Comatulid Crinoids in a Changing Ocean: Predation, Respiration, and Shifting Centers of Diversity

Abstract

The oceans have changed since the appearance of animals in the fossil record about 600 million years ago: mass extinctions and subsequent recoveries have repeatedly altered the composition of the marine biota, tectonic and glacio-eustatic shifts have limited the possible locations and richnesses of marine ecosystems, and organisms have become bigger, more metabolically intensive, and more predatory. Crinoids – suspension feeding echinoderms attached to the substrate by a stalk at some point in their development – were among the principal players in this marine evolutionary drama, and though they were prominent and widespread in shallow waters for much of the Phanerozoic, most lineages have subsequently been restricted to the deep sea by the intensification of shell-crushing predation that began in the Jurassic. The only crinoids remaining in shallow water today are the mostly stalkless comatulids, whose evolutionary success has been attributed to high motility, toxicity, and other anti-predatory features. They are the focus of this dissertation. I first consider respiratory physiology in comatulids and other crinoids. I show that a system of fluid-circulating body cavities in these organisms performs a role in respiration, that this role explains previously enigmatic features of skeletal anatomy in living and fossil comatulids, and that the respiratory demands of stalkless comatulids are probably greater than those of the less mobile, exclusively deep-sea stalked crinoids. Next, I analyze the modern geographic distributions of some anti-predatory features, recovering a curious and statistically robust increase in maximum arm number from the poles to the equator. Abiotic correlates of latitude such as temperature and productivity are poor explanations for this pattern; instead, ecological evidence points toward intense tropical predation as the cause. Finally, I show that neontological and paleontological data independently support an origin of comatulids near what is now the Mediterranean and subsequently elevated dispersal to their modern diversity ‘hotspot’ in the Indo-West Pacific, tracking the destruction and creation of shallow shelf area by tectonic activity. This dissertation suggests novel features of the history of marine life, including a causal link between predation and latitudinal gradients in functional richness and a movement of the marine richness hotspot by dispersal rather than by changing diversification rates. It also corroborates more general hypotheses on the changing marine fauna: the anti-predatory adaptations associated with persistence in shallow water despite intense predation, the importance of predator-prey interactions in the tropics, and the geographic shifts in the center of marine richness over the Cenozoic.