Muscle scars in euomphaline gastropods from the Ordovician of Baltica

A discrete pair of muscle scars is described for the first time on the umbilical wall of the open-coiled, hyperstrophic ophiletoidean gastropod Asgardispira, a close relative of the widely distributed Lytospira, from the middle Ordovician of the eastern Baltica. In a unique specimen of the euomphaloidean Lesueurilla of similar age and derivation, the muscles have coalesced into a single scar. A pair of pedal retractor muscles is characteristic of several major groups of gastropods both in the Lower Palaeozoic and at the present day, and was likely an ancestral character of the class. The consolidation of muscle attachment to a single site may reflect the tightening of the logarithmic spiral of the shell and is probably related to the increasing development of anisostrophic coiling and shell re-orientation during gastropod evolution.