THE FAUNA AND AGE OF THE UINTA “A,” UINTA FORMATION (MIDDLE EOCENE), NORTHEASTERN UTAH, USA

Informal subdivisions A-C of the Uinta Formation constitute the richly fossiliferous nominal stratotype of the Uintan North American Land Mammal “Age.” However, the lowermost subdivision of the Uinta Formation, the Uinta A, is sparsely fossiliferous, so its precise age is uncertain. Strata of early Uintan age are known to occur in the Turtle Bluff Member of the Bridger Formation in Wyoming, the Washakie Formation Sand Wash Basin sequence in Colorado, the middle unit of the Adobe Town Member of the Washakie Formation in the Washakie Basin in Wyoming, the lowermost beds of the Tepee Trail Formation in the East Fork Basin in Wyoming, the lower beds of the Devils Graveyard Formation in Texas, and the Friars Formation (lower tongue, Conglomerate Tongue, and upper tongue) and upper part of member B of the Santiago Formation in San Diego County, California.

The Uinta A is presumed to be of early Uintan age or possibly late Bridgerian age based on superpositional relationships. However, confusing stratigraphic nomenclature and museum locality records, combined with the few identifiable mammalian fossils that have been reported from this interval, have to date prevented a confident biochronologic assignment. Carnegie Museum of Natural History records for the general Wagonhound Canyon area (this area includes strata of the Uinta A and B1 as well as the underlying and interfingering Parachute Creek Member of the Green River Formation) include Dolichorhinus sp., Metarhinus earlei, Metarhinus sp., and Sthenodectes sp. An additional taxon, and one that can be more confidently assigned to the Uinta A based on a specimen in the collections of the Utah Field House Museum of Natural History, is Isectolophus sp. The perissodactyl-dominated fauna of the lowermost strata of the Uinta Formation is of little utility for a precise biochronologic assignment, but the presence of the Uintan brontotheres Dolichorhinus sp., Metarhinus sp and Sthenodectes sp. does rule out a late Bridgerian age, and suggests that the interval is correlative with one or more of the biochronologic intervals Ui1a, Ui1b, and Ui2. It is anticipated that future stratigraphically controlled collection efforts will yield more biostratigraphically useful smaller-bodied mammalian taxa from the Uinta A.