View - Kowalewski, M. - Virginia Tech

CHIN—ANALYSES OF COPROLITES PRODUCED BY VERTEBRATESBroken 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 sectionscan 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 on47