THE FIRST RECORD OF HIRNANTIAN ECHINODERMS FROM INDIANA AND OHIO

Hirnantian-Rhuddanian (latest Ordovician-early Silurian) echinoderm faunas are poorly known across eastern Laurentia owing to multiple extensive periods of erosion within and following this interval. The late Hirnantian (H2) unconformity records erosion of early Hirnantian strata, and later Silurian unconformities record erosion of later Hirnantian-earliest Rhuddanian strata. As a result, the effects of differential extinction in echinoderms through the major Late Ordovician mass extinction (LOME) interval in Laurentia are not fully recorded, nor is the recovery interval well documented. Echinoderms are a key component of the middle Paleozoic evolutionary fauna and their poorly understood history during the LOME is one of the major gaps in our current understanding of Paleozoic fossil communities.

A new low-diversity, locally dense echinoderm fauna was recovered from calcareous shale and siltstone in southern Indiana which appears to be a preserved remnant of the Centerville Member of the Whippoorwill Formation, previously mapped only in southern Ohio. The new echinoderm fauna is represented by five genera and five species (Ptychocrinus medinensis Brett, Tornatilicrinus sp. nov., Xenocrinus sp., Dendrocrinus sp., and undetermined ophiuroids.).This extends the known range of Ptychocrinus medinensis down into the Hirnantian, previously P. medinensis being recorded only from the Rhuddanian of New York and southern Ontario. The occurrence of Tornatilicrinus sp. nov. extends the genus from the Caradocian to the Hirnantian. Additionally, distal stem and holdfast morphology of the Tornatilicrinidae are preserved for the first time.

The newly described Centerville echinoderm fauna is the first record of late Hirnantian echinoderms in the immediate aftermath of the second pulse of the Hirnantian extinction in the United States and co-occurs with the cosmopolitan Edgewood-Cathay fauna. The low diversity-high dominance communities suggest stressed conditions, including frequent episodic sedimentation/turbulence events and possibly slightly low and fluctuating salinity. Similar, opportunistic dendrocrinid - dimerocrinitid associations persist as local dense patchy assemblages in otherwise barren facies upward into at least to the Early Devonian.