A LATE HIRNANTIAN FAUNA IN EROSIONAL REMNANT OF THE WHIPPOORWILL FORMATION IN SOUTHEASTERN INDIANA

Recent fieldwork along creek sections south of Greensburg, Indiana, has uncovered a Hirnantian-age faunal assemblage with strong affinities to the earlier recognized Centerville fauna in Ohio. Well preserved fossils occur in a newly recognized ~1.5 m interval of silty shale and sandy limestones. Brachiopods, corals, trilobites, and echinoderms of about 15 species were uncovered. Two age-diagnostic brachiopods, Mendacella sp. and Hindella ovoides, both occur in the Hirnantian aged Ellis Bay Formation of Anticosti Island, as well as the Edgewood Group of Missouri. Abundant specimens of the solitary rugose coral Streptelasma subregulare were also found. S. subregulare is known to occur in other Hirnantian strata, including the Keel and lowermost Cochrane formations in south-central Oklahoma, Leemon Formation in S Illinois and SE Missouri, the Cyrene and Bryant Knob formations in NE Missouri, Wilhelmi Formation in NE Illinois, and lower Mosalem Formation in NW Illinois and E Iowa. Recovered dalmanitid trilobite cephala and pygidia from this locality resemble the species Mucronaspis mucronata, which belongs to the globally widespread “Hirnantia Fauna.” Numerous, well-preserved specimens of Ptychocrinus sp. as well as unidentified species of cladid and disparid crinoids and ophiuroids were also found. The Ptychocrinus specimens resemble P. medinensis found in the Power Glen Shale of western New York, which is arguably earliest Rhuddanian in age.

Additionally, δ¹³C_carb values of samples from Greensburg, IN. revealed a positive excursion of up to ~2.5‰, comparable to that in the typical Centerville Member of the Whippoorwill Formation, from West Union and Fairborn, OH (~20 km NE of Centerville, OH). Given the paleontological context, this excursion is identified as a late phase of the Hirnantian Isotopic Carbon Excursion (HICE). We infer that the new unit is an erosional remnant of the Centerville Member possibly preserved in a paleochannel cut in underlying Richmondian strata. The sharp basal contact is identified as the Cherokee Unconformity. The megafauna combined with the HICE leads to new interpretations on the age of the Whippoorwill, as late Hirnantian. This is the first record of the HICE and of definite Hirnantian faunas in Indiana and Ohio, which elsewhere are truncated beneath a major unconformity at the base of the Silurian.