“PHOSPHATIZATION” OF ECHINODERM OSSICLES: INSIGHTS FROM ORDOVICIAN AND MODERN ENVIRONMENTS

Echinoderm ossicles are ubiquitous in Phanerozoic shelf carbonates. Because echinoderm microstructure reflects interpenetration by soft tissue, disarticulated ossicles can help reconstruct the distribution of connective tissue in extinct echinoderms. Unfortunately, growth of syntaxial calcite cement obscures stereomic microstructure, but deposition of phosphate (calcium fluorapatite, CFA) and other diagenetic minerals may more faithfully preserve primary microstructure signals.

To investigate the role of diagenetic minerals in preserving this microstructure, we examined thin sections and insoluble residue of Ordovician (Katian) crinoids from the Cincinnati, Ohio area, and modern Endoxocrinus, a stalked isocrinid, collected in the Florida Straits. Modern material included live specimens and decaying, disarticulated ossicles from sediment. We examined specimens in thin section, including EDS analysis.

Although literature includes many instances of ossicles “replaced” by phosphate, we found that this “phosphatization” process occurs in stages. Initially, CFA infills empty space within stereom as tissues decay, forming a complex three-dimensional internal mold. As the original calcareous skeleton dissolves, this new space also becomes infilled with additional CFA, forming a cast, although in many cases the skeleton dissolved incompletely. Pyrite and secondary calcite are often present, pointing to a complex oxygenation gradient within the ossicle micro-environment, fueled by decay of soft tissues and extending from the lumen outward (concentric diagenesis) in both phosphatized and non-phosphatized examples.

Hence, most “phosphatized” ossicles are micromolds of stereomic cavities, not true replacements, with the original stereom structure partially preserved. Acid dissolution obscures this by destroying original skeleton. In-sediment dissolution of skeletal calcite and subsequent filling of the void spaces by more phosphate indicates a relatively long residence of these grains in taphonomically active sediment.

Phosphatization is part of a complex process that molds and replaces echinoderm skeletons. Understanding these mixed diagenetic processes may help better reconstruct the history of shell bed accumulation.