COMPARING MORPHOLOGICAL DIFFERENCES IN MISSISSIPPIAN OPHIUROID ARM OSSICLES

The echinoderm group, Ophiuroidea, has a long fossil history but still remains largely understudied especially in the Paleozoic. Most of the documentationhas been through the identification of fully articulated specimens that show the gross overall morphology, but the detailed anatomy of the isolated skeletal elements as has been the primary focus of post-Paleozoic studies. Though this work has provided a general outline of ophiuroid evolution, there are large gaps in our knowledge, mainly in the Paleozoic. Recently, sediment collected from an Upper Mississippian (Chesterian) aged locality near Sulphur, IN was found to contain disarticulated ossicles of several genera of Paleozoic ophiuroids. This provides an opportunity to study the morphologyof the individual ossicles of the arm in several taxa. Numerous vertebrae and lateral arm plates (LAP) of these echinoderms have been found and an examined to characterize these species.

Currently, at least six taxa have been recognized. In most cases, the morphology of the vertebrae and LAPs are easily differentiated and can be investigated separately. There are two furcasterid taxa in the fauna. While the vertebrae and LAP morphologies share a number of similarities, these taxa also exhibit considerable variation along the length of the arms as well as ontogenetically. In furcasterid type 1 the vertebrae are rectangular, slightly longer than wide with a distinct median groove. These vertebrae are correlated with a J-shaped LAP with large identifiable spine articulations that range from very narrow to somewhat wider proximally to distally. Furcasterid type 2 vertebrae are rectangular in lateral view and triangular cross-section with large conspicuous podial basins. These vertebrae are associated with LAP’s that are more robust, having a rhombus-like shape and possess fewer spine articulations than the furcasterid type 1. These plates also show less variation than those of furcasterid type 1.

Proximal to distal variation of these vertebrateis qualitatively and quantitativelyillustrated.Differences seen among populations of these skeletal elements reflect proximal to distal changes within the arms and ontogenetic changes. Further work will expand these observations into character data to help infer the phylogeny of Paleozoic Ophiuroids.