CONCLUSIONS

Stalked forms dominate the rich fossil record of crinoids and have been the subject of most paleontological studies. On the other hand, extant stalked crinoids occur only at depths inaccessible by scuba, so that the shallow-water, stalkless comatulids have received most attention. However, the increased use of research submersibles since the 1970s has revealed much about basic aspects of extant stalked crinoid biology, including the crawling behavior previously known among the comatulids. In fact, our study reveals that crawling ability may be well developed among isocrinids, allowing them to travel at speeds of approximately 10 mm sec^-1, or two orders of magnitude faster than previously reported. At such speeds, crawling may play a much broader range of roles in the biology of stalked crinoids than had been suspected, including escaping from benthic predators such as cidaroid urchins. In this way, locomotion may play a similar role in isocrinids as in comatulids (Meyer and Macurda 1977).

The crawling abilities of some stalked crinoids may also impact our understanding of the evolutionary history of the crinoids. Arguably the major feature of their evolutionary history is the Permo-Triassic extinction. Prior to this event, crinoids were a common faunal element in shallow marine environments, so much so that together with brachiopods, rugose and tabulate corals, stenolaemate bryozoans and a few other groups, they have come to symbolize the Paleozoic fauna (Sepkoski 1981). For crinoids, the Permo-Triassic represented a major evolutionary bottleneck and, although they rebounded impressively in the Mesozoic and Cenozoic, their diversity did not return to Paleozoic levels. This study suggests that, for crinoids, the Permo-Triassic also represented a major functional threshold, with the predominantly sessile faunas of the Paleozoic being replaced, especially in the later Mesozoic and Cenozoic, by motile forms. This pattern is consistent with Vermeij's (1987) suggestion that among echinoderms "locomotor deterrence" became a more common mode of resistance from predators in the Mesozoic and Cenozoic, while "better armor" characterized the Paleozoic.