NEW DISTRIBUTION OF BRITTLE STARS THROUGH THE CARBONIFEROUS BASED ON ISOLATED ARM PLATES

New methods of identifying ophiuroid (brittle star) fossils from disarticulated elements is having a profound effect on our understanding of these organisms through the Late Paleozoic. Two lateral arm plates (LAPs) are present in each arm segment and because of their varied and important functions including such disparate things as housing the podial basins and tentacle notch for tube feet and bearing the bases for movable spines, they are morphology rich and useful taxonomically. Initial studies of Paleozoic LAPs, following protocols used for Mesozoic forms, have shown them to be both extremely abundant and diverse at most sampled localities. To date, seven formations have been sampled from the uppermost Meramacian through the lower Virgilian that bear ophiuroid LAPs. Between these formation some 20 genera (or species groups) have been identified and include a wide variety of both archaic and crown clade ophiuroid taxa. Stenuroids, virtually unknown post Devonian, are common at most localities and include two species groups. Several genera of protasterids/encrinasterids are present at all localities and in some cases up to four species per locality. Three different species groups of Furcaster are common at most localities including a large sickle-shaped form, a broad shield-shaped form, and a form with large diffuse spine bases with coarse reticulated ornamentation. Two genera of cheiropterasterids are present including Vandelooaster and Armathyraster. Modern clade ophiuroids are represented by four genera including Aganaster. These can be assigned to members of two of the three modern ophiuroid clades showing that the post Permian expansion of ophiuroids was already well under way in the Late Mississippian/Early Pennsylvanian.

This study documents LAPs from five of the nine North American Carboniferous stages with only the Upper Mississippian (Chesterian) represented by more than one formation. Only the Meramacian locality has low diversity but this may result from sparse sampling and low specimen abundance. Other formations have between 7 and 14 species making them among the most diverse Paleozoic brittle star occurrences. When compared to described North American whole specimens from the same interval we have more than quadrupled the diversity even with our very limited sampling.