The influence of antipredatory morphology on survivorship of the Owl Creek Formation molluscan fauna through the end-Cretaceous extinction

Abstract

The Hypothesis of Escalation states that organisms have changed throughout the Phanerozoic Eon by adapting to increased biotic hazards, such as predation or competition, by developing either physical or behavioral adaptations. One type of physical antipredatory adaptation among molluscs is the development of a thicker, more ornamented (escalated) shell. Although well-escalated organisms may have an advantage during intervals between mass extinctions, their thicker and/or more ornamented shells may provide a problem during extinction intervals when nutrients become scarce. Therefore, we propose that taxa with antipredatory adaptations are more prone to extinction during times of reduced productivity or nutrient availability. To test this hypothesis, upper Cretaceous bivalves and gastropods were collected from the Owl Creek Formation of Mississippi. Taxa were coded for the presence or absence of antipredatory characteristics, such as an overlapping margin and crenulated margin, among bivalve taxa, and apertural teeth, thickened lip and a narrow aperture, among gastropod taxa. The presence or absence of an umbilicus within gastropod taxa was also coded because it may increase a taxon's vulnerability to predation. A value between one and four was assigned to each taxon based on the degree of surface ornamentation and the thickness to height ratio was determined. The measured characteristics were analyzed statistically using the Fisher exact test, χ 2 test and Mann–Whitney U-test to determine whether their presence affected survivorship during the end-Cretaceous extinction. Survivorship was examined at the generic, subgeneric and specific levels. Two of the 36 tests revealed statistically significant results at the 0.05 level: the presence of a crenulated margin among bivalve genera ( p-value=0.031) and ornamentation among bivalve subgenera ( p-value=0.026). The statistical test for a crenulated margin suggests that bivalve genera with this adaptation were more likely to survive, contrary to our hypothesis, and the test for survivorship among bivalve subgenera with respect to ornamentation was significant in favor of our hypothesis. However, most of the statistical tests do not support the prediction that escalated molluscan taxa were more prone to extinction when compared to non-escalated taxa. Survivorship may not be predictable by analyzing only one aspect of an organism. Instead, it may be necessary to analyze statistically a combination of potential survivor strategies, such as morphology and trophic level or habitat.