Organic geochemistry of hard parts: Assessment of isotopic variability and indigeneity

Abstract

The indigenous organic fraction of fossils contains exciting information on the evolutionary biology, paleoecology, and taxonomy of organisms 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 modern organisms, we explore new applications of isotopes and protein separations for assessing the indigeneity of ancient organic fractions that have likely experienced extreme alteration. Small variation in δ 15N and δ 13C of high molecular weight organic (HMW) material from bones and teeth of an extant Caiman crocodile ( Caiman crocodilus, less than 0.5%) indicated that the bones and teeth were formed contemporaneously from similar dietary material. Isotopic values of HMW material from turtles of the family Trionychidae (7.9 to 12% and −23.9 to −21.5% for δ 15N and δ 13C, respectively), reflect habitat-associated variations in diet. The δ 15N and δ 13C of the HMW component from fossil teeth of alligators and gavials from clay-rich depositional environments 8.0–13.3% 0 and −24.1 to −21.3% 0, respectively, were similar to previously reported values for HMW material from bones of extant, high trophic level consumers. In contrast, the δ 15N compositions of teeth of ancient alligators from shell pits are conspicuously low (3.9% 0) and could be interpreted as evidence of contamination or extreme alteration. The presence of hydroxyproline, however, in one of the shell pit samples suggests 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 relationships. Consequently, we offer an alternative approach to assessing indigeneity of the organic fraction through isotopic analysis of amino acids enantiomers from individual protein fractions. To validate this approach data are presented that indicate that isotopic integrity of several amino acids is maintained during all steps of the preparatory procedure. The ability to document geochemical changes in individual proteins as a function of time and depositional environment will provide significant insight into specific mechanisms that occur during diagenesis.