GIANTS AMONG MICROMORPHS: PHOSPHATIC STEINKERNS ARE SMALL BECAUSE OF TAPHONOMIC SIZE-SELECTIVITY, NOT ECOLOGICAL STRESS

Small fossils are preserved as phosphatic (carbonate fluorapatite) micro-steinkerns (~0.5 mm diameter) in Upper Ordovician beds from Iowa to the Cincinnati area. Molluscs are common, but bryozoan zooecia, echinoderm ossicles and other taxa are present. These micromorphic mollusks have been interpreted as ecologically-dwarfed and adapted to oxygen-starved conditions, an interpretation with implications for ocean anoxia. An alternative explanation for small phosphatic steinkerns is taphonomic. Stable phosphate may have selectively filled small voids, preserving small fossils that include larval/young molluscs. Reworking would have concentrated small phosphatic steinkerns from multiple generations whose calcareous shells were destroyed along with larger unfilled shells, resulting in small fossils progressively replacing larger fossils.

With thin sections and insoluble residues, we document evidence suggesting that this taphonomically biased process created these fossil assemblages. Gastropods and monoplacophorans in thin section are often only partially filled with phosphate—starting at the apex—also evidenced by meniscus-like terminations common in whole steinkerns released by acid digestion. Umbilical steinkerns, much narrower than body-chamber fillings, are also present, and represent shells much larger than those of body-chamber steinkerns. Despite most steinkerns being similarly sized, stages of development are evidenced by progressive secondary nacre accretion that thickened the apical area of growing shells, again suggesting a size limit to these steinkerns. Bivalve steinkerns are similarly sized but without signs of partial filling because of their enclosed nature. However, phosphatic fillings between the hinge teeth of much larger bivalves suggest again that phosphate fillings were size-selective. Imprints of larger fossils are found in phosphatic particles apparently deposited within the sediment between grains. Bryozoans and echinoderms are preserved as zooecia/stereom fillings. The fossils are comparable to non-phosphatic counterparts, and were preserved because of their many small pores.

This evidence suggests that these fossil assemblages are taphonomically, not ecologically size limited. No evidence suggests size limitation by oxygen stress.