Shell fossil mollusc

Racemization rates (Table I) clearly differ among these seven amino acids. To compare results from the four proteins, rate constants were calculated from these data. For casein, D/L ratios were measured at 0, 1, 3, 8 and 24 hours. These results are plotted in Figure 2. The curves have two regions of different racemization rates. Rapid initial rates observable up to about 3 hours are followed by slower rates up to 24 hours. The amino acids apparently have not reached equilibrium by 24 hours of incubation. Theoretically for amino acids having one asymmetric center, the equilibrium D/L ratio is 1.0. This value has been observed in fossil bone protein (see 13) and in dry roasted proteins (10), but not in calcareous marine sediments (17) nor in fossil mollusc shell (18). 

Although the first demonstration that amino acid racemization took place in fossils used mollusc shells (23), the application of this reaction in dating these materials has been extensively investigated only recently (19,20). Work on Mercenaria (19), Chione (20), and other species (24) has tested the application of racemization dating to fossil mollusc shells from geological contexts and Indian shell middens. These and other studies have shown that there are problems with amino acid racemization dating of carbonaceous fossils which are not encountered with bone. Reversible first-order racemization kinetics which are observed in bone... 

Amino Acid Racemization Dating of Fossil Mollusc Shell... 

Pannella, G., 1976. Tidal growth patterns in recent and fossil mollusc bivalve shells a tool for the reconstruction of paleotides. Naturwissenschaften, 63 539—543. 

Oglobin, K.F. and Khalifa-Zade, C.M., 1974. Abundance of uranium in the shells of recent and fossil molluscs. Geochem. Int., 11 239—244. 

Fossil glycoproteins of the soluble organic matrix are present in an 80-million year old mollusc shell from the Late Cretaceous Period. Discrete molecular weight components, as determined by gel electrophoresis, are preserved. A particular repeating amino acid sequence (—Asp—Y—) found in contemporary mollusc shell proteins was identified in fossil glycoproteins425. ... 

Amino acid racemization dating of mollusc shell has not been investigated as extensively as that of fossil bone. Recently several studies have used d/l enantiomeric ratios to resolve stratigraphic relationships (24) and to date geological deposits by the calibration procedure (19,20). Racemization dating has also been applied to midden shell from archaeological sites with limited success (20). 

Various species of fungi are encountered in mollusc or barnacle shells where they exploit the orgemic matter laid down by the animal between the crystallites of CaCOs (see e.g. Plate 8 of Schneider, 1976). Several of these fungi have been described by Zebrowski (1936) and Porter and Zebrowski (1937) only on the basis of excavations and thus must be considered trace fossil taxa, since the living organisms have not been described. 

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