COMPARATIVE ANALYSIS OF LONGEVITY DATA FOR SPECIES AND GENERA OF MISSISSIPPIAN CRINOIDS FROM NORTH AMERICA

Comparisons between generic and species longevities in Mississippian crinoids were made to determine rates of evolution among and between the five major crinoid clades. The data are broken up into eleven time zones (biostratigraphic intervals) in North America ranging through the Kinderhookian to Chesterian. The total number of taxa included were 1157 species and 188 genera. Mean species longevities ranged from a low of 1.28 time units for the advanced cladids to a high of 1.66 for the primitive cladids, with the order of increasing species longevity, from lowest to highest: advanced cladids, camerates, flexibles, disparids, and primitive cladids. Mean generic longevity ranges from a low of 3.14 in the advanced cladids to a high of 4.03 in the camerates with the order of increasing generic longevity, from lowest to highest: advanced cladids, primitive cladids, flexibles, disparids, and camerates. The mean generic longevity is 2.60 times that of the mean species longevity. Could these differences in longevity within both species and genera be the result of evolutionary rates, preservational bias, or even taxonomic practice? More research is required to determine which of these, or if all of these, affected crinoid longevities during this time. Because advanced cladids have both the lowest species and generic longevities, they may have underwent rapid evolution, or possibly they were not as readily fossilized as other clades. But, there is independent evidence for their rapid evolution. Camerates may have had a greater generic longevity because of either slower evolution of their complex calyx, or their tendency to have better calyx preservation. It is likely that the greater longevity of primitive cladid species is real because it corresponds with the 1997 analysis that analyzed Osagean-Meramecian species. The 1997 analysis was based on a complete taxonomic review and revision of all treated species, unlike the current analysis extensively based on the literature. These data will continue to be analyzed to form more concrete hypotheses for the differences in Mississippian crinoid species and generic longevity.