Paleoecology and taphonomy of the Willow Tank Formation (Albian), Southern Nevada
Bonde, Joshua William.
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This study documents fossil remains from the Willow Tank Formation and places those remains into a taphonomic and sedimentological context in order to determine the paleoecology of southern Nevada during the Early Cretaceous (Albian). Recovered taxa include Lepisosteidae, Ceratodus, Holostean A, Naomichelys, Baenidae, c.f. Adocus, possible Trionychidae, Crocodyliformes, Thyreophora, Iguanodontia, Titanosauriformes, Tyrannosauroidea, Dromaeosauridae, indet. Theropoda, and two fern morphotypes. Sedimentology of the fossiliferous unit of the Willow Tank Formation suggests these taxa were deposited in an anastomosed fluvial system. Interpretation of an anastomosed fluvial system is based in part upon an overwhelming abundance of overbank fines, single storied channel fills, lack of lateral accretion structures, and common crevasse splay sandstones. Observed paleosols commonly contain carbonate nodules associated with mottled red-green mudrocks. The carbonate nodules are consistent with as seasonally arid environment and reddening of beds may suggest a well drained floodplain. Taphonomic modes include microsite, subaqueous bonebed, subaerial bonebed, and channel fill assemblages. Vertebrate fossils are found predominantly in overbank settings. The fauna of the Willow Tank Formation most resembles that of the Cedar Mountain Formation. Unlike the discrete, temporal, fauna assemblages of the various members of the Cedar Mountain Formation, the Willow Tank Formation fauna contains a mix of these different stratigraphic faunas. One example is the co-occurrence of Early Cretaceous iguanodon-grade and Late Cretaceous hadrosaur-grade teeth. Another example being the presence of a tyrannosauroid tooth in Albian beds of the Willow Tank Formation, where tyrannosauroids are not found in the Cedar Mountain Formation until the Cenomanian. Therefore, Willow Tank Formation strata may shed light on biogeographic and evolutionary relationships at the Early-Late Cretaceous boundary.