SHELL BREAKAGE IN PLEUROTOMARIOID GASTROPODS AND ITS RELATION TO SHELL MORPHOLOGY

The vetigastropod superfamily Pleurotomarioidea includes some of the first gastropods to appear in the fossil record in the Upper Cambrian and the group still exists today. It quickly became the most diverse group of gastropods and dominated the gastropod fauna throughout the rest of the Paleozoic. Ever since the extremely high diversity in the Paleozoic, pleurotomarioids have decreased in importance in fossil gastropod faunas, and today they live secluded at depths of several hundred metres. The group is considered to be primitive since pleurotomarioids retain characters such as paired internal organs and a deep mantle cavity. Another prominent feature is the emargination in the apertural margin of the shell where the two convergent exhalent water currents are expelled. This emargination may be a short, v-shaped sinus or it can be developed to form a deep, narrow slit which, when it is closed, forms a typical band in the shell called the selenizone.

This peculiar invagination of the apertural margin might be expected to affect the mechanical strength of the shell, decreasing shell strength in the apertural region. Since the slit-bearing pleurotomarioidean shell morphology still exists, the group offers an excellent opportunity to study the relationship between shell morphology and the frequency of repaired injuries through the geological record. Furthermore, changes in the form of shell fractures through geological time within the superfamily can be assessed.

This is a report on shell repair frequency in pleurotomarioid gastropods from the Paleozoic, Mesozoic and Recent. The frequency of repaired injuries increase with time as might be expected, from around 4 % in the Pennsylvanian species Glabrocingulum grayvillense to almost 100 % in the recent ones. The frequency of injuries is generally believed to be dependent on the presence and number of predators, but also on general shell morphology of the prey. In this case the open slit is an important factor.