Testing for escalation in Lower Mississippian camerate crinoids

Thompson, Jeffrey R. and Ausich, William I. (2015) Testing for escalation in Lower Mississippian camerate crinoids. Paleobiology, 41 (1), 89-107. (doi:10.1017/pab.2014.6).

Abstract

Crinoids were relatively unaffected by the end-Devonian Hangenberg mass extinction event. Major clades of Devonian durophagous fishes suffered significant extinctions, however, and the dominant surviving clades were biting or nipping predators. In part as a response to the Hangenberg event, early Mississippian crinoids underwent an adaptive radiation, while fish clades with a shell-crushing durophagous strategy diversified. Durophagous predators are inferred to have been more effective predators on camerate crinoids; and it is hypothesized, following the predictions of escalation, that through the early Mississippian, camerate crinoids evolved more effective anti-predatory strategies in response. We test this hypothesis of escalation by examining the changes in spinosity and plate convexity among camerate crinoids throughout this interval. A new method was formulated to test for an increase in convexity of the tegmen plates. Traits in Agaricocrinus, Aorocrinus, and Dorycrinus (Family Coelocrinidae) were tested for congruence to the escalation hypothesis, and results were mixed. Convexity of tegmen plates in Agaricocrinus, spine length/calyx diameter in Aorocrinus, calyx size in Aorocrinus, central spine length in Dorycrinus, and spine width in Dorycrinus did not have size increase trends supporting escalation. Rather than an increase in convexity, the variance of convexity in Agaricocrinus tegmen plates narrowed, which could reflect an optimum. Alternatively, morphological change consistent with the escalation hypothesis occurred in calyx size of Agaricocrinus and in lateral spine length and calyx size in Dorycrinus. Furthermore, central and lateral spine length, parameters of the spine width, and size trends support escalation when Aorocrinus and Dorycrinus are treated as a lineage. Thus, inferred escalation acted on traits differently within a single lineage and was relevant for both speciation and the diversification of a new genus.

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Contributors

Author: Jeffrey R. Thompson

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