SPINOSITY OVER TIME IN PALEOZOIC CRINOIDS AND ITS IMPLICATIONS FOR PREDATION PRESSURE

Predation intensity is hypothesized to have increased during the Phanerozoic, driving corresponding increases in anti-predatory defensive adaptations. The Mid-Paleozoic marine revolution has been identified as an episode during which durophagous nektonic fish predators became more common and, it has been argued, resulted in an increase of defensive adaptations in benthic organisms. Similarly, increases in predatory injuries in camerate crinoids from the Ordovician to the Mississippian correspond to an increase in frequency of camerate arm branching patterns that reduce the probability of arm loss during encounters with predators. Spines and nodes represent another commonly suggested adaptation that may have multiple anti-predatory functions, and it has been claimed that they increased among crinoids during the Mid-Paleozoic marine revolution. Such calycal growths have several possible defensive functions against predators and parasites, such as increasing effective size, distributing bite force, and disrupting parasite settling. Herein we describe patterns of change in the presence and location of spines and nodes in crinoids during the Paleozoic, based on genus descriptions compiled in the Treatise on Invertebrate Paleontology, and use them to discuss the possible relationships between defensive adaptations in the different Paleozoic crinoid suborders and the evolution of their predators and parasites.