TIMING THE TWILIGHT OF THE CRINOIDS: WHEN DID CRINOIDS CEDE ECOLOGICAL DOMINANCE?

The end-Permian extinction demarcates a major diversity bottleneck for crinoids following which they never regained pre-extinction diversity. Less clear is whether the end-Permian extinction also marks a major transition in crinoid ecology. Although modern crinoids are more environmentally restricted than in the past, the timing of the disappearance of substantial concentrations of crinoids from shallow habitats is unknown. Bioclastic accumulations can provide a window into the relative abundance (or ecological dominance) of constituent taxa. The crinoid fossil record is replete with examples of encrinites (bioclastic accumulations dominated by crinoid debris). A literature survey of encrinite occurrences from the Permian to the Cretaceous reveals the timing of the decline and eventual cessation of crinoid ecological dominance.

Encrinite production recovered to pre-extinction (Permian) levels in the Triassic and Jurassic, waned in the Upper Jurassic and plummeted in the Early Cretaceous. This timing strongly suggests that the decrease of crinoidal ecological dominance was not directly related to the end-Permian mass extinction. Although temporal links between the ecological decline of crinoids and the evolution/expansion of known crinoid predators (e.g. fishes, echinoids) are hazy, predatory escalation is likely to have been a contributory factor in crinoid ecological dominance loss.