analyses of coprolites produced by carnivorous …

CHIN—ANALYSES OF COPROLITES PRODUCED BY VERTEBRATES

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ANALYSES OF COPROLITESPRODUCED BY CARNIVOROUS VERTEBRATES

KAREN CHINMuseum of Natural History/Department of Geological Sciences, University of Colorado at Boulder,

UCB 265, Boulder, Colorado 80309 USA

ABSTRACT—The fossil record contains far more coprolites produced by carnivorous animals than by herbivores.This inequity reflects the fact that feces generated by diets of flesh and bone (and other skeletal materials)contain chemical constituents that may precipitate out under certain conditions as permineralizing phosphates.Thus, although coprolites are usually less common than fossil bones, they provide a significant source of informationabout ancient patterns of predation. The identity of a coprolite producer often remains unresolved, but fossilfeces can provide new perspectives on prey selection patterns, digestive efficiency, and the occurrence of previouslyunknown taxa in a paleoecosystem. Dietary residues are often embedded in the interior of coprolites, but muchcan be learned from analyses of intact specimens. When ample material is available, however, destructive analysessuch as petrography or coprolite dissolution may be used to extract additional paleobiological information.

INTRODUCTION

WHEN PREDATOR-PREY interactions cannotbe observed directly, fecal analysis provides the nextbest source of information about carnivore feedingactivity because refractory dietary residues oftenreveal what an animal has eaten. This approach isvery effective in studying extant wildlife, and it canalso be used to glean clues about ancient trophicinteractions. Yet, although fossilized carnivore fecesare often present in fossiliferous sediments, theirantiquity makes analysis more difficult. Diageneticalteration of specimens obscures the originaldigestive residues, and it is often impossible toascertain which animal produced the coprolite.Fossil feces also show significantly more variationin morphology and composition than skeletal fossils:animal diets are often highly variable, and soft fecalmaterial can assume a range of shapes—especiallyafter post-depositional deformation. In spite of suchcomplexities, coprolites provide importantinformation about ancient predator-prey interactionsthat is not available from body fossils. Coproliteanalyses, however, require different approachesfrom those used to study most skeletal fossils.Furthermore, different types of coprolites may

provide different types of information.Diet and depositional environment largely

determine which animal feces may be fossilized andthe quality of preservation of a lithified specimen.Significant concentrations of calcium and phosphorusin bone and flesh often favor the preservation ofcarnivore feces by providing autochthonous sourcesof constituents that can form permineralizing calciumphosphates (Bradley, 1946). Thus, althoughherbivores outnumber carnivores in terrestrialecosystems, carnivore feces are more likely to bepreserved. Preservation is also facilitated by rapidburial, so coprolites from aquatic taxa usually faroutnumber those from terrestrial animals. Thesetaphonomic biases explain why coprolites arerelatively rare in most terrestrial deposits, while fishcoprolites can be quite common.

WHAT CARNIVORE COPROLITESTELL US ABOUT PREDATOR-

PREY INTERACTIONS

The identity of the animal that produced acoprolite is very difficult, if not impossible, topinpoint because our knowledge of ancient faunasis incomplete and because fecal material is so

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variable. Although we know that spiral coprolites(or intestinal casts; see Williams, 1972) wereproduced by one of the groups of fish with spiralintestinal valves (e.g., sharks, lungfish, or gars;Gilmore, 1992), morphology usually provides littleinformation about the animal of origin becausemany animal droppings produced by different taxaare quite similar. Coprolite contents, composition,size, and stratigraphic placement can, however,constrain the number of likely perpetrators.Carnivore coprolites are usually easy todifferentiate from herbivore coprolites becausethey are typically phosphatic and often containskeletal inclusions. Coprolite size is also veryinformative, since fecal volumes generally scalewith animal size. While fecal amounts are variable,it’s clear that small animals cannot produce largeindividual fecal deposits. Field guides to modernscat provide analogs that can be used to roughlyapproximate the size range of animals that producefeces of a given mass. Thus, even when the taxa offecal producers are unknown, carnivore coprolitesprovide evidence that predators of an approximatesize range frequented a given habitat. This indicatesa trophic niche that may be further defined bydietary residues that reveal prey selection patterns.

In many cases, inclusions within a coproliteprovide more information about prey animals thanabout the coprolite producer itself. The integrityof included digestive residues depends on theircomposition and the extent of their exposure todigestive and diagenetic processes. Somecoprolites contain no recognizable inclusions, butrefractory skeletal constituents have been found innumerous carnivore coprolites (e.g., Hantzschel etal., 1968). When dietary residues are incompletelydigested, the morphology of elements such asmollusk shells, ganoid scales, and small bones mayallow identification of prey. Specific taxa ofmollusks (e.g., Speden, 1969; Stewart andCarpenter, 1990), crustaceans (Bishop, 1977; Sohnand Chatterjee, 1979), fish (e.g., Zangerl andRichardson, 1963; Waldman, 1970; Coy, 1995),reptiles (e.g., Parris and Holman, 1978), andmammals (e.g., Martin, 1981; Meng et al., 1998)have been recognized in coprolites. Furthermore,

fragments of larger bones may be ascribed tohigher-level taxonomic groups on the basis ofhistological analysis (e.g., Chin et al., 1998).

Coprolites may also show signs of ingested softtissues as organic residues or in the form of three-dimensional impressions. Some exceptionalspecimens have revealed evidence of feathers(Wetmore, 1943), fur (Meng and Wyss, 1997),insect exoskeletons (Northwood, 1997), andmuscle tissues (Chin et al., 1999). Such remarkablepreservation requires depositional conditions thatminimize diagenetic recrystallization.

These studies show that coprolites may containdietary residues that provide concrete evidence ofancient carnivory. It is clear, though, that thechallenges of coprolite analysis stem from thedifficulties involved in determining the animal oforigin and in identifying residual dietarycomponents. This analytical complexity reflects thefact that coprolites are relatively anonymouspackages that represent varying diets, digestiveprocesses, and diagenetic alteration. Such markedvariability necessitates that care be exercised indrawing conclusions from a limited number ofsamples. Even so, coprolites provide cumulativeclues that help flesh out our understanding ofpatterns of predation in ancient environments byidentifying prey species within a givenpaleoecosystem, and by indicating general size and/or age classes of prey animals (e.g., Zidek, 1980;Martin, 1981). The composition and integrity ofthese inclusions may also provide information aboutthe diet and digestive processes of the predator itself.

COPROLITE ANALYSIS

Documentation.—Both destructive and non-destructive techniques may be used to analyzecoprolites, and the choice of analytical methoddepends on the questions addressed by the researchproject. Regardless of experimental approach, eachstudy of coprolites must include carefuldocumentation of provenance and of the physicalcharacteristics of each specimen.

The collection of coprolites is similar to thecollection of vertebrate fossils where documentation


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of locality and stratigraphic information is of criticalimportance. In many cases, coprolites have erodedout of their encasing sediments and are collected asfloat. Although such specimens contribute notablebaseline information about a given formation, theinformative value of coprolites is greatly enhancedwhen they are collected in situ and can be correctlyplaced within a detailed stratigraphic column.Mapping specimens in place will also indicatecoprolite orientation and density. Such taphonomicinformation contributes important information aboutthe environment of deposition.

Photographic documentation of specimens isas useful for coprolite analysis as it is for researchon other fossils. Unfortunately, most coprolites arenot as strikingly photogenic as many skeletalfossils! Nevertheless, images of seeminglyamorphous coprolitic masses are quite usefulbecause they document the range of coprolite sizeand morphology, and help create search images forpaleontologists likely to encounter fossilized fecesin the field (Fig. 1). Such records become evenmore important when destructive analyses alter theoriginal form of a specimen. Photos should alwaysinclude a scale.

If a coprolite specimen is very important orhas an unusually distinctive morphology, standardpaleontological molding and casting methods (seeGoodwin and Chaney, 1994) can be used toreplicate the external form. Care should be taken,however, if this technique is applied to fragilespecimens; some coprolites are composed of softmaterials that can be easily scratched or gouged(possibly obscuring paleobiologically informativeimpressions), or may absorb compounds appliedas separators or mold release agents. This techniqueshould not be used on highly fractured specimens.

Non-destructive Analyses.—Intact coprolitespecimens can be characterized by morphology, size,and surface features. Although coprolites from manydifferent taxa can have similar traits, documentationof the physical characteristics of coprolite specimensis important because recurring features may revealdistinct morphological categories. Such forms maybe designated as coprolite morphotypes. One earlyclassification scheme differentiated spiral

coprolites into “heterpolar” or “amphipolar” types,depending on the spacing of coils along the longaxis of the specimen (Neumayer, 1904). Thissystem has been applied to other spiral coprolites,though there is debate as to whether thesemorphotypes have any taxonomic significance (e.g.Price, 1927; Zidek, 1980).

Linear dimensions of coprolites (e.g., diameterand length) provide rough approximations of fecalsize, but volumetric measurements give much moreinformative assessments. Volume can be measuredin several ways. The volume of small, densespecimens can be determined by measuring waterdisplaced by submerged samples; porousspecimens should be allowed to absorb waterbefore displacement is measured. This approachcan also be applied to large, fractured specimensby using water displacement to calculate thedensity of small fragments; volume can then bedetermined by extrapolation after weighing theentire specimen (e.g., Chin et al., 1998). In a fewcases a coprolite may be so large and fractured thatit remains in the plaster jacket in which it wascollected (e.g., Fig. 1a), and cannot be accuratelyweighed. The volume of such specimens can beapproximated by visualizing the mass as beingcomposed of one or more geometric shapes whosevolumes can be calculated.

When a large number of coprolites from agiven locality represents an unbiased sample,recurring size classes and other physicalcharacteristics may indicate specimens producedby a small number of taxa and/or age groups. Suchgroupings may be subtle, however. Edwards andYatkola (1974) analyzed 106 coprolites from theWhite River Formation and found no distinct sizeclasses within a continuum of coprolite diametersranging from 15 to 36 mm. But when the data wasre-analyzed using the mean diameter of in situcoprolites that occurred in small “clusters”(suggesting individual fecal deposits), they foundthree distinct size groups that may reflect the sizesof the carnivores that produced them. Correlatingcoprolite size with other physical attributes willrefine interpretations, though some features (suchas color and degree of flattening) probably provide


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little taxonomic information (Sawyer, 1981).Non-destructive analyses of coprolites should

also include careful examination of specimensurfaces for distinctive impressions or inclusions.These surfaces can be scanned with a stereomicroscope; if warranted, higher magnifications canbe obtained using scanning electron microscopy.Scrutiny of the outer surface of a coprolite mayreveal dietary inclusions protruding from the

coprolitic ground mass. Mineralized skeletalelements are commonly observed in carnivorecoprolites, and inclusions on outer surfaces may beaccentuated by weathering processes. Coproliteexteriors may also show three-dimensionalimpressions of the vegetation or detritus on whichthe feces were deposited. Such impressions are ofinterest, but should be distinguished from those thatrepresent dietary components.

FIGURE 1—Coprolites can have many morphologies, from easily recognizable forms to nondescriptmasses. (a) Very large Cretaceous coprolite from Alberta (Royal Tyrrell Museum, TMP 98.102.7). Thisspecimen was so fractured from weathering that it was removed in a plaster jacket. (b) Probable fishcoprolite from the Pennsylvanian black shale of Indiana (Field Museum of Natural History; FMNHPF2210; see Zangerl and Richardson, 1963). Photograph shows lateral view of sliced specimen; thespecimen originally appeared as a lump in the sediments, but was sliced through the center to revealthe coprolitic material sandwiched between the black shale. (c) Small coprolite with more typical,‘sausage’ fecal shape (Royal Tyrrell Museum, TMP 80.16.1098).


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Broken coprolites allow scrutiny of fracturedinternal surfaces. In most cases, recognizablefeatures evident in the interior of a specimen canbe confidently attributed to diet. This includesthree-dimensional impressions that may indicateundigested soft tissues.

Destructive Analyses.—As a general rule,damage of fossil specimens should be studiouslyavoided. At the present state of our technology,however, destructive analyses appear to providesome of the most effective means to extractpaleobiological information from coprolites.Techniques such as petrographic analysis or aciddissolution may reveal dietary components thataren’t evident on coprolite surfaces. Because suchanalyses destroy the morphological integrity of aspecimen, several factors should be consideredbefore a sample is altered. If numerous comparablecoprolites are available or if a specimen is verylarge and/or fragmented, the information obtainedfrom destructive tests is likely to compensate forthe loss of some coprolitic material. But thedecision of whether to perform destructive analysesbecomes more difficult if the tests will damage aunique specimen. When destructive analyses areplanned, they should be preceded by carefulmeasurements, photo-documentation, and scrutinyof accessible surfaces (see above).

Thin sections of coprolites provideexceptionally informative views of specimencontents because they permit analysis withcompound microscopes. Such analyses may revealdietary inclusions with considerable histologicaldetail. They also shed light on patterns of diageneticmineralization. The jumbled nature of fecalcontents makes thin section sampling ratherunpredictable, however, because identification ofdietary components depends on fortuitous slicesthrough recognizable structures. Fortunately, somefeatures diagnostic of certain taxonomic groups(such as patterns of bone vascularization) may beevident on small fragments.

Thin sections can be made from relatively smallpieces of coprolite, and careful scrutiny of coprolitefragments or intact specimens will help identifyoptimal sampling sites. When possible, thin sections

can be taken from the end of a specimen in order topreserve more of the original morphology. Althoughtechniques for preparing coprolite thin sections aresimilar to those for preparing standard petrographicsections of rock, more efforts are made to minimizedamage to and loss of coprolitic material (seeWilson, 1994 for a useful discussion of methods forpreparing fossil thin sections).

Saws with diamond-embedded blades are usedto reduce large samples to sizes that can be affixedto glass slides. A diamond saw is also necessary toshave off the thin sample slices that are mountedon slides (the sample can be sliced thin before orafter it is mounted on the slide). The use of aprecision saw with a thin diamond wafering bladewill facilitate more accurate cuts and help reduceloss of coprolite material during the cuttingoperation. In a few cases, indurate coprolites canbe cut with a precision saw without embedding,but fragile or fractured specimens should beembedded in or impregnated with an epoxy orpolyester resin before being cut.

The cut surface of a specimen must be groundsmooth before it is affixed to a microscope slidewith a strong epoxy bonding agent. Standardpetrographic slides are 27 × 46 mm, but specimenscan also be mounted on larger slides (or glass plates)as well. Grinding/polishing machines or lappingwheels are used to grind and polish the sample toan appropriate thickness (around 30–40 µm,depending on the nature of the sample). Slides canthen be cover-slipped for examination with a lightmicroscope, or finely polished for chemical analyseswith a microprobe or scanning electron microscope.

In a few studies, coprolites have beenmechanically disaggregated in order to releasedietary residues from the ground mass. Thistechnique will be most effective when the prey havebeen poorly digested, and it may reveal thepresence of small prey taxa that are not otherwiserepresented in a faunal assemblage. Aciddissolution may also facilitate chemical andmorphological studies of amorphous organicresidues (e.g., Hollocher et al., 2001). Some of thetechniques used in the acid preparation ofvertebrate fossils may be modified for use on


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coprolites (see Rutzky et al., 1994). In suchprocedures, the exposed portions of skeletalelements that are encased in well-lithifiedsediments are carefully coated with a hardener forprotection before the specimens are immersed inweak acid solutions (usually phosphate-bufferedacetic or formic acids). As this process is repeated,embedded skeletal elements are gradually releasedfrom the sediments.

Because the ground mass of most coprolites isphosphatic, acid solutions prepared for dissolvingthis material should not have a phosphate buffer(unless it is very weak). The acid dissolutionprocedure will be easier when the ground mass of acoprolite is more susceptible to acid than the dietaryinclusions. The operation will be more challenging,however, when both included skeletal elements andthe coprolitic ground mass show similarsusceptibility to the acid solution. Because thephosphatic ground masses of different coprolites canhave widely varying compositions, it is clear thatexperimentation will be necessary to identify themost effective methods for releasing inclusions froma given type of coprolite. Some authors (Sohn andChatterjee, 1979) have used formic acid to releaseostracods from coprolites. Others (Burmeister et al.,

1999) report that using ultrasonication with weakacetic acid is effective in freeing inclusions fromcoprolites that have a significant carbonatecomponent. Mechanical disaggregation alone mayalso be used on coprolites that have a softer groundmass (e.g., Parris and Holman, 1978).

CONCLUSIONS

Coprolite analysis is quite different from thestudy of fossil skeletal elements. Becausemorphology is usually not diagnostic, the chemicaland physical composition of a coprolite assumesgreater importance and may provide as much (ormore) paleobiological information than size andshape. The ambiguity of coprolite morphology alsomakes it difficult to unequivocally associate acoprolite with the animal that produced it.

Despite these challenges, some carnivorecoprolites provide unique perspectives on ancientpredator-prey activities. Although they may notprovide complete information about the carnivoroushabits of specific animals, coprolites can supplyimportant fossil evidence that reveals prey selectionpatterns, digestive efficiency, and the occurrence ofsmaller fauna in a given paleoenvironment.

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