THE ROLE OF ANTI-PREDATORY MORPHOLOGY IN MOLLUSCAN SURVIVORSHIP THROUGH THE END-CRETACEOUS MASS EXTINCTION

The Hypothesis of Escalation states that during the Phanerozoic biological hazards, such as predation, have increased, as have adaptations to those hazards. Anti-predatory adaptations, however, may require more energy for maintenance and mobility. As a result, taxa with enhanced anti-predatory adaptations may be more prone to extinction during a mass extinction compared to less escalated taxa. This hypothesis is heavily debated, with one side supporting the above argument and the opposing side stating that, although survivorship during an extinction is not random, patterns among survivors can not be easily predicted or attributed to a single cause.

To test this hypothesis, Upper Cretaceous molluscs were collected from the Owl Creek Formation of Mississippi and identified to the lowest possible taxonomic level. Taxa were coded for the presence or absence of five anti-predatory characteristics (overlapping margin and crenulated margin among bivalves and apertural teeth, thickened lip, and narrow aperture, among gastropods). The presence or absence of an umbilicus within gastropods was also coded because an umbilicus can lower shell strength and make the taxon more vulnerable to predation. Values from one to four were assigned to each taxon based on its surface ornamentation and the thickness:height ratio was determined. The measured characteristics were then analyzed statistically to determine whether their presence enhanced or hindered survivorship across the end-Cretaceous extinction.

The results of two out of thirty-six tests were statistically significant at the 0.05 level. The presence of a crenulated margin among bivalve genera, with a p-value of 0.031, was significant against the hypothesis, and surface ornamentation among bivalve subgenera, with a p-value of 0.026, supported the hypothesis. The remaining thirty-four tests were not significant (i.e. the presence of other anti-predatory adaptations did not influence molluscan survivorship). Consequently, survivorship may not be predictable by analyzing only one aspect of an organism. Instead, it may be necessary to statistically analyze a suite of survivorship strategies, such as morphology and trophic level or habitat.