Fossil intra-crystalline biomolecules of brachiopod shells: diagenesis and preserved geo-biological information

Intra-crystalline macromolecules isolated from the shells of four brachiopod species, collected from successive horizons representing the last 1.47 million years, have been studied by immunoassay and amino acid analysis. Patterns in both immunological responses and free/total amino acid ratios indicated that degradation of these macromolecules proceeded rapidly within the first 0.13 Myr, followed by a plateau phase (0.13-0.62 Myr). Antigenic determinants could not be detected from the oldest (1.47 Myr) specimens studied, suggesting the presence of a third phase of degradation. Canonical variate analyses (CVA) of the amino acid determinations demonstrated separations of two plausible taxonomic groups among living and fossil samples, and revealed progressive diagenetic trends in each group. CVA also gave an explicit separation of the species examined based on the compositions of the free amino acids present in living and fossil shells, suggesting that the intra-crystalline microenvironment approximates a semi-closed system.