Shell repair and shell form in Jurassic pleurotomarioid gastropods from England

Abstract

Specimens of the vetigastropods Pleurotomaria Defrance, 1826 and Pyrgotrochus Fischer, 1885, from the Jurassic of England were separated into three morphological groups and surveyed for traces of shell repair. Two measures of shell repair frequencies were calculated: 1) ratio of the number of specimens with at least one repaired injury to the total number of specimens; 2) ratio of the total number of injuries to the total number of specimens in the sample. The Pleurotomaria anglica (Sowerby, 1818) group containing large, high spired trochiform shells showed the lowest repair frequency with 28.8% and 75.3%, respectively. The shell repair frequency in the low spired trochiform Pleurotomaria actinomphala Deslongchamps, 1848 group was 44.4% and 81.0%, respectively. Lastly, the Pyrgotrochus group containing conoidal trochiform shells showed the highest frequency calculated by both methods, 46.2% and 92.3%. Three types of injuries were found in all morphological groups, although in different proportions. Breaks across the entire whorl constitute the majority of repaired injuries in shells of the Pleurotomaria anglica group; in the P. actinomphala group the figure is about 50% and in Pyrgotrochus only 35%. The coniodal shell form and a deep slit in the aperture margin probably proved to be a defensive strategy for Pyrgotrochus, with many fractures terminating at the margin of the slit, whereas Pleurotomaria anglica was protected by its size.