ESCALATION AND SPECIATION IN LOWER MISSISSIPPIAN CAMERATE CRINOIDS

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 part as a 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, and following the Mid-Paleozoic Marine Revolution, camerate crinoids evolved more effective anti-predatory strategies in order to compensate for the more effective predatory strategy of these Mississippian durophagous fishes. More convex plates and longer spines are commonly regarded to provide more effective anti-predatory strategies, and first became widespread in crinoids following the Mid-Paleozoic Marine Revolution. 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. Dorycrinus showed significant evidence for directional change through time, which provides evidence for escalation as a response to fish predation. Aorocrinus displayed stasis or a random walk, and did not appear to display evidence of escalation. Agaricocrinus did not show any evidence for distinct directional evolution, but instead displayed a decrease in variance in convexity values through time, possibly demonstrating an anti-predatory minimum threshold in convexity values. This study demonstrates that although the three examined genera were exposed to similar predation pressures, their evolutionary responses to this predation in the aftermath of the Mid-Paleozoic Marine Revolution differed.