Testing for escalation in Lower Mississippian camerate crinoids

Abstract

AbstractCrinoids 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 inAgaricocrinus,Aorocrinus, andDorycrinus(Family Coelocrinidae) were tested for congruence to the escalation hypothesis, and results were mixed. Convexity of tegmen plates inAgaricocrinus, spine length/calyx diameter inAorocrinus, calyx size inAorocrinus, central spine length inDorycrinus, and spine width inDorycrinusdid not have size increase trends supporting escalation. Rather than an increase in convexity, the variance of convexity inAgaricocrinustegmen plates narrowed, which could reflect an optimum. Alternatively, morphological change consistent with the escalation hypothesis occurred in calyx size ofAgaricocrinusand in lateral spine length and calyx size inDorycrinus. Furthermore, central and lateral spine length, parameters of the spine width, and size trends support escalation whenAorocrinusandDorycrinusare 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.