Shell breakage in two pleurotomarioid gastropods from the Upper Carboniferous of Texas, and its relation to shell morphology

Abstract

Shells of two widespread Palaeozoic pleurotomarioid gastropods from North America, Worthenia tabulata (Conrad, 1835) and Glabrocingulum grayvillense (Norwood & Pratten, 1855), were scanned for evidence of shell breakage in the form of injuries repaired during the snail's life. Such injuries can result from a variety of agents but failed predation is normally believed to be the most common cause. Characteristic of these gastropods and other pleurotomarioids is the open slit in the aperture margin, which generates a selenizone. The length of the slit is often thought to influence the strength of the shell in the aperture area and through this, the frequency of shell repair. However, the large difference in shell repair frequency observed between Worthenia and Glabrocingulum cannot be attributed to the length of the slit alone. Apparently, Worthenia was much better suited to withstand attack than Glabrocingulum because of its shorter slit and general shell morphology. A higher spire and the relatively prominent angulations in the aperture seem to limit the extent of fractures effectively. The slit and the selenizone strongly influence the shape and location of shell injuires. The observed patterns show a remarkable similarity with repaired injuries in extant pleurotomarioids.

Keywords:

• Pleurotomarioidea
• shell repair
• predation
• slit
• selenizone