Microevolutionary Response in Lower Mississippian Camerate Crinoids to Predation Pressure
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Date
2013-05
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The Ohio State University
Abstract
Crinoids were relatively unaffected by the end-Devonian Hangenberg event, but the major clades of Devonian durophagous fishes suffered significant extinctions. These dominant Devonian fishes were biting or nipping predators. In response to the Hangenberg event, Lower Mississippian crinoids underwent an adaptive radiation, while fish clades with a shell-crushing durophagous strategy emerged. Durophagous predators are inferred to have been more effective predators on camerate crinoids and it is hypothesized that through the Lower Mississippian, camerate crinoids evolved more effective anti-predatory strategies in order to compensate for the more effective predatory strategy of the durophagous fishes. More convex plates and longer spines are commonly regarded to provide more effective anti-predatory strategies. Did convexity and spinosity increase among camerate crinoids during the Lower Mississippian? A new method was formulated to test for an increase in convexity of the calyx plates among species of the genus Agaricocrinus. Spine length was analyzed in the genera Aorocrinus and Dorycrinus and is a simple linear measurement standardized to calyx diameter. Data were analyzed using Kolmogorov-Smirnov and Mann-Whitney U tests to determine if morphological change was statistically significant. Dorycrinus showed the most significant evidence for directional change through time, which provides evidence for escalation as a response to fish predation. Aorocrinus seemed to most likely display stasis or a random walk, whereas Agaricocrinus did not show evidence for distinct directional evolution, but rather showed a decrease in variance in convexity values through time.
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Crinoid, Predation, Escalation, Mississippian