ECHINODERM COMMUNITY EVOLUTION IN THE MIDDLE PALEOZOIC

Crinoids and blastoids reached an apex in generic richness and abundance during the Mississippian. Explanations include: 1) Rising biodiversity of advanced cladids during the transition from the Middle Paleozoic to the Late Paleozoic Crinoid Macroevolutionary Fauna, 2) establishment of widespread carbonate ramps after the demise of Late Devonian reef communities and 3) Response to declining predation pressure caused by extinctions of major groups preying on crinoids at the Hangenberg extinction event.

Although the causes of the Mississippian crinoid explosion are probably multifaceted, climatic events in the Devonian likely played a major role. Crinoid and blastoid generic richness reached a peak in the Early Devonian reflecting the success of the camerate-dominated Middle Paleozoic Crinoid Macroevolutionary Fauna during an interval of widespread reefal facies. Throughout most of the Devonian, diversity paralleled the reefal diversity, which paralleled patterns of low latitude sea surface temperature. This pattern changed after Late Devonian extinction events. Famennian echinoderm communities are dominated by cladid crinoids and more closely resemble Late Paleozoic rather than Middle Paleozoic Crinoid Macroevolutionary Faunas. The demise of reef communities in the Late Devonian led to the development of widespread carbonate ramps in the Mississippian and for a time the resurgence of the camerate-dominated MPCMF.

Riding (2009) suggested that significant changes in Devonian atmospheric chemistry in the Devonian led to changes in phytoplankton communities which led to significant increases in abundance of suspension-feeding echinoderms beginning in the Late Devonian and accelerating into the Mississippian.

Although much work remains to understand the dynamics of echinoderm community evolution in the Devonian, dramatic climate change had a major impact. Crinoid communities were able to be successful both in the Lower Devonian greenhouse world and in the Mississippian icehouse world because different clades were able to adapt to changing climates and the demise of reef ecosystems and flourish. Whether this success was the result of adaptation to changing sedimentological regime or changing patterns of plankton diversity is unclear at the present time.