EVOLUTIONARY PALEOECOLOGY OF THE MID PALEOZOIC CRINOID TRANSITION

Published in 1971, N. G. Lane's seminal paper “Crinoids and Reefs” clearly illustrated environmental preferences for major crinoid clades and spatiotemporal changes in their relative abundance. Beginning in the Middle Devonian, camerate crinoids began to lose their former ecologic, numeric, and taxonomic dominance while cladids gradually replaced them by the Late Mississippian. The beginning and the end of this transition were marked by significantly high extinction rates, but this revolution was not the result of mass extinction but of rapid faunal turnover.

A database consisting of 578 crinoid species distributed among 10 Middle Devonian—Late Mississippian formations provides additional insight into the nature of the Mid Paleozoic crinoid transition: 1) camerates numerically and taxonomically dominated most facies during the Middle Silurian-Middle Devonian; 2) the Late Devonian was characterized by greatly reduced camerate diversity coupled with environmental expansion and taxonomic diversification of cladids; 3) during the Early Mississippian (Kinderhookian), camerate crinoids regained some of their former dominance, especially in carbonate packstone-grainstone facies; 4) this camerate expansion peaked on the Burlington Shelf, where not only monobathrids, but also flexibles and blastoids reached their taxonomic zenith; 5) extinctions following Burlington deposition resulted in cladids attaining greater genus diversity than camerates, even though the latter retain species and numerical dominance on carbonate shelves; 6) diplobathrid camerates suffered local (North American) extinction and monobathrids continued to decline relative to cladids, which attained species dominance at the Osagean-Meramecian boundary; 7) monobathrids lost numerical dominance in the Late Mississippian (lower Chesterian), resulting in the complete domination of cladids in all environments, thus completing the transition to the late Paleozoic crinoid fauna.

Causes for the Mid Paleozoic crinoid transition remain largely untested, but several hypotheses have been proposed to explain it, including increased siliclastic influx, selective predation against camerates, and competitive advantages gained by cladids following the evolution of muscular articulations in their arms.