DISCOVERY OF EARLY JURASSIC AMMONOIDS FROM THE DEATH VALLEY NATIONAL PARK (CALIFORNIA, USA): IMPLICATIONS FOR BIOSTRATIGRAPHY, PALEOBIOGEOGRAPHY AND POST-EXTINCTION RECOVERY

The number of Jurassic ammonoid localities known from western North America is far less than those in Europe and their fauna remains less studied. However, analysis of faunal assemblages may add crucial information to our understanding of the tectonically complex orogen of the North American Cordillera. The new locality presented here contributes to our knowledge of the Early Jurassic Eastern Pacific ammonoid faunas and their biostratigraphy and paleobiogeographic distribution. The new data may also be used to constrain reconstructions of paleogeography and tectonic history. The studied assemblages provide insight into the ammonoid diversification following the devastating end-Triassic extinction.

The Death Valley National Park, within the Basin and Range Province near the border of California and Nevada, features bedrocks ranging in age from the Neoproterozoic to present. The new locality at Butte Valley exposes the Mesozoic formation of the same name, first described by B. K. Johnson in 1957. The age of the formation was originally thought to be Early Triassic, on the basis of a few poorly preserved ammonoids. However, here we present newly obtained fossil collections of Jurassic age from the topmost part of the formation. Nearly 200 specimens were collected at the re-discovered fossil locality of Johnson and compared to the existing museum collection. Although species-level identification is hampered by the poor state of preservation, ammonite biostratigraphy allows assignment of the studied section to the early-middle Hettangian, using quantitative biostratigraphic methods (Unitary Associations and Horizon Annealing). The lower part falls into the Minutus Zone, whereas the upper part represents the Coronoides Zone of the North American regional zonation. The genera present in the collection are best compared with those from the Gabbs Valley area in Nevada and Haida Gwaii (Queen Charlotte Islands) in British Columbia. At each of the three areas cosmopolitan genera (e.g. Kammerkarites and Franziceras) dominate the assemblages, supplemented by few other East Pacific taxa (e.g. Eolytoceras).

Previous misidentification and erroneous age assignment reflect morphological similarities of post-extinction assemblages. The revised Early Jurassic, rather than Early Triassic age of the uppermost Butte Valley Formation requires reconsideration of depositional, paleogeographic and tectonic models of the Mojave Desert region.