ORDOVICIAN SMALL SHELLY FAUNA FROM THE ELGIN MEMBER OF THE MAQUOKETA: ECOLOGICALLY DWARFED OR TAPHONOMICALLY BIASED?

The Cambrian “small shelly faunas” are assemblages that include millimeter-sized phosphatic internal molds of shells. While similarly-preserved small fossils range through to the Holocene, post Cambrian occurrences are generally poorly studied or overlooked, but show identical preservation. The Upper Ordovician Elgin Member of the Maquoketa Shale in Iowa contains phosphorites dominated by similar diminutive molluscan assemblage of phosphatic steinkerns. The small size of the mollusks in this assemblage has been interpreted as an adaptation or response to anoxic conditions, and taken as evidence for oceanic anoxic events during the Ordovician.

This interpretation may be unwarranted, because small (<5mm) molluscan species and juveniles are ubiquitous in modern marine environments, and shelled organisms tend to be absent, not dwarfed, in modern oxygen minimum zones.

An alternative hypothesis is that phosphatic steinkerns formation preferentially preserves smaller (mm-scale) fossils. If true, then the presence of normal-sized individuals in these deposits would argue against dwarfism in the original fauna.

To test the competing hypotheses of ecological dwarfism vs. small-pass taphonomic bias, rock samples were collected from two phosphorite beds at Graf, Iowa. Polished slabs and thin sections were made from these samples. Remaining samples were dissolved in acetic acid and the residues picked for phosphatized fossils. Thin sections show moldic porosity that indicates diagenetic destruction of calcareous shells, and incomplete phosphate filling of internal shell spaces. Acid residues contain normally-sized specimens of bryozoan zooecial molds, crinoid columnals, and umbilical molds of snails, all of which are small parts of larger animals. The position or absence of muscle scars and palial line in half-shell bivalve steinkerns indicates incomplete preservation of larger individuals. These findings suggest that phosphate preferentially precipitated in small pores and could not fully fill larger ones. Therefore, preservational bias favored either small shells, or small parts of larger shells. This finding suggests that the original fauna may not have been dwarfed, but that larger shells were not preserved as phosphatic molds before being destroyed.