Assessment of indigeneity in fossil organic matter: amino acids and stable isotopes

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Abstract
Whereas the presence of amino acids in fossil materials is clearly related to biosynthesis, an indigenous relationship with the fossil may be suspect. At present, attempts to establish the indigeneity of amino acids in fossils are based on distribution and stereochemistry. However, fossil systems are not closed and racemization may be retarded in organic materials within a fossil matrix. The advent of new technologies has resulted in alternative methods for evaluating the authenticity of fossil organic matter. A comparison of the stable carbon isotope compositions of the D- and L-enantiomers of individual amino acids facilitates the evaluation of indigeneity. Here we report the application of this method for determining the origin(s) of amino acids in fossils and extraterrestrial samples. Further, nitrogen isotope compositions of fossil materials should reflect trophic order, with increasing enrichment in $\delta ^{15}$N with progression up the food chain. Establishment of the trophic position of a fossil would further reinforce the appraisal of indigeneity based on the isotopic composition of its residual organic matter. Results of initial studies on Cretaceous age vertebrates which are consistent with the suggested feeding morphologies of these organisms are also presented.