PATTERNS OF GENUS RICHNESS WITHIN MISSISSIPPIAN CRINOID CLADES BETWEEN NORTH AMERICA AND THE BRITISH ISLES

Although both present-day North America and the British Isles were part of Laurentia during the Mississippian, their crinoid faunas were largely isolated from each other by the emerging Appalachian Mountains. The only available migration route was around the southern margin of the Appalachians and through the narrow, elongate remnants of the Rheic Ocean.

In general, crinoid habitat space was much more extensive and genus richness was much higher in North America than the British Isles. Therefore, rarefaction plots, which adjust for sample sizes, were used to compare genus richness among clades from each region. Rarefaction adjusted comparisons indicate two groupings of clades. Genus richness was similar between regions for diplobathrid camerates, advanced cladids, and flexibles (they are also considered comparable for disparids); whereas, monobathrid camerates and primitive cladids had approximately twice the richness in North America versus the British Isles. The characteristics of these two groups are contrasted through comparison of the two clades with the highest genus richness, advanced cladids and monobathrid camerates. They are compared through consideration of the origination of new genera versus holdovers from the Devonian, number of endemic versus cosmopolitan genera, fate of genera originated in each region, consideration of facies availability for migration, relative assemblage sizes between regions, and the specific contrast of crinoids from the Burlington Limestone to other regions of North America versus the British Isles. At a minimum, the data supports the hypothesis that diplobathrids, advanced cladids, and flexibles were able to freely migrate across the Rheic Ocean, whereas monobathrids and primitive cladids were less able to migrate, possibly because of different larval types. Their much greater diversity in North America suggests an evolutionary center for these latter two clades.