BIOSTRATIGRAPHIC AND PALEOBIOLOGIC IMPLICATIONS OF A PLATE OF UINTACRINUS SOCIALIS GRINNELL 1871 FROM THE SMOKY HILL MEMBER OF THE BLUE GATE SHALE (MANCOS GROUP, UPPER CRETACEOUS) IN EAST CENTRAL UTAH

The crinoid Uintacrinus is an index fossil for the Upper Santonian (Upper Cretaceous) worldwide. In the U.S. western interior, the species U. socialis is restricted to the Upper Santonian and most commonly found with the ammonites Desmoscaphites erdmanni and D. bassleri, which respectively define the last two ammonite zones of the Santonian. We here document a new plate of U. socialis, the first reported from Utah since its first report and discovery by O. C. Marsh in 1870. The crinoid was found and collected by the second author in 2009, about 13km southeast of Green River, Utah, in the Smoky Hill Member of the Blue Gate Shale (Mancos Group, Upper Cretaceous).

Identifiable index fossils recently found associated with the crinoid are the ammonite Scaphites leei I, and the bivalve Sphenoceramus lundbreckensis. Scaphites leei I, found in a concretion ~8 meters below the level of the crinoid plate is most often found in the middle Upper Santonian D. erdmanni ammonite zone. Sphenoceramus lundbreckensis, found on top of a nearby concretion ~1 meter above the crinoid level and also in a concretionary bed ~6 meters above, is found in middle and upper Upper Santonian rocks throughout the Euramerican biogeographical region. Considering these data and reports of D. bassleri from higher in the section near the base of the Prairie Canyon Formation at Hatch Mesa, we consider the crinoid plate and its containing rocks to be from the middle Upper Santonian.

Uintacrinus socialis is now considered an epibenthic, gregarious crinoid. The current plate has 7 or 8 visible subspherical calyxes from 4 to 6 cm in diameter that would have had arms up to 1 meter in length. The thickness of the present plate ranges from 4 to 24 mm, averaging 11mm. At this time, we are unable to reconstruct the entire plate but can estimate the size to be ~75cm², very much smaller than some of the plates found farther east in Colorado and Kansas, one measuring ~93m². The articulated parts of the crinoid are preserved in a bed of crinoidal limestone made of disarticulated calyx plates and brachials surrounded by the dark gray fissile shale. The small size of this specimen and paucity of other crinoid colonies in the eastern Utah area remains to be understood. This may be attributable to the water depth or the amount of siliciclastic sediment; with the more eastern sediments containing more chalk.