STRATIGRAPHIC DISTRIBUTION OF LOWER CARBONIFEROUS CRINOIDS IN THE UNITED KINGDOM AND IRELAND

Crinoid faunas were abundant in a limited number of facies from the Tournaisian to the Serpukhovian of the United Kingdom and Ireland. These crinoid faunas are distributed through ten time zones. During this time period there was the transition between the Middle Paleozoic Crinoid Evolutionary Fauna (CEF) (with different clades dominant in different facies) and the late Paleozoic CEF (with cladids dominant). The transition in the United Kingdom and Ireland was quite abrupt, due in part to the limited facies occurrences of crinoid faunas in Times 6 and 7.

During the Ivorian (Time 3), crinoids were common along a south-facing carbonate ramp that extended from southern Ireland, to southern Wales, and through Somerset. The Hook Head, Ireland fauna is representative, and it has a generic richness of 32, with the distribution of genera as follows Camerata (Ca), 38%,; Cladida (Cl), 59%; Flexibilia (F), 3%; and Disparida (D), 0%. Crinoid associations changed along the ramp. Generic richnesses are as follows: deep ramp 8 (Ca, 13%; Cl, 87%; F, 0%; and D, 0%); intermediate ramp 12 (Ca, 42%; Cl, 50%; F, 8%; and D, 0%); and shallow ramp 20 (Ca, 50%; Cl, 50%; F, 0%; and D, 0%). During the early Chadian (Time 4) crinoid faunas dominated by camerates flourished on the flanks of Waulsortian mudmounds that were common down slope buildups with a total generic richness of 28 (Ca, 50%; Cl, 36%; F, 11%; and D, 4%). The other two major times of crinoid generic richness include the Asbian (Time 8) with 22 genera (Ca, 14%; Cl, 73%; F, 14%; and D, 0%) and, the Brigantian (Time 9) faunas with 42 genera (Ca, 10%; Cl, 60%; F, 19%; and D, 12%). Faunas from both of these times are principally known from Scotland where they occur in cyclothemic sequences with marine bands composed of thin shales and limestones.

Increasing evidence points to significant migration of crinoid taxa between North America and western Europe. Out of 122 genera of crinoids known from the Lower Carboniferous of western Europe, 58 genera, or 48%, are known from North America. North America was apparently an evolutionary center as over 200 genera occurred there during the Lower Carboniferous, much greater than in western Europe. Presently, data are being assembled to analyze the direction(s) of migration for various genera and to search for trends within and between clades.