AN ASYNCHRONOUS MESOZOIC MARINE REVOLUTION: DRILLING VERSUS DUROPHAGY IN POST-PALEOZOIC ECHINOIDS

Although predation influences structure and dynamics in modern ecosystems, the importance of biotic interactions on evolutionary scales remains contentious. While echinoids are common in the fossil record, they remain understudied relative to mollusks, offering a means of independently evaluating the hypothesis of escalation, which posits that predators and prey engage in a co-evolutionary arms race. In addition, the Mesozoic diversification of infaunal echinoids may have represented an evolutionary response to increasing predation pressure.

Here we examine the association between the diversity of infaunal echinoids and their predators, as well as the evolution of antipredatory morphologies that may have contributed to defense against durophagy. The radiation of infaunal echinoids pre-dates the diversification of cassid gastropods, and drilling predation intensified ~100 million years after the peak of the Mesozoic Marine Revolution. However, the radiation of sea stars, crustaceans, sharks, and marine reptiles coincides with the infaunalization of echinoids, suggesting that the rise of shell crushing predators may have led to an increase in predation pressure. In addition, we identified six antipredatory morphologies in post-Paleozoic echinoids including internal struts and the length, width, shape, ornamentation, and density of spines. The appearances of these morphologies relative to the diversification of both post-Paleozoic echinoids and shell crushing predators was examined. Preliminary results based on the examination of 15 epifaunal echinoid genera and 59 species indicate that a radiation of echinoids with long spines or variable spine shapes took place during the Early Cretaceous when genera expressing long or non-cylindrical spines appeared, coinciding with the diversification of crustaceans, marine reptiles, sharks, and ray-finned fishes.

These preliminary findings suggest that the infaunalization of echinoids took place at the time when shell crushing predators diversified. If similar trends are found with other antipredatory morphologies, this would indicate that escalatory trends associated with the Mesozoic Marine Revolution can be identified in echinoids, and that escalation involving echinoids was driven by durophagous rather than drilling predators.