Platyschismatinae

Platyschisma helicoides, from Knight et al. (1960).

In an earlier post on this site, I commented on some of the various ways that gastropods deal with the fact that their development tends to put their anus uncomfortably close to their mouth. A common solution is the development of a sinus or slit in the shell that provides spaces for the anus to be moved backwards.

One of the major gastropod groups exhibiting such a feature is known as the Pleurotomarioidea. In the modern fauna, pleurotomarioids are not hugely abundant, with living species restricted to deep waters. However, they were one of the dominant gastropod groups back in the Palaeozoic when they were represented by a number of families. One Palaeozoic pleurotomarioid group is the Platyschismatinae, known from the Lower Ordovician to the Middle Permian (Knight et al. 1960). Platyschismatines went with the sinus option, with a sinus present at or above the midpoint on the outer edge of the shell opening. Knight et al. (1960) included five genera in the Platyschismatinae. The type genus, Platyschisma, has a slightly flattened spiral and a relatively thin shell. Some of the other platyschismatines were also relatively flat.

REFERENCE

Knight, J. B., L. R. Cox, A. M. Keen, R. L. Batten, E. L. Yochelson & R. Robertson. 1960. Gastropoda: systematic descriptions. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt I. Mollusca 1: Mollusca—General Features, Scaphopoda, Amphineura, Monoplacophora, Gastropoda—General Features, Archaeogastropoda and some (mainly Paleozoic) Caenogastropoda and Opisthobranchia pp. I169-I331. Geological Society of America, and University of Kansas Press.