Article: Contrasting lifestyles in Lower Jurassic crinoids: a comparison of benthic and pseudopelagic Isocrinida
Michael J. Simms
Lower Jurassic pentacrinitids have been regarded either as pseudopelagic in habit, living suspended beneath floating objects such as driftwood, or as strictly benthic, living much like all known representatives of their sister group the Isocrinidae. The taphonomy of Lower Jurassic pentacrinitids differs significantly from that of contemporary isocrinids in their environments of preservation, extent of disarticulation, occurrence in debris accumulations, frequency of association with driftwood, and the size and position of the driftwood relative to the crinoids. Unlike contemporary Isocrinidae, Lower Jurassic pentacrinitids have a low overall population density, wide geographical distribution, and rapid growth to reproductive maturity, indicated by the growth lines on brachials and by the high filtration efficiency of the endotomous arm branching. A high fecundity is implied by the large size of adult pentacrinitids and the high concentration of larval attachment discs on driftwood. These taphonomic, palaeobiological and morphological features of pentacrinitids are consistent with those which might be anticipated for pseudopelagic organisms suspended beneath temporarily floating objects of limited availability and subject to wide dispersal. The evolutionary stasis shown by the pentacrinitid Seirocrinus subangularis (Miller), which persisted from the Carixian into the mid-Toarcian, is remarkable considering the profound faunal changes which occurred in the early Toarcian and suggests that this species was not influenced by changes in the benthic environment.