The lost Permo-Carboniferous vertebrate deposit of Horseshoe Bend near Danville, Vermilion County, Illinois

Abstract One of the earliest discoveries of Permo-Carboniferous terrestrial vertebrates in North America occurred in 1875 along Horseshoe Bend, a cutbank on the Salt Fork of the Vermilion River west of Danville, Vermilion County, east-central Illinois. The discovery was soon eclipsed by the description of similar but much more complete remains from the Lower Permian of Texas in 1878. The deposit itself was obliterated by slumping and erosion in the earliest 1900s and has not been collected since despite repeated efforts. Previously unreported outcrop records and subsurface data indicate that the deposit originated as a paleochannel fill in the Inglefield Sandstone Member of the Patoka Formation, which underlies the Macoupin Limestone Member (early Missourian Stage of the Midcontinent, early Kasimovian Stage of global Carboniferous time scale). In addition to aquatic to terrestrial tetrapods, teeth of lungfishes ( Sagenodus Owen, 1867, Conchopoma Cope, 1877a, Gnathorhiza Cope, 1883a) and teeth, occipital spines, and coprolites of a xenacanth shark ( Orthacanthus Agassiz, 1838) are known from Horseshoe Bend. The teeth of the marine petalodont shark Janassa Münster, 1839, also are present in the collection but presumed to have been derived from one of the beds on the cutbank that produced brackish to marine invertebrate fossils. Alhough not diverse, the tetrapod assemblage is significant in that it contains the oldest diplocaulid amphibian ( Diplocaulus salamandroides Cope, 1877a), fragmentary remains of the oldest diadectid and limnoscelid stem reptiles, and possibly the oldest captorhinid eureptile, all of which have not been adequately described. The ophiacodontid synapsid Clepsydrops Cope, 1875, is the most common fossil at Danville, which could be an artifact of primitive collecting methods that did not promote the recovery of articulated material. An accurate stratigraphic placement of the Horseshoe Bend deposit and a review of other late Carboniferous tetrapod localities reveals that this important Illinois locality combines an overlooked vanguard of terrestrial taxa regarded as Permo-Carboniferous (Kasimovian-Asselian) and amphibious to aquatic forms known from older, Moscovian deposits.