AMORPHOUS CALCIUM CARBONATE AND ITS ROLE IN THE PRESERVATION OF DECAPOD CUTICLE

Amorphous calcium carbonate (ACC) is more prevalent within extant decapod cuticle than previously recognized, and this calcium carbonate is significant in the ultimate preservation of cuticle that is often considered a ‘soft part’ in terms of its preservation potential. Decapod cuticle can contain calcium carbonate in both crystalline (calcite) and amorphous (ACC) phases. The distribution of calcite within the organic framework of extant decapod cuticle, because of its crystallinity, can be easily detected under cross-polarized light. In contrast, regions of (ACC) in extant cuticle must be detected using other methods such as energy dispersive spectroscopy; in conjunction with transmitted polarized light both composition and phase can be ascertained. We have demonstrated the distribution of both phases of calcium carbonate in the crab Callinectes sapidus and within the lobsters, Nephropsis aculeata and Nephrops binghami. In cuticle from extant decapods, phosphorus is present in higher concentrations in regions containing ACC than in adjacent calcitic regions. Based on observations of fossilized cuticle which is predominantly apatitic but shows isolated regions of calcite, we hypothesize that the ACC in conjunction with the associated higher levels of phosphorus provides the necessary source material to facilitate the formation of apatite. Portions of the cuticle that originally contained calcite during life may remain relatively unaltered. Similarities in the morphology of calcitic zones in cuticle from extant decapods, which may appear as isolated pillows or bands, with that of calcitic zones in otherwise apatitic fossil cuticle, help support the hypothesis that the distribution of these calcitic zones results from the chemical arrangement during life. We propose that regions of the cuticle that were calcified during life can be preserved, and recognized, in otherwise apatitic fossil cuticle. The link between calcification and preservation has long been recognized; the role that ACC plays is less well known, and the extent and distribution of ACC within decapod cuticle is virtually unknown. Thus, we hypothesize that the presence and the distribution of ACC can be recognized in fossilized cuticle and is key to its preservation. This work was supported by NSF grant EF 0531670 to Feldmann and Schweitzer.