ARCHAEOCIDARID ECHINOID JAW ELEMENTS FROM THE UPPERPENNSYLVANIAN BARNSDALL FORMATION OF NORTHERN OKLAHOMA:MORPHOLOGY AND DISTRIBUTIONAL PATTERNS

The fossil record of Paleozoic regular echinoids is notoriously poor, leading to an incomplete understanding of their paleogeographic and paleoenvironmental distribution. The recent discovery of echinoid material from the Upper Pennsylvanian (Kasimovian) Barnsdall Formation near Copan, Washington County, northeastern Oklahoma, has improved the record of echinoids in the North American midcontinent. This material consisted of isolated archaeocidarid spines and interambulacral test plates recovered from 7 horizons within the Copan crinoid Lagerstätte interval of the middle, mudstone-dominated portion of the Barnsdall Formation. Reinvestigation of micropaleontological residues derived from sieving of the sampled interval revealed previously undescribed echinoid jaw elements. A total of 91 isolated, relatively slender hemipyramids with fine serrations were present, overwhelmingly in the 1-2 mm size fraction. Hemipyramid abundance ranged from 0 to 4.44 (mean 1.88) specimens per kilogram, with microstratigraphic distribution generally mirroring overall echinoid ossicle abundance. Any major patterns involving echinoid abundance relative to crinoid abundance were not discernable. No hemipyramids were encrusted or broken, in contrast to co-occurring spines and test plates. Interestingly, the expected relative abundance of echinoid ossicle types (spines > test plates > hemipyramids) is observed in the gravel-sized fraction but not in finer fractions. This study suggests that regular echinoid jaw elements may be present but unreported in Paleozoic mudrocks and are likely to be less taphonomically altered than other echinoid ossicles. Moreover, it highlights the significance of isolated ossicles in improving the record of echinoids in the Paleozoic.