DIFFERENTIATING SPECIES OF THE LATE PALEOZOIC OPHIUROID FURCASTER IN NORTH AMERICA

While the use of ophiuroid lateral arm plates (LAPs) to differentiate species is well established in Mesozoic and Cenozoic strata, this line of research is in its infancy among Paleozoic taxa. Among the most common Late Paleozoic brittle stars, Furcaster offers an opportunity to study species differentiation and proximal to distal variation within this clade. Furcaster LAPs are characterized by an arcuate to triangular main plate body, a well-defined vertebral articulation along the proximal-ventral interior plate corner, well-defined spine facets along the distal margin and a series of large groove spine facets along the ventral margin. Plate shape changes along the length of the arm with more arcuate plates proximally, becoming more triangular distally while bearing fewer spines along the distal margin. However, relative size and orientation of spine bases and surface ornamentation remain relatively constant along the length of the arm.

Large collections of isolated ophiuroid plates have been collected from numerous Carboniferous - Permian formations in the North American Mid-continent showing an unexpected diversity and abundance. Within these collections, three tentative Furcaster species groups can be identified that all persist minimally from the middle Mississippian through Lower Permian. Group 1 LAPs are large and broad, bearing well defined spine bases along a scalloped distal plate margin and a generally smooth plate surface externally. Proximally, these plates are arcuate and transition to broadly triangular distally. Group 2 LAPs are similar to Group 1, but smaller, with more distally directed spine bases and stronger scalloping along the ventral margin for the groove spines. These are morphologically discrete from Group 1 and do not represent an earlier ontogenetic stage. Group 3 LAPs are more triangular, similar to distal Group 1 LAPs, but bear a strongly pitted exterior surface ornament. The spine bases are large but subtle, and the plates lack a strongly scalloped distal margin. Groove spines are also more weakly developed. Within species groups individual species can be discriminated by surface ornamentation, size distribution of spine bases and spine base counts on proximal elements. Similar studies are underway with other stem lineage ophiuroid genera.