner, 1980;

Blome and Albert, 1985; Müller, 1985; Babcock et al., 1987; Criss et al., 1988; Martill, 3

1988; Pye et al., 1990; Allison and Pye, 1994; Ahlberg, 1998; Hellstrom and Babcock,

2000; Borkow and Babcock, 2003; Maas et al., 2006; Schwimmer and Montante, 2007,

2012; Arena, 2008; Hall and Savrda, 2008; Dabard and Loi, 2012; Gaines et al., 2012;

Álvaro et al., 2013; Calner et al., 2013). Concretions are formed syndepositionally and are enriched in certain minerals compared to the surrounding sediment (Dabard and Loi,

2012), resulting in higher resistance to compaction during later sediment diagenesis

(Blome and Albert, 1985; Criss et al., 1988; Hellstrom and Babcock, 2000; Ciampaglio et al., 2006; Maas et al., 2006; Schwimmer and Montante, 2007; Calner et al., 2013; Wilson and Brett, 2013).

Concretionary masses in sedimentary rock have been known for centuries. Efforts to describe particular instances date at least to the work of Hitchcock (1841), who described concretions as “clay stones.” The first known attempt to classify different morphologies of concretions was that of Abbott (1907). Abbott restricted his classification to those concretions that exhibited: (1) a definite geometrical shape; (2) parallel banding; and (3) symmetry. Abbott (1907) acknowledged that concretions may exhibit a wide range of morphologies, even within a single formation. Many concretions, therefore, would be excluded from his classification system.

The interpretation of concretion development has changed over time. Concretions were once thought to form slowly as water percolated through sediment or rock, leaching out soluble materials and afterward gradually concentrating and reprecipitating them around a central nucleus (Abbott, 1907). This helped explain the parallel banding that some concretions possess, as it was thought that each band formed during a discrete 4 generation of mineral precipitation. Today, however, this explanation has been replaced in favor of the concept that concretion development is related to microbial decomposition of organic matter within an aqueous environment (Berner, 1968; Pye et al., 1990;

Schieber, 2002; Borkow and Babcock, 2003; Ciampaglio et al., 2005, 2006; Babcock and

Ciampaglio, 2007; Schwimmer and Montante, 2007, 2012; Arena, 2008; Das and

Tripathi, 2009; Ahn and Babcock, 2012; Gaines et al., 2012; Wilson and Brett, 2013).

When organic matter decays in a body of water, it is surrounded by a three- dimensional halo of microorganisms, called a biofilm, which consists of a consortium of bacteria and fungal hyphae (Borkow and Babcock, 2003). Some of the bacteria within the fungal framework initiate mineral precipitation and are capable of autolithifying the biofilm, eventually forming the concretion. This stable network of microbial growth maintains a sturdy, supportive structure and enhances mineral precipitation in part by distributing decompositional gases through the biofilm (Borkow and Babcock, 2003;

Babcock and Ciampaglio, 2007; Schwimmer and Montante, 2007; Ahn and Babcock,

2012). Furthermore, the presence of extracellular polymeric substances (EPS), or slime, allow nutrients to disseminate through the developing concretion as if through a three- dimensional conduit (Borkow and Babcock, 2003).

A fungal framework is crucial to developing a concretion, as one must be present in order to maintain the structural support and restriction of gasses necessary. Biofilms that consist solely of bacteria will instead result in mineral crusts or sheets (Borkow and

Babcock, 2003).

As not all organic matter that decays in an aqueous environment will produce a concretion, more than the presence of a biofilm is necessary for concretion development; pore water within the sediment must also be rich in necessary ions. Pyrite, for example, is a mineral commonly found in concretions due to sulfate reduction by bacteria in iron-rich and sulfur-rich waters (Babcock and Speyer, 1987; Allison and Pye, 1994; Schieber,

2002; Borkow and Babcock, 2003; Schwimmer and Montante, 2007; Wilson and Brett,

2013). Pyrite requires a low pH to precipitate, which occurs due to the production of acids during decomposition that then localize and are contained within the biofilm

(Wilson and Brett, 2013). When concretions consist of multiple minerals, such as pyrite in carbonate concretions, ions necessary for both minerals must be present in the pore water with reducing bacteria for both available within the biofilm. In the case of pyrite and carbonate minerals, the pyrite precipitates first due to the initial reduction in pH, with the carbonate minerals following after pH again rises after byproducts, such as bicarbonate and ammonia, are released during decomposition (Weeks, 1967; Martill,

1988; Wilson and Brett, 2013). This process is influenced by the fact that a biofilm, during the normal processes of decay, will consume available oxygen within the decay halo. This results in a localized dysoxic or anoxic microenvironment (a “chemical microenvironment”), which is necessary for harboring reducing bacteria (Babcock and

Speyer, 1987).

The amount of time it takes for a concretion to form is essentially instantaneous on a geologic timescale. Experimental and comparative fossil evidence suggests that concretion development is initiated in a matter of days to weeks (Borkow and Babcock, 6

2003; Babcock et al., 2005, in press; Babcock and Ciampaglio 2007; Arena, 2008,

Wilson and Brett, 2013), and Pye et al. (2013) showed that a carbonate concretion can fully form in a natural setting within six months, and in laboratory settings between two and four weeks. It has been hypothesized that in some instances, concretions may continue to grow, perhaps for years (Allison and Pye, 1994), centuries, or millennia

(Berner, 1968; Canfield and Raiswell, 1991; Wilson and Brett, 2013). Whereas a time- frame of centuries or longer may help explain exceptionally large concretions, the work of Pye et al. (2013) shows that it is not necessary for the development of smaller concretions. However, it does tend to corroborate earlier interpretations that concretions form beneath the sediment-water interface, and likely require a temporary hiatus in the deposition of overlying sediments in order to fully develop (Canfield and Raiswell, 1991;

Mozley and Burns, 1993; Borkow and Babcock, 2001; Wilson and Brett, 2013).

Because concretions form from the decay of organic matter, develop in short time spans, and are inherently resistant to compaction, they have the capacity to exhibit a remarkable degree of fossil preservation. Organisms retaining only soft parts or non- biomineralized parts, which do not fossilize under most circumstances, can be preserved as a result of the rapid development of concretions during the time of active decay. Some organisms, such as the enigmatic Tullimonstrum gregarium (Carboniferous of Illinois), are known only from fossils preserved in concretions (Foster, 1979; McCoy et al., 2016).

Other organisms, while containing biomineralized parts, commonly disarticulate soon after death and are usually found in sediments as isolated elements or fragments.

Sponges, for example, have a fossil record stretching from the Proterozoic (Rigby, 1987; 7

Braiser and Green, 1997; Müller et al., 2007), but articulated specimens are relatively rare until after the Cambrian, when fused spicule tips became prevalent (DeLaubenfels,

1955; Rigby, 1987). In Cambrian strata, the rapidity of concretion development helps account for the presence of articulated sponges. In other cases, fossils represented in concretions can also be found complete in the surrounding host rock, but they are flattened or otherwise distorted due to compaction from overlying sedimentary layers

(Schwimmer, 1989; Ahn and Babcock, 2012; Gaines et al., 2012). As concretions are resistant to compaction, the same types of organisms preserved within them commonly retain their original three-dimensionality.

Conasauga Formation

The Conasauga Formation (or Group) was identified and named by Hayes (1891) for exposures of Cambrian shale and limestone along the Conasauga River, Dalton quadrangle, Georgia. Previously, rocks now recognized as Conasauga had been described under the names Coosa and Flatwoods, but most authors have accepted Conasauga as the name for the formation (Munyan, 1951). Before this designation, Walcott (1886) referred to rocks that include the Conasauga Formation as the “Georgia shales.”

The Conasauga Formation (or Group) is a Cambrian sedimentary unit recognized through the states of Georgia, Alabama, Tennessee, and Virginia. The unit consists of shale, limestone, and dolostone, and is of varying thickness. In Georgia and Alabama, the

Conasauga Formation is dominated by olive-green to yellow shales and, lower in the unit, thin interbeds of fine, siliciclastic limestones. In Tennessee and Virginia, where the 8

Conasauga is termed a Group, it consists largely of fine-grained carbonate beds (Walcott,

1898; Munyan, 1951; Hasson and Haase, 2013). Tectonic deformation in the Coosa River fault zone have made it difficult to develop precise stratigraphic maps, particularly in

Georgia and Alabama. Munyan (1951), however, has characterized the formation as comprising three or four lithostratigraphic zones; zone A, the lowermost interval consisting of shale; zone B, an interval that consists of shale and thin limestones; zone C, another shale interval; and zone D, the uppermost interval consisting of limestone and dolomite. Due to the tectonic deformation of the region, Munyan (1951) did acknowledge that zone A may be a repetition of zone C.

In Georgia, where most of the concretions studied here were found, the

Conasauga is best known from the Dalton quadrangle (Fig. 1). Here, the formation is exposed primarily adjacent to the Conasauga River. Most exposures are sub-parallel, trending north and slightly east, and tend to occur in narrow bands constrained in width and size by faults and topography. When fresh, Conasauga shale is typically yellowish to olive-green in color, but can appear more brown to red when weathered. It contains little silt (Munyan, 1951).

In the Dalton quadrangle and neighboring areas of Georgia and Alabama, the

Rome Formation (Cambrian) underlies the Conasauga and is composed of limestone, sandstone, and shale. Some beds of the Rome exhibit striking bands of bright colors such as red, green, yellow, purple, and white, which can be used as identifying markers of the formation and permits differentiation from the Conasauga. The Knox Group (Cambrian-

Ordovician) overlies the Conasauga and is composed of gray to white dolostone of 9 variable microfacies, oolitic chert, and thin shale or clay interbeds. It is mainly identified by exposures of chert, as fresh outcrops of the Knox are rare in the Dalton quadrangle

(Munyan, 1951).

During the latter part of the Paleozoic, the Conasauga Formation in the Coosa

River Valley region was deformed, producing numerous thrust and transform faults.

Attempts at mapping these faults have been made with varying results (Munyan, 1951;

Woodward et al., 1988; Schwimmer and Montante, 2012). In Tennessee, the Conasauga

Group experienced less substantial deformation, allowing the stratigraphy to be studied in detail (Woodward et al., 1988). However, these units represent an area more proximal to a carbonate platform rather than the open-shelf environment of Georgia and Alabama, and the two lithofacies vary considerably in both lithologic composition and stratigraphic thickness.

The age of the Conasauga Formation has been constrained by open-shelf trilobite assemblages (Schwimmer, 1989) indicative of the Bolaspidella Zone (Drumian to

Guzhangian Stages; Schwimmer, 1989; Babcock et al., 2007), the Cedaria Zone

(Guzhangian Stage; Peng et al., 2009), and the Glyptagnostus reticulatus Zone (Paibian

Stage; Peng et al., 2004). Concretion-bearing layers range from Drumian to Guzhangian.

In addition to both agnostiod and polymerid trilobites, the Conasauga Formation hosts a rich assemblage of body and trace fossils. These include hyoliths, salterellids, chancelloriids, brachiopods, graptolites, mollusks, echinoderms (ossicles), dinocaridids

(limb fragments), algae, sponges, an undetermined frondose fossil, coprolites, and cololites (Walcott, 1886, 1896, 1898, 1916; Resser, 1935, 1937, 1938; Ciampaglio et al., 10

2005, 2006; Babcock and Ciampaglio, 2007; Schwimmer and Montante, 2007, 2012).

These fossils, including the trilobites, are broadly comparable to those described from the

Great Basin (e.g., Robison, 1964; Robison and Babcock, 2011; Robison et al., 2015).

Body fossils found on concretion surfaces often retain their three-dimensional shapes, whereas those found in the shale tend to be flattened and compressed.

Furthermore, fossils in the shale commonly are surrounded by halos of goethite

(secondary after pyrite) and, in the case of trilobites, presumed carcasses retaining librigena are particularly likely to exhibit goethite halos (Schwimmer and Montante,

2007, 2012). Because concretions form in association with the decomposition of organic matter, and iron sulfide is a byproduct of decomposition, it is possible these sites were locations where decomposition and incipient concretion formation were occurring, but the process was halted before completion. In the Conasauga Formation, concretions are usually found weathering loose from shale, but when discovered in situ, they are draped by overlying fissile beds. This implies very early diagenesis and lithification of the concretions, at a time prior to mud compaction (Schwimmer and Montante, 2007).

Paleontological studies of the Conasauga Formation date at least to the time of C.

D. Walcott during the late 1800s. Walcott studied hundreds of concretions, many of which, termed “star cobbles,” are flattened and have a radial appearance (Walcott, 1896,

1898). Although Walcott’s (1896, 1898) eventual interpretation of the star cobbles as fossilized medusae was much later deemed incorrect (Ciampaglio et al., 2005, 2006), his contributions to the Conasauga concretions remain substantially unmodified.

Furthermore, his research on the fossils within the host rock of the Conasauga contributed 11 greatly to our understanding of Cambrian biotas (Walcott, 1886, 1916). C. E. Resser continued studies on Conasauga fossils and described material in a number of subsequent publications (Resser, 1935, 1937, 1938). In more recent times, Conasauga fossils and concretions have been discussed by a number of workers (e.g., Schwimmer, 1989;

Ciampaglio et al., 2005, 2006; Babcock and Ciampaglio, 2007; Schwimmer and

Montante, 2007, 2012).

Sponge Morphology

The Porifera, or sponges, are simple, multicellular organisms that show a cellular grade of organization (e.g., DeLaubenfels, 1955; Rigby, 1987). They are predominantly marine animals that normally live attached to or anchored in the substrate and feed by filtration of seawater. Their bodies consist of an internal body wall, an internal cavity

(spongocoel), and an external body covering (pinacoderm). Water, carrying organic nutrients, enters the sponge through pores (ostia) along the surface of the pinacoderm.

The water is then carried through incurrent canals until it reaches specialized cells that absorb the organic particles, then continues through excurrent canals into the spongocoel.

From here, the nutrient-depleted water then exits through a large opening called an osculum. Three sponge body forms, all variations on this basic structure, are known

(Rigby, 1987). They are (1) ascon, a simple form where the spongocoel is lined with a layer of flagellate choanocytes (flagellate-collared cells); (2) sycon, a moderately complex body form with separate flagellate chambers that open directly into the

12 spongocoel; and (3) leucon, a complex body form with flagellate chambers connected to incurrent and excurrent canals.

The skeleton of a sponge is composed of small elements that either attach together to form a rigid framework or remain loose within the sponge (e.g., Rigby, 1987). These elements, which are termed spicules or sclerites (plates), can be composed of either opaline silica or calcium carbonate, and range in size from 0.01 to 0.1 mm if they are microsclerites, and 0.1 to 1 mm or more if they are megasclerites. They are classified based on the number of axes or rays they possess and, if none are present, on their general shape (Rigby, 1987; Boury-Esnault and Rützler, 1997).

Various groups of Porifera are known, some of which are commonly regarded as classes (e.g., Rigby, 1987). These include Demospongea, Hexactinellida, and Calcarea, which are all represented in the fossil record. Archaeocyatha, a group of sponges found in

Cambrian strata, may represent an additional class. Apart from this group, placement in a sponge class is based on the shape and mineralogical composition of skeletal elements.

Sponges of the class Demospongea, which date to at least the earliest Cambrian, possess a skeleton composed of either spongin (collogen microfibrils), spicules composed of opaline silica, or both. Paleozoic forms, such as Choia and close relatives (which were common in the Cambrian), are presumed demosponges that possessed siliceous spicules.

Demosponges are the most diverse group of Holocene sponges, but they have a relatively poor fossil record due to the tendency of spongin to decompose quickly after death

(DeLaubenfels, 1955, Rigby, 1987). Demosponge spicules range in variable shapes and sizes, but are usually found as straight monaxons or four-rayed tetraxons, although any 13 rayed demosponge spicule possesses angles of either 60˚ or 120˚, which easily differentiates it from spicules of other groups (Boury-Esnault and Rützler, 1997).

Sponges of the class Hexactinellida, also known as Hyalospongea, have a stratigraphic record extending from the Ediacaran Period (Brasier and Green, 1997;

Müller et al., 2007). They are commonly called glass sponges because their spicules are composed entirely of opaline silica. Often, especially in the post-Cambrian, the siliceous spicules were able to attach together to form a rigid skeletal framework for the sponge, allowing whole skeletons to be preserved as fossils (DeLaubenfels, 1955, Rigby, 1987).

Hexactinellid spicules usually have triaxon shapes, which often possesses six rays

(always with three axes) at 90˚ angles. Sometimes, one axis is lost during the course of evolution, producing a four-rayed spicule in which each ray is 90˚ from each other and within a single plane (Boury-Esnault and Rützler, 1997).

Sponges of the class Calcarea, also known as Calcispongea, date to at least the earliest Cambrian (Rigby, 1986, 1987). They are composed of calcium carbonate (in the form of calcite or aragonite), and their skeletal elements vary considerably. Their spicules include straight monaxons, three-rayed triactin, and many-rayed tetraxon (Boury-Esnault and Rützler, 1997).

Material and Methods

Study Material

A total of 120 concretions from the Consasauga Formation, 117 from Georgia and three from Alabama, were examined in this study. Specimens were collected from the localities listed below. For comparative purposes, twenty concretions of various ages and from other localities (15 siliceous, 3 siderite, 1 calcite, 1 barite), eight Holocene poriferans, and six fossil poriferans were also examined. Samples are in collections at

The Ohio State University, Columbus, Ohio, USA.

Specimens were studied using standard techniques, including light microscopy, and their interiors were scanned using X-ray computed tomography (XCT). Specimens were photographed macroscopically using a Canon Rebel Xsi digital camera.

Localities

1. Outcrops along Melson Road and adjoining roads, south of the Coosa River,

Georgia.

a. 34°09’921”N / 85°23’333”W

b. 34°09’063”N / 85°23’480”W

c. 34°09’076”N / 85°23’292”W

d. 34°09’079”N/ 85°22’979”W

e. 34°08’294”N / 85°23’264”W

2. Outcrops along Blacks Bluff Road, Foster’s Mill section, Georgia; 34°09’921”N /

85°19’994”W.

3. Road cut and field along McGee Bend Road., near Spann’s Farm, Georgia;

34°11’106”N / 85°23’338”W.

4. Yancey’s Bend to Centre, Cherokee, Alabama; currently located under Weiss

Lake.

XCT-Scanner

X-ray computed tomography (XCT) scanning for this paper was performed on a portable Neurologica CereTom XCT-scanner at The Ohio State University School of

Earth Sciences. The CereTom emits numerous x-rays at the specimen, which are then processed by eight x-ray detectors. This allows the CereTom’s software to compile image slices into two-dimensional models that can be digitally dissected anteriorly, dorsally, and laterally in order to study possible internal structures. These slices are compiled into three-dimensional models that can be rotated and dissected at any angle in order to further understand the shapes of internal structures.

Scanning was performed at a resolution of 0.5 x 0.5 x 0.625 mm in order to provide the best possible resolution for study. Images were examined by using the

CereTom’s software and the computer program ImageJ.

Fig. 1. The Coosa River Valley, northwest Georgia, and adjacent part of eastern

Alabama, USA showing localities of siliceous concretions described herein (from

Ciampalgio et al., 2006, and modified from Allen and Lester, 1954).

Research

Conasauga Concretions

Concretions from the Conasauga Formation range in size, shape, and texture, but all exhibit similar coloration, from beige to a light brown. Commonly, concretions display one shade of color on their surfaces, but broken concretions show a dark to light color transition extending from the surface to the interior. Concretion size, which is on average 5.5 cm along the long axis, varies greatly and can range from 2.2 cm to 15 cm long. Shape varies from nearly round to elongate to amorphous, the latter usually due to knobby protrusions or branch-like extensions on an otherwise round shape. Some concretions are nearly spherical or egg-shaped, whereas others exhibit varying degrees of compression resulting in a slightly compressed to flattened shape. Texture also varies, often due to the presence or absence of external fossils or dissolution cavities. Many concretions are extremely rough whereas others, which are devoid of obvious macrofossils or cavities, are almost perfectly smooth (Fig. 2). Texture generally falls somewhere between the two extremes, and concretions that are beige in color tend to be more fossiliferous, and therefore rougher, than those that exhibit the darker brown coloration. Two of the 120 studied Conasauga concretions also exhibit twinning, where the concretion appears to have originally been two separate specimens fused into one at

18 an unknown point during diagenesis (Fig. 3). Externally, twinning could be identified in concretions that differed in size, shape, texture, and/or color, and internally from the change of internal structures or the presence of a higher density boundary line.

Porosity- Almost every concretion studied with the XCT-scanner shows evidence of porosity in the interior. Pore spaces usually exhibit much lower density than the surrounding silica, though sometimes these regions have been infilled by a higher density mineral. Both situations are easily identified in the two-dimensional scans as well as the three-dimensional models. Porosity can be divided into three types: (1) thin, meandering tunnels spread out within the whole of the concretion; (2) large, straight tunnels that cut through the concretion; and (3) a dense cluster of thin tunnels in a central location.

Individual concretions can exhibit more than one type of porosity and, of these, the most common is type 1 (observed in 114 concretions).

The sinuous tunnels of type 1 porosity are generally small and numerous, reaching scores to hundreds in some specimens. Tunnels sometimes reach the edge of the concretion, but in other examples disappear far before the outer boundary is breached

(Fig. 4A). Type 2 porosity is far less common than type 1, and occurs in only a total of 13 concretions. These pores resemble tunnels that, almost without exception, extend through the whole of the concretion along a straight and linear path. They are distinct from type 1 in that they are far less numerous (a concretion will usually have only a single example amid dozens or scores of type 1) straight or nearly straight in orientation, and are much wider at 3 to 4 times the width (Fig. 4B).

Type 3 porosity is interesting, although it is also uncommon. These pores resemble thin tunnels, like the first type, except they never continue to the edge of the concretion and weave through one another in a dense cluster at a particular location within the concretion (Fig. 4C). Of the 23 specimens that exhibit this, 5 show the centralized porosity within a region of lower density than the surrounding silica. This does not occur with any other porosity type and, as this is characteristic of fossilization in concretions, implies that the porosity is a result of the preservation of an already porous nucleus.

Hyoliths and their impressions have been identified on the surfaces of many concretions. It was considered likely, therefore, that hyoliths would be found within at least a few concretions. Identifying these animals could not easily be done with the 2-D slices, as their bodies left behind cavities or cones of lower density minerals that resembled either type 1 or type 3 porosity. By studying the 3-D models, however, hyoliths became apparent due to their particular, easily identifiable conical shape. In some examples that appeared to be type 3 porosity, the 3-D model showed that the pore spaces were nothing more than a dense collection of hyolith conchs. Once this was realized, all porous concretions were checked for a possible hyolith explanation. As of yet, no opercula or helens of hyoliths have been identified in any concretion.

Density Layers- Another internal structure of note is the existence of density layering starting from a concretion’s center. The contrast between the layers of differing density is apparent in at least 23 concretions, and the layers are almost always centralized. Two or three layers are common, but the total number may reach six. As with 20 porosity, more than one type of density layering is apparent. The first type is in the form of spherical layers, often similar in shape to the concretion itself. These layers, of which there can be many, usually surround a core of low density such as a low density mass or concentrated porosity (Fig. 5A, B). In the case of one specimen (Appendix A, specimen

2), they enclose a structure of high density which, given the odd shape of the object, is possibly a trace fossil and the probable nucleus of the concretion (Fig. 5C). Another type of density layering resembles intersecting oval-shaped structures that may have once been spherical before being altered. The ovals are misshapen and often stretch from one side of the concretion to the other. They are present with the first and second types of porosity, with one or more of the tunnels following within the low density formed from the ovals (Fig. 6).

Other structures of low density are apparent in seven concretions. These are areas of either low density minerals or void spaces and are round and solid to patchy and amorphous in shape (Fig. 7A, B). In the case of two specimens (Appendix A, specimens

13, 22), the low density areas surround locations of higher density, making them appear to ‘float’ when viewed in 3-D. In most cases, however, the areas contain no structures other than errant porosity. Most are in three-dimensions, allowing for easy study with 3-

D models.

Finally, three concretions exhibit exceptionally slight density transitions. Two specimens show apparent layers emanating from the concretion center and one shows a slight density change around a type 2 tunnel. Furthermore, in nine concretions, at least

21 one example of type 1 or type 2 porosity is surrounded by a higher or lower density sheath. This occurs even when no other pore in the concretion exhibits this (Fig. 7C).

Sponge Spicules- Among other small fossils, four-rayed (stauract) sponge spicules of hexactinellid type, about 3 to 5 mm long, have been identified on the surfaces of concretions (Fig. 8A). Stauract spicules also have been imaged in the interiors of seven concretions. Of these, four exhibit spicules 1.5 to 2 mm in length, and three have spicules about 5 mm in length (Fig. 8B, C, D). One specimen (Appendix A , specimen 3), which contains multiple examples of the larger spicule size, also exhibits two exceptionally large spicules, the largest being 15 mm in length (Fig. 8E).

Other Concretions

Concretions from other localities and ages were scanned for comparison with the

Conasauga Formation concretions. They include seven siliceous concretions from the

Sicasica Formation (Devonian) of the Bolivian Altiplano, seven siliceous concretions from the Valtorres Formation (Cambrian) of Spain, one siliceous concretion from the St.

Louis Limestone (Carboniferous) of Illinois, three siderite concretions from Mazon

Creek-type deposits (Francis Creek Shale, Carboniferous of Braidwood, Illinois, and

Durkee’s Ferry, Indiana), an orsten-type calcite concretion from the Huaqiao Formation of Hunan, China, and a barite concretion of unknown provenance collected as a glacial erratic at Columbus, Ohio. These comparisons were intended to determine if any internal structures are a result of concretion genesis, and can therefore be found in concretions of

22 diverse mineralogy and origin. The internal structures of particular interest are porosity and density layering.

Sicasica Formation concretions- Studied concretions from the Sicasica Formation are composed of silica, presumably biogenic in origin, are a uniform gray color, texturally smooth, and 3.5 to 4 cm long. They show no macroscopic fossils on their exteriors but do yield large fossils internally. Some fossils extend to the edges of, or beyond, the concretion margins. Because of this, it is often possible to identify the preserved organisms where they are exposed along the concretion exteriors. Seven of these concretions were scanned, one of which was already broken open, and six of which could be determined to have formed around polymerid trilobites.

All seven examined concretions from the Sicasica Formation showed type 1 porosity consisting of numerous, thin, meandering tunnels. Pores were either left as empty cavities or later infilled with higher density material. In four of the six unbroken concretions, fossils on the insides were outlined by a different density (either higher or lower) than the surrounding silica, and could be identified in the two-dimensional images.

Three of these, all previously known to be polymerid trilobites, showed the axial and pleural lobes of the animal extending the lengths of the concretions. The other concretion revealed a large, conical organism identified as a conulariid (Fig. 9A).

Valtorres Formation concretions- Siliceous concretions from the Valtorres

Formation (Cambrian) of the Iberian Chains, Spain, were recently described by Álvaro et al. (2013). They show considerable similarity to concretions from the Conasauga

Formation of the southeastern United States in composition, fossil content, and age 23 within the Cambrian. Seven specimens from the Valtorres Formation were studied. They are 4 to 5 cm long, have a slightly medium-dark brown to gray color, range in texture from smooth to rough, and are generally ovoid in shape. The interiors are reported to host an assemblage of fossils including agnostoid and small polymerid trilobites, hyoliths, and hexactinellid sponge spicules. These concretions evidently formed from biogenic silica, and are of inferred Guzhangian Age (Álvaro et al., 2013).

All types of porosity observed in the Conasauga Formation concretions are also present in Valtorres Formation concretions. All six specimens contain type 1 porosity, three contain type 2 porosity, and one contains type 3 porosity. All but one of the concretions exhibit a lower density center surrounded by higher density silica (Figs. 9D,

E).

St. Louis Formation concretion- A chert (siliceous) concretion from the St. Louis

Limestone (Carboniferous) of Illinois was examined. It is spherical, light gray in color, smooth, and 8.1 cm in maximum dimension. Two-dimensional images of this specimen show few small pore spaces and two long cracks not visible from the concretion’s surface. The scans reveal internal density layering near the concretion’s center, similar to what is seen in the Conasauga specimens (Fig. 9B).

Mazon Creek-type concretions- Siderite concretions from the Francis Creek Shale of Illinois and Indiana were examined. They are larger than the Conasauga specimens, 8 cm long on average, but are a similar beige to yellow color. They possess a slightly rough texture that is not due to surface fossils, and are more ovoid in shape. None of the three siderite concretions showed porosity. One has patches of lower density, but they do not 24 resemble tunnels nor do they appear to be infilled pore spaces. This concretion, already broken open, contains the holotype of Euproops colletti, a horseshoe crab. The fossil is of greater density than the siderite surrounding it, which suggests that fossils have the capacity to be detected on the insides of siderite concretions using density differences

(Fig. 9C). The other two concretions show no such density differences; one revealed numerous fractures not visible from the surface, and the other was a solid, uniform mass.

Orsten-type and barite concretions- Other concretions, one a Cambrian orsten- type concretion from the Huaqiao Formation of Hunan, China, and the other a barite concretion collected from glacial drift in Ohio, were also scanned using the XCT-scanner, but neither shows any structures on the interiors and instead appear to be solid masses.

Comparison of Concretions with Holocene and Fossil Poriferans

Eight Holocene and six fossil sponges were analyzed for structural comparison with the Conasauga Formation concretions. Sponges are largely porous animals, and hexactinellid forms in addition to other forms including demosponges (e.g., choiids) could have provided the biogenic silica involved in concretion diagenesis. The siliceous spicules of hexactinellids and choiids are composed of opal (non-crystalline SiO2), and dissolution of some siliceous spicules followed by reprecipitation as microcrystalline quartz (chert) or cryptocrystalline quartz (chalcedony) is hypothesized as a source of silica in concretions. One test of this possibility would be to identify instances in which original structures of the sponges, such as porosity consistent with canals in a poriferan and undissolved spicules, are preserved in essentially unaltered conditions. 25

Examples of hexactinellid sponges, which secrete opaline silica spicules, as well demosponges and calcisponges, were analyzed by XCT. Although spicules found associated with concretions are primarily of hexactinellid sponges, studying sponges of the classes Demospongea and Calcarea was deemed worthwhile as a means of better understanding the structure and positions of interior canals within poriferans. Choiids, which have an unusual, low, pin cushion-type morphology, were not analyzed, as three- dimensional fossils that would be amenable for study of internal structure are unknown.

Holocene Poriferans- Of the eight Holocene poriferans studied, one is from the class Hexactinellida, six are from the Demospongea, and one is from the Calcarea.

Studied demosponges span a range of general morphologies: vase-shaped, globular, arborescent, tubular, infundibuliform, and columnar. The hexactinellid sponge has an elongate-conical morphology, and the calcisponge has no defined morphology.

XCT-scans of most of the Holocene poriferan specimens revealed fine details of the structure of interior canals. With one exception, all of the demosponges showed the interior canals to be a dense collection of tunnels; interior canals were not evident from scans of the round, arborescent demosponge. Where evident, diameters of interior canals vary from <1 mm to >1 cm (Fig. 10A, B). Directions of the interior canals were difficult to determine from 2-D images alone, but for the most part seem to be directed toward the center of the sponge (Fig. 10B). The spongocoel is well-pronounced in the 2-D images and resemble large, interior cavities oriented top to bottom through the sponges’ interiors.

In the vase-shaped and infundibuliform specimens, the spongocoel appears to occupy

26 nearly the same volume as the sponge tissue. In the other demosponges, the spongocoel is much thinner.

The hexactinellid specimen is of ascon-type construction, and consists of a thin, mesh-like skeleton surrounding a central cavity. The interior canals are hardly discernable for most of the length of the skeleton except near its base, where it thickens substantially (Fig. 10C).

The calcareous sponge contains large, prominent interior canals, about 2.5 mm in diameter, which resemble the canals of the demosponges in all characteristics except size.

Its spongocoel is slightly larger than that of the surrounding canals and identifiable as a spongocoel only in top-bottom orientation (Fig. 10D).

Fossil Poriferans- Fossil poriferans chosen for XCT-scanning are two Hydnoceras

(Hexactinellida, Devonian of New York), two Astylospongia (Demospongea, Silurian of

Indiana), and two Astraeospongia (Calcarea, Silurian of Tennessee).

The Hydnoceras specimens are steinkerns of compressed-coniform skeletons with large annular swellings. XCT-scans reveal them to be semi-solid masses. They contain a few small pores, but they are unlike the tunnel-like porosity in the Conasauga concretions

(Fig. 11A).

The Astylospongia fossils are globular in shape, and ostia are identifiable from their external surfaces. XCT-scans reveal the positions of myriad interior canals that were infilled with a higher density material than what replaced the body fossils (Fig. 11B). The canals tend to be oriented toward the interior of the sponge and, in most images, appear as radiating lines originating from the center. This structure resembles the canals in the 27 vase-shape, globular, and especially the elongate-conical Holocene demosponges that were scanned.

The Astraeospongia fossils resemble low bowls or saucers and have distinctive flower-like spicules visible externally. Scans of these specimens reveal porosity nearly identical to the type 3 porosity of the Conasauga concretions. These pores, indicative of interior canals, are all small, dense, and tunnel-like (Fig. 11C). In one specimen, most porosity is evident just below the surface of the concave bowl, whereas in the other specimen, the fossil is porous is most areas.

Fig. 2. Surface textures of siliceous concretions from the Conasauga Formation

(Cambrian), Coosa River Valley, Georgia. A, Smooth texture, lacking macrofossils and dissolution cavities; specimen 2, 45 mm wide. B, Rough texture due to fossils and dissolution cavities; specimen 6, 49 mm wide.

Fig. 3. Twinned siliceous concretions from the Conasauga Formation (Cambrian), Coosa

River Valley, Georgia. A, External view, specimen 37, 110 mm long. B, Two- dimensional XCT-slice of specimen 78, 83 mm wide, showing internal differences between halves. C, Two-dimensional XCT-slice of specimen 37 showing internal differences between halves.

Fig. 4. Porosity types identified using XCT-scanning in concretions from the Conasauga

Formation (Cambrian), Coosa River Valley, Georgia. A, Type 1 porosity, numerous tunnels and holes; specimen 86, 26 mm wide. B, Type 2 porosity, a single long, straight tunnel, specimen 7, 27 mm wide in the image. C, Inverted three-dimensional model of specimen 7 (see Fig. 4B) showing a portion of the type 2 pore. D, Type 3 porosity, a tight cluster of voids; specimen 114, 33 mm wide.

Fig. 5. Examples of multiple diagenetic generations imaged using XCT-scanning in siliceous concretions from the Conasauga Formation (Cambrian), Coosa River Valley,

Georgia. A, Faint concentric bands and multiple type 1 pores; specimen 11, 57 mm wide in the image. B, Multiple generational bands; specimen 31, 99 mm wide in the image. C,

Halved three-dimensional model of specimen 2, 45 mm in diameter, showing density differences between bands surrounding a probable trace fossil.

Fig. 6. Low density, oval-shaped structures stretching across a siliceous concretion from the Conasauga Formation (Cambrian), Coosa River Valley, Georgia, imaged using XCT; specimen 62, 56 mm wide. A-C, Two-dimensional slices from various positions in the concretion. Black voids are oblique slices of a type 2 tunnel.

Fig. 7. Types of internal density differences identified using XCT in siliceous concretions from the Conasauga Formation (Cambrian), Coosa River Valley, Georgia. A, Solid, low density mass 25 mm long, specimen 85. B, Irregular, low density mass 36.5 mm across containing patches of higher density; specimen 18. C, Type 1 pore surrounded by higher density area, indicated by arrow; specimen 100, concretion is 31.55 mm wide in the image.

Fig. 8. Stauract spicules in siliceous concretions from the Conasauga Formation

(Cambrian), Coosa River Valley, Georgia, identified on concretion surface (A) and inside concretions using XCT-scanning (B-E). A, External surface of concretion showing spicules 3 to 5 mm long (two spicules circled); specimen 59. B, XCT-slice of specimen 6 showing spicules about 2 mm long (one is circled) within type 3 porosity. C, XCT-slice of specimen 37 showing numerous spicules about 5 mm long. D, E, XCT-slices of specimen 3 showing spicules 5 mm long (D), and about 15 mm long (E).

Fig. 9. Internal structures of concretions from varied localities imaged using XCT- scanning. A, XCT-scan of siliceous concretions from the Sicasica Formation (Devonian),

Altiplano, Bolivia, showing density differences around a conulariid, Paraconularia ulrichi, Sic-5, 30 mm long (left), and a trilobite, Phacops oruruensis, Sic-6, 30 mm long

(right). B, XCT-slice of chert concretion from the St. Louis Limestone (Carboniferous)

36 showing multi-generational concentric bands, 79 mm wide in the image. C, Three- dimensional model of siderite concretion from the Francis Creek Shale (Carboniferous),

Durkee’s Ferry, Indiana, preserving a horseshoe crab in high density minerals distinct from the surrounding concretion; Maz-1, 65 mm wide. D, E, Siliceous concretions from the Valtorres Formation (Cambrian), Spain, showing a centralized mass of low density

(D), Val-6, 41.5 mm wide; and showing type 3 porosity (E), Val-7, 52 mm wide.

Fig. 10. XCT-slices of Holocene poriferans. A, Demosponge, globular shaped leucon,

Hol-2, 99 mm long in the image. B, Demosponge, tubular shaped sycon, Hol-4, 108 mm long in the image. C, Hexactinellid, elongate-conical ascon, Hol-7, 253 mm long in the image. D, Calcisponge, leucon with an undefined shape, Hol-8, 70 mm long in the image. 38

Fig 11. XCT-slices of fossil poriferans. A, Hydnoceras bathense, “Chemung Group”

(Devonian), southwestern New York, Hydno-1, preserved as a steinkern in siltstone and appearing as a solid mass in XCT-scan, 98.5 mm long in the image. B, Astylospongia praemorsa, a leucon-grade sponge from the Waldron Shale (Silurian), Indiana, Astyl-1,

31.5 mm long in the image, preserving interior canals as high density lines trending toward center. C, Astraeospongia meniscus, a leucon-grade sponge from the Beech River

Formation (Silurian), Decatur, County, Tennessee, Astra-2, 59 mm wide in the image, with canals preserved as void spaces of type 3 porosity.

Discussion

Density Differences in Conasauga Formation Concretions

Observing abrupt and transitional density differences using the XCT-scanner is essential to understanding the preserved internal structures within concretions. During concretion development, preserved elements may dissolve to leave casts, remineralize to a material with a different density, or retain the cavities present in life as voids. In many cases, the exterior of the fossil is preserved as a thin outline composed of higher or lower density minerals, preserving the overall shape of the fossil. Such density differences may have been established early, perhaps at the time of initial mineral precipitation in the biofilm.

Examples of density differences can be seen in concretions from strata other than the Conasauga Formation. Fossils preserved in the Mazon Creek-type siderite concretions exhibit exceptionally high density as compared to the surrounding siderite. In one split concretion, a compressed horseshoe crab fossil shows high density minerals that are not represented elsewhere in the concretion (Fig. 9C). Likewise, siliceous concretions from the Sicasica Formation of Bolivia reveal outlines of identifiable fossils inside (Fig. 9A).

In the Conasauga Formation concretions, density differences indicate the presence of fossils, secondary mineralization perhaps due to burrowing behavior through slushy

40 concretions during development, the boundaries between ‘twinned’ (combined) concretions, or boundaries between successive generations of concretion development.

Trilobites

The Conasauga Formation yields a rich assemblage of agnostoid and polymerid trilobites (Walcott, 1886, 1916; Resser, 1935, 1937, 1938; Schwimmer, 1989).

Concretion surfaces preserve numerous disarticulated fragments, including loose cephala with free cheeks attached or absent, thoraxes, and pygidia. No articulated trilobites were studied.

Trilobites are important biostratigraphic tools through much of the Cambrian, and certain taxa help place age constraints on concretionary intervals of the Conasauga

Formation. The most prolific trilobite identified from concretions is Cedaria prolifica

Walcott, 1924, a guide fossil to the Guzhangian Stage (Babcock et al., 2014; see also

Pratt, 1992; Peng et al., 2009). Some concretions preserve examples of Aphelaspis, which suggest correlation with the Paibian Stage (Babcock et al., 2011; see also Pratt, 1992;

Peng et al., 2004). Other trilobites identified are Proagnostus bulbus Butts, 1926,

Ptychagnostus sp. Jaekel, 1909, Deiracephalus aster (Walcott, 1916), and Norwoodia sp.

Walcott, 1916. A large moldic cephalon is referred to Olenoides. Together, this suite of trilobites is consistent with inferred ages of the concretionary intervals as Guzhangian to

Paibian.

Trilobite fossils are present both on concretion surfaces and in their interiors. A number of Guzhangian to Paibian polymerids were spinose, and recognition of trilobite 41 fossils in the interiors of concretions is limited primarily to disarticulated spines.

Excellent examples of spinose trilobites include Deiracephalus, Norwoodia, and

Olenoides (Robison and Babcock, 2011; Robison et al., 2015; herein Fig. 12).

Hyoliths

Hyoliths are an important component of the Conasauga Formation biota.

Numerous species have been identified (Resser, 1938), although most are in need of revision. In the study concretions, most hyoliths are identifiable from voids of the conchs, and possibilities for identification are limited. However, the general shapes of the voids show that both hyolithides and orthothecides are present. One hyolithide conch preserved on the surface of a concretion is referred to ‘Hyolithes’ partitus Resser, 1938.

Hyoliths occur within at least 24 studied concretions from the Conasauga

Formation. Another four concretions show elongate-conical features resembling hyoliths.

Hyoliths have been found in abundance on concretion surfaces and, in 19 of these concretions, are also preserved in the interiors. Individual concretions may preserve a single hyolith, to tens of hyoliths or more. The conchs range from intact to partly broken, and some have been breached by tracemakers, possibly scavengers. Hyolith helens and opercula were not identified in or on any of the concretions.

In concretion interiors, hyolith conchs are identifiable by conical voids, although many specimens are the same density as the surrounding silica and are differentiated by a higher or lower density sheath-like outline, interpreted to be the shell of the animal. In two specimens, two hyoliths are preserved partially as a solid body with an outline, and 42 partially as a void. In one specimen, the void is smaller than the outline (shell), and indicates the preservation of the hyolith’s internal cavity (Fig. 13A-C). The other specimen contains a void the same size as the outline, and indicates dissolution of the animal (Fig. 13D-F).

Conch proximity and orientation provide indications as to how the hyoliths accumulated. Hyoliths in seven studied concretions show bidirectional orientation, suggesting storm-related deposition (Fig. 14A). Four concretions exhibit clumps of more randomly oriented hyoliths numbering in the tens to hundreds, and are inferred to be coprolites (Fig. 14B). Priapulid cololites, interpreted from hyoliths positioned unidirectionally in a narrow band, have been identified in the Conasauga Formation

(compare Conway Morris, 1977; Babcock and Robison, 1988; Schwimmer and

Montante, 2007) but were not observed in the concretions studied.

Trace fossils

The depositional environment of the Conasauga Formation supported an active infauna. Most burrowers apart from arthropods are not known from body fossils, but their traces were preserved both on the surfaces and in the interiors of concretions. General occurrences of traces that have been identified from studied concretions include (1) surficial traces; (2) traces evidently forming the nuclei of concretions; and (3) traces that breached concretionary masses after they had begun to mineralize.

Surficial traces have been identified in 13 concretions. Of these, seven represent traces in sediment that were replicated in concretions. Traces preserved in this manner 43 include Planolites, Chondrites, and Rusophycus (Fig. 15A, B). One Rusophycus is preserved in intimate association with a Planolites trace, and represents a hunting trace

(Fig. 15C; compare Jensen, 1990; Brandt et al., 1995; Babcock, 2003; English and

Babcock, 2007).

Traces that appear to have served as nuclei for concretions preserve internal details that are lacking in the surficial traces. The most common traces of this type are probable coprolites (11 concretions), identifiable using XCT to observe dense assemblages of hyoliths, trilobite spines, and other skeletal debris in irregular clumps varying in shape and size, or identifiable from low density patches interpreted to be originally organic material. One concretion is broken, revealing fragmentary trilobite sclerites in its interior (Fig. 15C). A coprolite origin is inferred, but this was one concretion not determined to be a coprolite from XCT-scanning. This suggests that evidence of coprolites may not always be ascertained by the XCT-scanner.

Other traces as the nuclei of concretions include round burrows and a trace network (Chondrites?). The burrows have round cross sections, and scans reveal preservation of layered density in type 3 porosity (Fig. 16A). This is also observed in sections of the trace network (Fig. 16B), which may indicate a similar origin. The trace network varies in tunnel diameter from 3.6 mm to 16.5 mm and is branched and labyrinthine in shape (Fig. 16C, D). Due to the flattened morphology of the concretion it occurs in, it likely formed horizontal to bedding. All other concretions with traces at their nuclei are interpreted to be small sections of burrowing tunnels.

Traces that breached developing concretions have either type 1 or type 2 porosity.

Type 1 porosity, which resembles thin, meandering tunnels (Fig. 4A), may be largely the result of traces left by deposit-feeders or infaunal scavengers. Type 1 porosity was identified in 114 studied concretions, suggesting a well-oxygenated environment that supported abundant life near the sediment-water interface. Type 2 porosity, which consists of large, straight tunnels (Fig. 4B) occurs in 13 studied concretions. In flattened or compressed specimens where bedding lines can be interpreted, the traces often intersect the concretion either parallel or perpendicular to bedding.

Both type 1 and type 2 traces evidently formed during the time of concretion development but before lithification. The tracemakers likely sought organic material in the incipient concretions, as most traces seem to penetrate structures identified as organic remains. One example of a concretion showing type 2 porosity includes a trace that penetrates the length of the concretion directly through the preserved organics (Fig. 6,

17). The edges of the trace have become mineralized even beyond the concretion margin, resulting in bumps on either side of the concretion. This shows that concretions can continue to grow until full lithification has taken place.

In some cases, density structures appear to have formed as a direct result of secondary traces. Low density, intersecting ovoid structures were identified in four concretions; they extend from one side of the concretion to the other (Fig. 6). In each case, a type 1 or type 2 trace has been identified entering and continuing through part of the ovoid structure. As none of these structures have been found without secondary

45 traces, it is interpreted that the traces may have formed the ovoid structures or deformed preexisting structures.

Nine concretions exhibit rare type 1 or type 2 traces that are encased in higher density minerals, resembling sheaths (Fig. 7C). The mineral sheaths likely formed as the result of early diagenesis of organic material, such as fecal matter or mucous, lining the walls of the traces (compare Ahn and Babcock, 2012).

Poriferans in the Conasauga Formation

Walcott (1898) noted the presence of four-rayed (stauract) hexactinellid spicules, composed of silica and identified as Protospongia, on the surfaces of concretions and in thin sections. He (Walcott, 1898) attributed the silica forming the concretions to siliceous sediments and siliceous skeletal elements including those of sponges. Later work also showed the presence of sponges belonging to the classes Demospongea and Calcarea in the Conasauga Formation (Schwimmer and Montante, 2007). Among the demosponges present is Choia, which has siliceous spicules. Brooksella, a star-shaped concretion that

Walcott (1896, 1898) originally classified as a fossil jellyfish, has been, after further work, reinterpreted as an articulated hexactinellid sponge (Ciampaglio et al., 2005, 2006).

In this study, dense clusters of tunnels termed type 3 porosity were observed in two-dimensional XCT-scans of some Conasauga concretions (Fig. 4C). Most of these are interpreted as interior canals of articulated poriferans. XCT-scans of Silurian

Astraeospongia show porosity clearly attributable to interior canals that is essentially

46 indistinguishable from the type 3 porosity of the Conasauga concretions; they compare favorably in size, shape, and distribution (Fig. 11C).

Sponge spicules have been identified both surficially and in two-dimensional scans of concretions. The spicules are four-rayed stauracts ranging from 1.5 mm to 15 mm long (Fig. 8). A stauract shape indicates a hexactinellid origin for such spicules.

Other spicule shapes were not identified in this study.

Differences in spicule size and differences in concretion shapes open the possibility that more than one type of siliceous-spicule-forming sponge contributed biogenic silica involved in the development of the Conasauga Formation concretions.

Two specimens contain numerous large spicules 5 mm or so in length. One specimen, which shows type 3 porosity, has spicules 15 mm long near the boundaries of the interior canals (Fig. 8E). These are interpreted as basal spicules, and indicate that the concretion was scanned upside-down compared to life position of the sponge. One specimen is a twinned concretion with spicules in both halves, and the lack of interior canals (Fig. 3C) is interpreted to be the result of skeletal collapse before diagenesis of the concretion began.

Several concretion specimens seem to be made largely of spicules, 1.5 to 2 mm in maximum length. Two of them preserve canals in the interior that are observed within low density regions and appear to indicate the skeletal shapes of sponges (Fig. 8B). The density contrast in one specimen is sufficient that the inferred sponge skeleton could be separated from the surrounding concretion using ImageJ and compiled into a three- dimensional model (Fig. 18A, B). This visualization procedure reveals the skeletal shape 47 to be bowl-shaped with a protrusion extending from one side. Another concretion that preserves a sponge is shaped in a similar manner, suggesting that the concretion’s shape mimics the shape of the sponge skeleton inside (Fig. 18C).

XCT-scans of Holocene poriferans reveal the relationship between skeletal morphology of a sponge and the distribution and orientation of canals in the interior.

Canals in the interior are remarkably similar to the type 3 porosity observed in Conasauga

Formation concretions. Concretions showing intertwining canals are consistent with the leucon structural grade body form (compare Rigby, 1987). More simple, ascon- and sycon-grade, skeletons have not been observed.

Previous work showed that star cobbles from the Conasauga Formation, commonly identified as Brooksella alternata Walcott, 1896 (or a junior synonym), have an origin as hexactinellid sponges (Ciampaglio et al., 2005, 2006), but the present work hints at a more diverse fauna of siliceous sponges. Concretions identified as Brooksella often retain shapes resembling sponge skeletons, show ostia, oscula, and internal canals, and show siliceous spicules both on the surfaces and in the interiors (Ciampaglio et al.,

2005, 2006). Most concretions studied in the present work show much less complex shapes than those usually referred to as Brooksella, yet they show structures internally that are similar in most details. Indications are that the depositional environment of the

Conasauga Formation hosted a rather diverse assemblage of siliceous sponges, including hexactinellids and probably demosponges. That inferred diversity is understated by the name Brooksella alternata. Walcott (1896) may have recognized some of this apparent diversity in star cobbles by erecting the ‘species’ B. alternata, Brooksella confusia, and 48

Laotira cambria. However, illustrated cobbles (concretions) show a spectrum of shapes

(Walcott, 1896, 1898) and the ‘species’ may simply represent morphological or taphonomic end members. At least two general poriferan skeletal structures seem to be represented in the Conasauga Formation concretions, and neither resemble Brooksella. It is unlikely that any of the names erected by Walcott reflects biological species, rather they represent concretion morphologies whose external shapes reflect a combination of biological variables and taphonomic factors.

Isolated stauract spicules in Cambrian strata are commonly referred to

Protospongia (e.g., Walcott, 1898; Rigby, 1966, 1978; Ciampaglio et al., 2006; Robison et al., 2015). The stauracts associated with concretions in the Conasauga Formation, including Brooksella cobbles, are of Protospongia type. Rigby (1978) noted that

Protospongia is commonly used as a descriptor of spicule shape, and that Protospongia- type spicules were likely secreted by species assigned to more than one hexactinellid genus based on general skeletal shape. Robison et al. (2015) characterized Protospongia as a “wastebasket” taxonomic name for disarticulated, siliceous stauract spicules, but also noted that Brooksella and Diagonella have similar spicules.

Generations of Concretion Development

Concentric layering, or parallel banding, has long been documented in concretions

(e.g., Abbott, 1907; Buchardt et al., 1997; Hellstrom and Babcock, 2000; Calner et al.,

2013). Abbott (1907) attributed parallel banding to different generations of mineral precipitation around a nucleus. Others have shown that some concentric rings on the 49 surfaces of concretions are related to other factors such as sedimentary layers or successive stages of concretion dissolution (Buchardt et al., 1997). Borkow and Babcock

(2003) provided cogent evidence that concretions form through autolithification of microbial biofilms, but this explanation does not necessarily apply entirely to large, or what appear to be multi-generational, concretions (see Wilson and Brett, 2013; compare also Berner, 1968; Canfield and Raiswell, 1991). In such examples, initial concretion development may have been mediated by a biofilm, but concretion growth must have continued beyond this initial phase of development.

XCT-scans of some concretions from the Conasauga Formation show concentric

‘layering,’ suggesting punctuated episodes of growth, with each band or ‘layer’ representing an individual diagenetic generation. Among studied concretions, five show distinct concentric bands related to successive generations of concretion growth. The number of bands varies from two to six. Boundaries between bands are of either higher or lower density (Fig. 19).

Trace fossil evidence places constraints on the timing of development of successive generations of concretions. In two studied concretions, meandering traces penetrate each of the bands (Fig. 5A), which indicates that none of them had been fully lithified when the traces were constructed. Pye et al. (2013) showed that concretions can become fully lithified in as little as one to six months, so the multiple generations illustrated in Conasauga Formation concretions may all have formed within a matter of months. A contrasting scenario is illustrated by one concretion that shows two growth bands (Fig. 20). In this example, meandering traces can be observed extending through 50 the outer band or core band toward the generational boundary, but do not penetrate it.

This suggests that the core band formed first and was at least partially lithified prior to lithification of the subsequent band.

Twinned Concretions

Most Conasauga Formation concretions are single entities, but two studied examples appear to have formed from the fusion of two separate concretions each. Fusion probably occurred at an early stage of concretion development, and prior to full lithification. Twinned concretions can show different shapes, colors, and textures between the two halves. They also show distinct density-related boundaries between the halves (Fig. 3A-C). In one concretion, one half shows a dense assortment of fossil fragments, and the other shows few fossils (Fig. 3B).

One hypothesis for the origin of twinned concretions is that adjacent biofilms were close enough that the concretions they spawned merged near their edges. Rigby

(1987) stated that sponges are commonly gregarious, and it is likely that such gregarious organisms lived in close enough association for this to happen.

Additional Organic Material Preserved in Concretions

General- Some concretions from the Conasauga Formation are not readily identifiable as having formed around sponge skeletons. It is possible that some resulted from microbially mediated early diagenesis of silica whose origin was largely with siliceous sponges. In some cases, breakdown of sponge remains may have led to

51 remobilization of biogenic silica. Biofilms associated with the decay of varied bodily remains, fecal matter, or associated with burrowing traces, may have nucleated some of the concretions. This is likely, especially in examples where the fossils are preserved inside the concretions. Fossils preserved on the exterior surfaces of concretions could have simply been trapped in biofilms as the concretions were developing in sediment, or perhaps, in some examples, they represent skeletons around which biofilms had formed, leading to concretion nucleation.

Decomposed Organic Material- Seven studied concretions from the Conasauga

Formation exhibit centralized regions usually of low density. These regions are interpreted to be the result of breakdown of non-biomineralized organic material that nucleated the concretions. Traces are common in such concretions, suggesting scavenging activity. Where few traces are present, the organic material tends to be preserved as a solid mass (Fig. 7A), but where many traces are present, organics are preserved inconsistently (Fig. 7B).

Dissolved Skeletal Remains - The surficial texture of Conasauga Formation concretions is influenced primarily by the presence or absence of fossils, and exceptionally rough-textured surfaces show molds formed by dissolution of skeletal elements. Taxa identified from molds include hyoliths, trilobites, sponge spicules, the cap-shaped fossil Scenella (Fig. 21A), and square or rectangular voids attributed to helicoplacoid ossicles (Fig. 2B). Scenella have also been identified in concretion interiors using XCT (Fig. 21B-D).

Valtorres Formation Concretions

Siliceous concretions from the Valtorres exhibit similar features to those from the

Conasauga Formation. They are composed of silica, presumably biogenic in origin, are round to oval in shape, and average 5 cm in length. Internally, they exhibit many of the same features as in the Conasauga concretions, including fossil content and low density cores interpreted to be organic remains.

According to Álvaro et al. (2013), the Valtorres Formation concretions harbor fossils such as polymerid and agnostoid trilobites, brachiopods, hyoliths, and hexactinellid sponge spicules. Spicules range in size from 0.1 to 4 mm in length and are described (Álvaro et al., 2013) as monaxons and triaxons, although figured specimens are stauractins. Trilobites present in the concretions are inferred shelf-dwellers, and indicative of the Guzhangian Stage (Álvaro et al., 2013).

XCT-scans reveal type 1 and 2 porosity (due to trace fossils) in the Valtorres concretions. Mineral sheaths were identified around two type 2 traces. Low density cores, inferred to reflect organic remains, are common in the Valtorres concretions. Some concretions show the traces penetrating the cores. In two examples, a type 2 trace enters the core without continuing through to the other side of the concretion (Fig. 22A). The

Valtorres Formation evidently supported an active infauna, as did the Conasauga

Formation.

Type 3 porosity also has been identified in one concretion from the Valtorres

Formation using XCT-scanning (Fig. 9E). Spicules, however, have not been clearly identified in any of the scans. The clustering and morphology of the voids resemble what 53 is expected of the interior canals of a sponge, however, and thus the specimen is interpreted to be a hexactinellid sponge.

One concretion preserves an inferred cololite in the form of preferentially- oriented hyolith conchs (Fig. 22B). Cololites owing to the work of priapulids have been identified from the Conasauga Formation (Schwimmer and Montante, 2007), although not from concretions. This suggests that priapulid worms were a component of the

Valtorres biota, just as they were in the Conasauga biota.

Fig 12. Trilobite spines in a siliceous concretion, specimen 65, from the Conasauga

Formation (Cambrian), Coosa River Valley, Georgia, imaged using XCT. A, XCT-slice showing nearly straight, thin voids about 10.5 mm long. B, Inverted three-dimensional model showing the voids illustrated in A as solid masses.

Fig. 13. Density-related outlines surrounding hyoliths in siliceous concretions, specimens

36 and 78, from the Conasauga Formation (Cambrian), Coosa River Valley, Georgia, imaged using XCT. A-C Specimen 36, 66.5 mm wide in A. D-F, specimen 78, 84.5 mm wide in D. Arrows point to example hyoliths.

Fig. 14. Assemblages of hyoliths in siliceous concretions from the Conasauga Formation

(Cambrian), Coosa River Valley, Georgia. A, External view of concretion showing conchs in bidirectional orientation, specimen 76, 60 mm wide. B, XCT-slice of concretion containing numerous hyoliths in random orientation, interpreted as a coprolite, specimen 110, 135.5 mm wide.

Fig. 15. Trace fossils preserved on external surfaces of siliceous concretions from the

Conasauga Formation (Cambrian), Coosa River Valley, Georgia. A, Chondrites network

(fine scale traces) associated with Planolites traces (wider traces); specimen 41, 145 mm wide. B-C, Rusophycus, each about 35 mm long, associated with Planolites, specimen

45. B, View showing three Rusophycus associated with Planolites. C, Detail of one

Rusophycus showing scratch marks made by trilobite appendages and impressions on ventral anatomy; Rusophycus intersects a Planolites trace at front (arrow). D, Interior of broken concretion showing inferred coprolite; specimen 59, 60 mm wide.

Fig. 16. Trace fossils at centers of siliceous concretions, and perhaps forming their nuclei, from the Conasauga Formation (Cambrian), Coosa River Valley, Georgia, imaged using XCT. A, Concretion showing traces at nucleus, density layers, and type 3 porosity; specimen 24, 26.5 mm wide in image. B, Concretion with trace network (Chondrites?) at center; density layers and type 3 porosity also present; specimen 46, 52.5 mm wide in image. C, Concretion with trace network (Chondrites?) at center; specimen 46, 103.5 mm wide in image. D, Inverted three-dimensional model of section in 16C, indicated by arrow, showing a U-shaped tunnel; specimen 46.

Fig. 17. Growth on siliceous concretion related to presence of a trace fossil, specimen 62, from the Conasauga Formation (Cambrian), Coosa River Valley, Georgia. A, External view, showing growths on two sides of the concretion, 56 mm long. B, Inverted three- dimensional model imaged from XCT showing a type 2 trace extending through the concretion including the areas of growth at sides. C, Same model as in B but showing the altered lower density structures surrounding the trace.

Fig. 18. Fossilized sponges preserved in siliceous concretions from the Conasauga

Formation (Cambrian), Coosa River Valley, Georgia, imaged using XCT. A, XCT-slice showing contrast between the sponge fossil and the remainder of the concretion; specimen 6, 37.5 mm long. B, Three-dimensional model of sponge extracted from scans of specimen 6, compiled using ImageJ. C, Concretion showing rough morphology; specimen 58, 66 mm wide in the image (compare with B).

Fig. 19. Inverted three-dimensional model of siliceous concretion, specimen 31, showing multi-generational diagenetic banding, from the Conasauga Formation (Cambrian),

Coosa River Valley, Georgia, imaged using XCT. Four generations of diagenesis are evident; concretion (generation 4) is 65 mm wide. A, XCT-slice emphasizing concretion nucleus. B-D, XCT-slices illustrating diagenetic bands of generation 1 (B) surrounding nucleus, generation 2 (C), and generation 3 (D). 62

Fig. 20. Concretion showing two diagenetic generations and trace fossils, from the

Conasauga Formation (Cambrian), Coosa River Valley, Georgia, imaged using XCT; specimen 50, 71 mm wide. Center of concretion (generation 1) is a low density area.

Arrows indicate type 1 traces that extend through generation 2 but stop at the boundary between generations 1 and 2.

Fig. 21. Cap-shaped fossils preserved in siliceous concretions from the Conasauga

Formation (Cambrian), Coosa River Valley, Georgia. A, Exterior of concretion showing

Scenella, 9 mm in diameter, near center of photograph, and other fossils; specimen 6. B,

XCT-slice of specimen 61 showing half-moon structure, 9.5 mm wide, a probable cap- shaped fossil. C, XCT-slice of specimen 102 showing probable cap-shaped fossil, 7 mm wide. D, Inverted three-dimensional image of specimen 61 (see B) showing probable cap- shaped fossil.

Fig. 22. XCT-slices of siliceous concretions from the Valtorres Formation (Cambrian),

Spain, showing fossils. A, Concretion showing type 2 porosity and a trace fossil, about 11 mm long, extending to the low density center; Val-1. B, Assemblage of hyolith conchs, probably a coprolite or cololite; Val-7, 23.5 mm wide.

References

Abbott, G. 1907. Concretions. S. E. Union of Scientific Societies Transactions, 1907:67- 76.

Ahlberg, P. (ed). 1998. Guide to Excursions in Scania and Västergötland, Southern Sweden. IV Field Conference of the Cambrian Stage Subdivision Working Group. International Subcommission on Cambrian Stratigraphy. Sweden, 24-31 August 1998. Lund Publications in Geology 141, 1-47 p.

Ahn, S. Y., and L. E. Babcock. 2012. Microorganism-mediated preservation of Planolites, a common trace fossil from the Harkless Formation, Cambrian of Nevada, USA. Sedimentary Geology, 263:30-35.

Allison, P. A., and K. Pye. 1994. Early diagenetic mineralization and fossil preservation in modern carbonate concretions. Palaios, 9:561-575.

Álvaro, J. J., S. Zamora, D. Vizcaïno, and P. Ahlberg. 2013. Guzhangian (Mid Cambrian) trilobites from siliceous concretions of the Valtorres Formation, Iberian Chains, NE Spain. Geological Magazine, 150:123-142.

Arena, D. A. 2008. Exceptional preservation of plants and invertebrates by phosphatization, Riversleigh, Australia. Palaios, 23:495-502.

Babcock, L. E. 2003. Trilobites in Paleozoic predator-prey systems, and their role in reorganization of early Paleozoic ecosystems, p. 55-92. In Kelley, P. A., Kowalewski, M., and Hansen, T. A. (eds.), Predator-Prey Interactions in the Fossil Record. Kluwer Academic/Plenum Publishers, New York.

Babcock, L. E., M. T. Baranoski, and A. E. Cook. 2014. Cambrian (Guzhangian Stage) trilobites from Ohio, USA, and modification of the Cedaria Zone as used in Laurentia. GFF 136, 6-15.

Babcock, L. E., R. M. Feldmann, M. T. Wilson, and M. Suárez-Riglos. 1987. Devonian conulariids of Bolivia. National Geographic Research, 3:210-231.

Babcock, L. E., and C. N. Ciampaglio. 2007. Frondose fossil from the Conasauga Formation (Cambrian: Drumian Stage) of Georgia, USA. Memoirs of the Association of Australasian Palaeontologists, 34:555-562.

Babcock, L. E., W. P. Dong, and R. A. Robison. 2005. Agnostoid trilobite with phosphatized anatomy and microbial biofilm from the Cambrian of China. Acta Micropalaeontologica Sinica, 22 (Supplement):10-12.

Babcock, L. E., J. M. Kastigar, V. G. Gunther, A. E. Cook, and R. A. Robison. In press. Exceptionally preserved Cambrian trilobite in a concretion from Utah, USA. Palaeo Down Under 2, Abstracts. Australasian Association of Palaeontologists.

Babcock, L. E., S. C. Peng, C. E. Brett, M. Y. Zhu, P. Ahlberg, M. Bevis, and R. A. Robison. 2015. Global climate, sea level cycles, and biotic events in the Cambrian Period. Palaeoworld, 24:5-15.

Babcock, L. E., and R. A. Robison. 1988. Taxonomy and paleontology of some Middle Cambrian Scenella (Cnidaria) and hyolithids (Mollusca) from Western North America. University of Kansas Paleontological Contributions, 121:1-22.

Babcock, L. E., R. A. Robison, and S. C. Peng. 2011. Cambrian stage and series nomenclature of Laurentia and the developing global chronostratigraphic scale. Museum of Northern Arizona Bulletin, 67:12-26.

Babcock, L. E., R. A. Robison, M. N. Rees, S. Peng, and M. R. Saltzman. 2007. The global boundary stratotype section and point (GSSP) of the Drumian Stage (Cambrian) in the Drum Mountains, Utah, USA. Episodes, 30:84-94.

Babcock, L. E., and S. E. Speyer. 1987. Enrolled trilobites from the Alden pyrite bed, Ledyard Shale (Middle Devonian) of western New York. Journal of Paleontology, 61:539-548.

Berner, R. A. 1968. Calcium carbonate concretions formed by the decomposition of organic matter. Science, 159:195-197.

Blome, C. D., and N. R. Albert. 1985. Carbonate concretions: an ideal sedimentary host for microfossils. Geology. 13:212-215.

Borkow, P. S., and L. E. Babcock. 2003. Turning pyrite concretions outside-in: role of biofilms in pyritization of fossils. The Sedimentary Record, 1:4-7.

Boury-Esnault, N., and K. Rützler (eds.). 1997. Thesaurus of Sponge Morphology. Smithsonian Contributions to Zoology, 596:1-56.

Brasier, M., and Green, O. 1997. Ediacarian sponge spicule clusters from southwestern Mongolia and the origins of the Cambrian fauna. Geology, 25:303-306.

Brandt, D. S., D. L. Meyer, and P. B. Lask, P. B. 1995. Isotelus (Trilobita) "hunting burrow" from Upper Ordovician strata, Ohio, Journal of Paleontology, 69:1079- 1083.

Bremner, J. M. 1980. Concretionary phosphorite from SW Africa. Journal of the Geological Society, 137:773-786.

Briggs, D. E. G., D. H. Erwin, and F. J. Collier. 1994. The Fossils of the Burgess Shale. Smithsonian Institution Press, Washington, D.C., 238 p.

Buchardt, B., A. T. Nieldsen, and N. H. Schovsbo. 1997. Alun Skiferen i Skandinavien. Geologisk Tidsskrift, 1997(3):1-30.

Calner, M., P. Ahlberg, O. Lehnert, M. Erlström (eds.). 2013. The Lower Paleozoic of Southern Sweden and the Oslo Region, Norway. Field Guide for the 3rd Annual Meeting of the IGCP Project 591. Sveriges Geologiska Undersökning, Rapporter och Meddelanden 133, 96p.

Canfield, D. E., and R. Raiswell. 1991. Carbonate precipitation and dissolution, its relevance to fossil preservation, p. 411-453. In Allison, P.A., and D. E. G. Briggs (eds.). Tahponomy: Releasing the Data Locked in the Fossil Record. Plenum Press, New York.

Ciampaglio, C. N., Wellman, C., Brunswick, H., York, A., and Babcock, L. E. 2005. Reinterpretation of Brooksella from the Conasauga Formation (Cambrian) of Georgia and Alabama, USA. Acta Micropalaeontologica Sinica, 22 (Supplement):21-23.

Ciampaglio, C. N., L. E. Babcock, C.L. Wellman, A.R. York, and H.K. Brunswick. 2006. Phylogenetic affinities and taphonomy of Brooksella from the Cambrian of Georgia and Alabama, USA. Palaeoworld, 15:256-265.

Conway Morris, S. 1977. Fossil priapulid worms. The Palaeontological Association, Special Papers in Palaeontology 20, 155 p.

Criss, R. E., G. A. Cooke, and S. D. Day. 1988. An organic origin for the carbonate concretions of the Ohio Shale. U.S. Geological Survey Bulletin 1836, 21p. 68

Dabard, M. P., and A. Loi. 2012. Environmental control on concretion-forming processes: examples from Paleozoic terrigenous sediments of the North Gondwana margin, American Massif (Middle Ordovician and Middle Devonian) and SW Sardinia (Late Ordovician). Sedimentary Geology 267:93-103.

Das, S. S., and M. K. Tripathi. 2009. Trace fossils from Talchir carbonate concretions, Giridih Basin, Jharkhand. Journal of Earth System Science 118:89-100.

DeLaubenfels, M. W. Porifera, p. 21-112. In R. C. Moore (ed.). 1955. Treatise on Invertebrate Paleontology, Part E. Archaeocytha and Porifera. Geological Society of America and University of Kansas Press, New York and Lawrence, Kansas.

English, A. M., and L. E. Babcock. 2007. Feeding behaviour of two Ordovician trilobites inferred from trace fossils and non-biomineralised anatomy, Ohio and Kentucky, USA. Memoirs of the Australasian Association of Palaeontologists, 34:537-544.

Foster, M. W. 1979. A reappraisal of Tullimonstrum gregarium, p. 269-301. In Nitecki, M. H. (ed.). Mazon Creek Fossils, Academic Press, New York.

Gaines, R. R., D. E. G. Briggs, P. J. Orr, and P. Van Roy. 2012. Preservation of giant anomalocaridids in silica-chlorite concretions from the Early Ordovician of Morocco. Palaios, 27:317-325.

Hall, J. T., and C. E. Savrda. 2008. Ichnofossils and ichnofabrics in syngenetic phosphatic concretions in siliciclastic shelf deposits, Ripley Formation, Cretaceous, Alabama. Palaios, 23:233-245.

Hasson, K. O., and C. S. Haase. 2013. Lithofacies and paleogeography of the Conasauga Group, (middle and late Cambrian) in the Valley and Ridge Province of east Tennessee. Geological Society of America Bulletin, 100:234-246.

Hayes, C. W. 1891. The overthrust faults of the southern Appalachians. Geological Society of America Bulletin, 2:141-154.

Hellstrom, L. W., and L. E. Babcock. 2000. High-resolution stratigraphy of the Ohio Shale (Upper Devonian), Ohio. Northeastern Geology and Environmental Sciences, 22:202-226.

Hitchcock, E. 1841. Final report on the Geology of Massachusetts. J. H. Butler, Northampton, 299 p.

Jensen, S. 1990. Predation by Early Cambrian trilobites on infaunal worms---evidence from the Swedish Mickwitzia Sandstone, Lethaia, 23:29-42. 69

Martill, D. M. 1988. Preservation of fish in the Cretaceous Santana Formation of Brazil. Palaeontology, 31:1-18.

Martinsson, A. 1974. The Cambrian of Norden, p. 185-283. In C. H. Holland (ed.). Lower Paleozoic Rocks of the World.. 2. Cambrian of the British Isles, Norden, and Spitsbergen. John Wiley & Sons.

Maas, A., A. Braun, X. Dong, P. C. J. Donoghue, K. J. Müller, E. Olempska, J. E. Repetski, D. J. Siveter, M. Stein, D. Waloszek. 2006. The ‘orsten’—more than a Cambrian Konservat-lagerstätte yielding exceptional preservation. Palaeoworld, 15:266-282.

McCoy, V. E., E. E. Saupe, J. C. Lamsdell, L. G. Tarhan, S. McMahon, S. Lidgard, P. Mayer, C. D. Whalen, C. Soriano, L. Finney, S. Vogt, E. G. Clark, R. P. Anderson, H. Petermann, E. R. Locatelli, and D. E. G. Briggs. 2016. The “Tully monster” is a vertebrate. Nature, 16992:1-4.

Mozley, P. S., and S. J. Burns. 1993. Oxygen and carbon isotopic composition of marine carbonate concretions: an overview. Journal of Sedimentary Petrology, 63:73-83.

Müller, K. J., and Walossek, D. 1985. A remarkable arthropod fauna from the Upper Cambrian “Orsten” of Sweden. Transactions of the Royal Society of Edinburgh, 76:161-172.

Müller, W. E. G., J. Li, H. C. Schröder, L. Qiao, and X. Wang. 2007. The unique skeleton of siliceous sponges (Porifera; Hexactinellida and Demospongiae) that evolved first from the Urmetazoa during the Proterozoic: a review. European Geosciences Union, 4:385-416.

Munyan, A. C. 1951. Geology and mineral resources of the Dalton Quadrangle, Georgia- Tennessee. Geological Survey of Georgia Bulletin 57, 128 p.

Neuendorf, K. K. E., J. P. Mehl Jr., and J. A. Jackson (eds.). 2005. Glossary of Geology. American Geological Institute, Alexandria, Virginia, 779 p.

Nitecki, M. H. (ed.). 1979. Mazon Creek Fossils. Academic Press, New York, 563 p.

Peng, S. L. E. Babcock, R. A. Robison, H. L. Lin, M. N. Rees, and M. R. Saltzman. 2004. Global standard stratotype-section and point (GSSP) of the Furongian Series and Paibian Stage (Cambrian). Lethaia, 37:365-379.

Peng, S., L. E. Babcock, J. Zuo, H. Lin, X. Zhu, X. Yang, R. A. Robison, Y. Qi, G. Bagnoli, and Y. Chen. 2009. The global boundary stratotype section and point (GSSP) of the Guzhangian Stage (Cambrian) in the Wuling Mountains, Northwestern Hunan, China. Episodes, 32:41-55.

Pratt, B. R. 1992. Trilobites of the Marjuman and Steptoean stages (Upper Cambrian), Rabbitkettle Formation, southern Mackenzie Mountains, northwest Canada. Palaeontographica Canadiana 9, 179 p.

Pye, K., J. A. D. Dickson, N. Schiavon, M. L. Coleman, and M. Cox. 1990. Formation of siderite—Mg-calcite—iron sulphide concretions in intertidal marsh and sandflat sediments, north Norfolk, England. Sedimentology, 37:325-343.

Resser, C. E. 1935. Nomenclature of some Cambrian trilobites. Smithsonian Miscellaneous Collections, 93(5), 46 p.

Resser, C. E. 1937. Second contribution to nomenclature of Cambrian trilobites. Smithsonian Miscellaneous Collections, 95(4), 29 p.

Resser, C. E. 1938. Cambrian System (restricted) of the Southern Appalachians. Geological Society of America, Special Paper 15, 140 p.

Revelle, R., and K. O. Emery. 1951. Barite concretions from the ocean floor. Geological Society of America Bulletin, 62:707-724.

Rigby, J. K. 1966. Protospongia hicksi Hinde from the Middle Cambrian of western Utah. Journal of Paleontology, 40:549-554.

Rigby, J. K. 1978. Porifera of the Middle Cambrian Wheeler Shale, from the Wheeler Amphitheater, House Range, in western Utah. Journal of Paleontology, 52:1325- 1345.

Rigby, J. K. 1986. Sponges of the Burgess shale (Middle Cambrian), British Columbia. Palaeontographica Canadiana 2, 1-105.

Rigby, J. K. 1987. Phylum Porifera, p. 116-139. In R. S. Boardman, , A. H. Cheetham, and A. J. Rowell (eds.). Fossil Invertebrates. Blackwell Scientific Publications, Boston.

Robison, R. A. 1964. Late middle Cambrian faunas from western Utah. Journal of Paleontology, 38:510-566.

Robison, R. A., and L. E. Babcock, L. E. Systematics, paleobiology, and taphonomy of some exceptionally preserved trilobites from Cambrian Lagerstätten of Utah. Paleontological Contributions 5: 1-47

Robison, A. R., L. E. Babcock, and V. G. Gunther. 2015. Exceptional Cambrian Fossils from Utah: A Window into the Age of Trilobites. Utah Geological Survey Miscellaneous Publication 15-1, Utah, 97 p.

Schieber, J. 2002. Sedimentary pyrite: a window into the microbial past. Geology, 30:531-534.

Schwimmer, D. R. 1989. Taxonomy and biostratigraphic significance of some Middle Cambrian trilobites from the Conasauga Formation in western Georgia. Journal of Paleontology, 63:484-494.

Schwimmer, D. R., and W.M. Montante. 2007. Exceptional fossil preservation in the Conasauga Formation, Cambrian, northwestern Georgia, USA. Palaios, 22:360- 372.

Schwimmer, D. R., and W.M. Montante. 2012. An Aphelaspis zone (upper Cambrian, Paibian) trilobite faunule in the central Conasauga River Valley, North Georgia, USA. Southeastern Geology, 49:31-41.

Walcott, C. D. 1886. Second contribution to the studies on Cambrian faunas of North America. U.S. Geological Survey Bulletin, 30:1-369.

Walcott, C. D. 1896. Fossil jelly fishes from the Middle Cambrian terrane. Proceedings of the United States National Museum, 18:611-614.

Walcott, C. D. 1898. Fossil Medusae. U.S. Geological Survey Monograph 30, 201 p.

Walcott, C. D. 1916. Cambrian trilobites: Cambrian geology and paleontology III. Smithsonian Miscellaneous Collections, 64:303-451.

Weeks, L. G. 1967. Origin of carbonate concretions in shales, Magdalena Valley, Colombia. Geological Society of America Bulletin, 68:95-102.

Westergård, A. H. 1922. Sveriges olenidskiffer. Sveriges Geologiska Undersökning Ca, 18:1-205.

Wilson, D. D., and C. E. Brett. 2013. Concretions as sources of exceptional preservation, and decay as a source of concretions: examples from the Middle Devonian of New York. Palaios, 28:305-316. 72

Woodward, N. B., K. R. Walker, and C. T. Lutz. 1988. Relationships between Early Paleozoic facies patterns and structural trends in the Saltville thrust family, Tennessee Valley and Ridge, Southern Appalachians. Geological Society of America Bulletin, 100:1758-1769.

Appendix A: Conasauga Concretions

Appendix B: Comparison Concretions

Appendix C: Porifera