ENVIRONMENTAL ADAPTATIONS OF FLEXIBLE CRINOIDS DURING THE MISSISSIPPIAN PERIOD, MIDCONTINENT, USA

Flexibles were one of the five major clades of Paleozoic crinoids. In the Mississippian they were a minor, but ever present, group within crinoid faunas. A total of 12 genera, containing 64 species, have been reported from Mississippian rocks in Iowa, Missouri, Illinois, Indiana, Ohio, Kentucky, and Tennessee. Chronostratigraphic, lithostratigraphic, geographic, and facies occurrences were compiled for these taxa. Using this data set, the environmental adaptations of Mississippian flexible crinoids were analyzed by comparison between facies occurrences, generic durations, and arm branching and calyx morphology.

Three genera, Taxocrinus, Onychocrinus, and Forbesiocrinus, stand out from the other genera in terms of overall facies range and duration. These generalists have the “classic flexible” morphology and were characterized by complex ramules at the end of the main arms, and an open arm branching pattern. The remaining genera were specialists, typically with a shorter durations, and tended to have different morphology from the generalists. Deep-water specialists, such as Gaulocrinus and Mespilocrinus, had fewer arms, simpler branching patterns, wider ambulacral grooves, and simple cups. The shallow-water siliciclastic specialist Nipterocrinus was a cyathocrine homeomorph, whereas Dactylocrinus was a melocrinid camerate homeomorph with its bi-endotomous arm branching. The shallow-water carbonate specialist Parichthyocrinus with its large number of fixed brachials had a calyx similar to an actinocrinid camerate, which were abundant in that facies. These examples of convergent evolution provide evidence of optimal adaptations among crinoids for specific environments.

In summary, generalists typically have larger and more complex calyces than specialists found in deeper water. They have longer, wider arms with clusters of complex ramules. In contrast, deeper-water specialists usually have smaller, simpler calyces, shorter arms, and relatively wider ambulacra. Some specialists found in shallow-water facies had morphology convergent upon non-flexible crinoids also found in those facies.