Predators and their prey are often locked in a constant evolutionary battle as they strive for survival. As metabolism and energetics has increased over time, many predators have also become larger and more powerful (known as escalation). Evidence for escalation has been found in shellfish groups, and the ratio between the size of predators and their shellfish prey has been increasing for the past 100 million years. Here we explore this phenomenon in another important group, sea urchins. We looked for evidence from the past 170 million years for whether predatory sea snails have become more powerful over time and preferentially selected larger urchin prey. Predatory snails attack their prey by scraping a hole in the prey exoskeleton using their sharp-toothed tongue. This produces a hole through which the snail can access the internal tissue to feed. These drill holes can be easily identified on fossil urchins, and are direct evidence of predator-prey interactions. Drill hole size is also known to correspond to the size of the predator, and can be used as a proxy for predator size. We used the EAT (Echinoid Associated Traces) Database which houses measurements of thousands of fossil urchins and these traces of predation to compare the size of drill holes, a proxy for predator size, and the size of the urchin prey over time. We found that the size of predatory snails has increased since the Jurassic Period (Pearson’s correlation r = 0.20, p = 0.05), i.e., evidence of escalation, and that the size-ratios of predators to their urchin prey has also been increasing over time (supported by a directional evolutionary model from the Cretaceous onwards). This is the first time evidence of this nature for escalation has been documented in urchins, and supports the idea that predation is an important evolutionary force.
Author: Andrew Selm
Faculty Advisor: Carrie Tyler, Geology and Environmental Earth Science
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