CLIMATE CHANGE VERSUS PREDATION: DRIVERS FOR MACROEVOLUTIONARY TRANSITIONS WITHIN PALEOZOIC CRINOIDS

What are the drivers responsible for transitions between the three crinoid evolutionary faunas (CEF) of the Paleozoic? The transition from the lower to the middle CEF was coincident with the end-Ordovician extinction event. Crinoids experienced a single extinction associated with the onset of glaciation. Thus, crinoid extinctions were presumably the result of global climate change and habitat loss due to a drop in sea level. Crinoid biodiversity recovered and exceeded pre-extinction levels after approximately five million years. Although jawed fishes may have existed during this time, there impact on ecosystems is considered minimal.

In contrast, a combination of abiotic and biotic factors drove the change to the late Paleozoic CEF. After the Middle Devonian end-Givetian extinction among crinoids, crinoid biodiversity increased into the Mississippian until the end of the Osagean (early Visean) when the middle Paleozoic CEF began changing to that of the late Paleozoic by the early Chesterian (late Visean). Crinoid biodiversity continued to rise and the overall community composition remained the same despite the Frasnian-Famennian extinction, the end-Devonian glaciation and associated Hangenberg extinction, and the end-Kinderhookian (middle Tournaisian) glaciation. The late Osagean transition was characterized by rapid faunal turnover, rather than mass extinction; and it was associated with both the loss of expansive carbonate ramps and the continued diversification of shell-crushing fishes. The late Paleozoic CEF never regained the biodiversity of the Osagean, presumably due to shell-crushing fishes. Thus, recovery after each event also differed.

We conclude that macroevolutionary changes in crinoids during the early Paleozoic were more influenced by environmental changes, whereas by the middle to late Paleozoic biologic interactions played a greater role. The middle to late Paleozoic transition appears to have resulted from increasingly complex predator-prey dynamics starting with the Middle Paleozoic Marine Revolution. Thus, biological and physical changes through time result in a changing landscape on which macroevolutionary changes occur.