SYMMORIIFORM SHARKS OF THE INDIAN CAVE SANDSTONE (PENNSYLVANIAN OF NEBRASKA) AND THEIR STRATIGRAPHIC CONTEXT

Symmoriiform chondrichthyans are a diverse group of marine Paleozoic vertebrates ranging from the late Devonian Period to Middle Permian Period. From the Indian Cave Sandstone of Nebraska, which was deposited in the upper Pennsylvanian (Gzhelian), three Symmoriiforms have been reported to date: Denaea saltsmani Ginter and Hansen, 2010, Stethacanthus concavus Ginter, 2018, and Stethacanthus sp. Additionally, symmoriiform denticles from the formation may represent a fourth symmoriid similar to the genus Cobelodus Zangerl, 1973. While the original stratigraphic information for these sharks was not reported upon their discovery, our petrographic and faunal analysis of fossil rich zones in the Indian Cave Sandstone indicates that these chondrichthyans were deposited in carbonate lenses or concretions, in a medium-grained, cross-bedded sandstone channel. They co-occur with a mixed assemblage of marine and freshwater vertebrates such as ctenacanth and hitherto unreported symmoriid sharks as well as fragmentary plant fossils. This suggests that this formation held a somewhat richer symoriid community in an estuarine environment, with influxes of nutrients and organisms coming from both the terrestrial and marine realms.