178-14 A New Echinoderm Fauna From The Hirnantian Of Indiana

Session: Paleontology, Diversity, Extinction, Origination

Presenting Author:

Authors:

Abstract:

The Hirnantian Stage (Uppermost Ordovician) is a critical interval as it records two major pulses of the Late Ordovician Mass Extinction Event (LOME), the first of Raup and Sepkoski’s “Big Five” mass extinctions. Yet the details of recovery in its immediate aftermath remain poorly constrained. Hirnantian echinoderm faunas are particularly poorly represented across Laurentia owing to several overlapping unconformities, which truncate rocks from this interval and a general absence of articulated material. Due to the poor representation of echinoderms during the Hirnantian of North America, gaps exist in the record of this group’s extinction and recovery following the LOME.  

A new exceptionally well-preserved, low-diversity echinoderm fauna has been recovered from two sections in southeastern Indiana in a 1.5-2m calcareous shale-siltstone and packstone interval representing a local remnant of the Centerville Member of the Whippoorwill Formation, a unit previously only recognized in Ohio. Biostratigraphy of conodonts, brachiopods and corals and C-isotope chemostratigraphy have demonstrated that the Centerville Member in both areas is of late Hirnantian age and has affinities with the “Edgewood-Cathay fauna” of the Mississippi Valley. The new fauna is characterized by patchy, locally dense, clusters of the crinoids Ptychocrinus medinensis Brett and Tornatilicrinus sp. nov., as well as isolated specimens of Dendrocrinus sp., square columnals attributable to the genus Xenocrinus, and scattered occurrences of Protasterid ophiuroids. The new fauna extends the range of P. medinensis into the Hirnantian and includes the first definitive representative of the Tornatilicrinidae within the Katian-Hirnantian interval. Though undescribed ophiuroids have been recovered from the older Girardeau limestone, the new occurrence is the first from the latest Hirnantian Edgewood fauna of North America.  

The newly recognized Centerville echinoderm fauna postdates the well-known Girardeau fauna and records a “recovery fauna” in the immediate aftermath of the second pulse of the LOME. Taphonomic and paleoecological evidence demonstrates that the Centerville fauna occurred in somewhat stressed shallow subtidal conditions, subject to frequent episodic sedimentation influx/turbidity, and possibly fluctuations in salinity. Though elements of the Centerville crinoid fauna do not persist much further into the later Silurian, the association of euryptopic Dimerocrinitid-Dendrocrinid crinoids persists from Hirnantian to Early Devonian. These long ranging survivors persist as opportunistic species that persisted with little change in unstable environments for > 25 myr.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10125

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