NEW INFORMATION on THE MORPHOLOGY AND MODE OF LIFE OF THE CURIOUS UNIVALVE MOLLUSK PTEROTHECA
Pterotheca is a common faunal constituent of the Blackriveran age Forreston Member of the Grand Detour Formation (Platteville Group) in the stateline area of northern Illinois/southern Wisconsin where it is preserved as highly detailed molds and casts in fine grained dolostone. For this study, specimens were collected from two locations: Stateline Quarry located at South Beloit, Winnebago County, Illinois, and Karl’s Quarry, located just west of Beloit, in Rock County, Wisconsin.
An unusual feature observed on steinkerns of Pterotheca from the Forreston Member are three prominent and closely spaced ridges (or prongs) on internal casts at the apex of the body cavity. These are interpreted as muscle scars. Muscle scars have not been previously described in any of the known pterothecines and attest to the presence of powerful retractor muscles in the living animal. In addition, the surface area of the shell of Pterotheca is disproportionately large in comparison to the volume of living space within it. A large surface area to volume ratio is commonly associated with mollusks that live on soft sediments. In fact, many Forreston Member mollusks display features that could be interpreted as adaptions to living on soft sediments (wide flaring apertures and disjunct coiling among the univalves for example).
One possible explanation for the presence of large retractor muscles at the apex of a shell with a small internal volume could be that the living animal resembled a modern moon snail in its ability, when extended, to pump water through its mantle and foot, thereby “inflating” its mass to a much larger size than the shell alone could accommodate. If threatened, Pterotheca could expel the water from its body, enabling it to retract its body inside the shell.