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