ORGANIC GEOCHEMISTRY OF HARD PARTS - ASSESSMENT OF ISOTOPIC VARIABILITY AND INDIGENEITY

The indigenous organic fraction of fossils contains exciting informati on on the evolutionary biology, paleoecology, and taxonomy of organism s which can be deciphered using geochemical techniques. This approach requires an understanding of isotopic variability and an assessment of indigeneity. In addition to assessing isotopic variability among mode rn organisms, we explore new applications of isotopes and protein sepa rations for assessing the indigeneity of ancient organic fractions tha t have likely experienced extreme alteration. Small variation in delta (15)N and delta(13)C of high molecular weight organic (HMW) material f rom bones and teeth of an extant Caiman crocodile (Caiman crocodilus, less than 0.5 parts per thousand) indicated that the bones and teeth w ere formed contemporaneously from similar dietary material. Isotopic v alues of HMW material from turtles of the family Trionychidae (7.9 to 12 parts per thousand and -23.9 to -21.5 parts per thousand for delta( 15)N and delta(13)C, respectively), reflect habitat-associated variati ons in diet. The delta(15)N and delta(13)C of the HMW component from f ossil teeth of alligators and gavials from clay-rich depositional envi ronments 8.0-13.3 parts per thousand and -24.1 to -21.3 parts per thou sand, respectively, were similar to previously reported values for HMW material from bones of extant, high trophic level consumers. In contr ast, the delta(15)N compositions of teeth of ancient alligators from s hell pits are conspicuously low (3.9 parts per thousand) and could be interpreted as evidence of contamination or extreme alteration. The pr esence of hydroxyproline, however, in one of the shell pit samples sug gests that a remnant of the original protein has been retained in the fossil. Isotopic analysis of bulk organic material will provide only a first approximation in understanding paleoecological and taxonomic re lationships. Consequently, we offer an alternative approach to assessi ng indigeneity of the organic fraction through isotopic analysis of am ino acids enantiomers from individual protein fractions. To validate t his approach data are presented that indicate that isotopic integrity of several amino acids is maintained during all steps of the preparato ry procedure. The ability to document geochemical changes in individua l proteins as a function of time and depositional environment will pro vide significant insight into specific mechanisms that occur during di agenesis.