SHELLS, SHARKS AND STORMS: COMBINING FOSSIL AND SEDIMENTARY RECORDS TO DESCRIBE AN UPPER CRETACEOUS NEAR-MARINE FOSSIL ASSEMBLAGE FROM NORTHWEST COLORADO

The Upper Cretaceous Williams Fork Formation is the uppermost member of the Mesaverde Group of northwestern Colorado and northeastern Utah. Largely understudied paleontologically, previous work performed at a locality near Rangely, Colorado known as the J&M site suggests that it represents an Upper Cretaceous fluvio-deltaic deposit near the Western Interior Seaway. Sediment and fossil samples collected at the J&M site include unionid mussels, gastropods, lepisosteid ganoid scales, amiid teeth, alligatoroid teeth, and large amounts of turtle shell fragments; all of which support a fluvial environment. Dinosaur and mammal microfossils are also known from the site, but taphonomic processes have worn these down more than the aquatic specimens. The fossiliferous sediment at the J&M site occurs in a cross-bedded sandstone that was deposited in an approximately one meter deep scour into the underlying fine-grained strata. Vertebrate bones and teeth are abundant at the lower 0.5 meters of this scour, which we interpret as an avulsive event, possibly the progradation of a delta lobe. There is a layer about one meter below this that is abundant with plant material and lignite. One sample of sediment, collected approximately two meters below the scour, appears to represent hummocky cross-stratification, indicative of large storm waves in shallow marine waters. These hummocks lie just below the main riverine deposits, and as such show a recently receded coastline in the region. Along with the sediment data, there are several teeth recently recovered from the Williams Fork Formation belonging to selachian taxa that affect our depositional interpretation. Among these are the rhinobatoid rays Myledaphus bipartitus, Cristomylus sp. and Pseudomyledaphus madseni, the hybodont shark Lonchidion griffisi, and the orectolobiformes Cantioscyllium markaguntensis and Chiloscyllium sp. Of these M. bipartitus, Chiloscyllium sp., and C. markaguntensis are believed to be freshwater in origin. P. madseni and Cristomylus sp. have been reported from marine paleofaunal assemblages, while L. griffisi is thought to have inhabited estuarine locations. The presence of these euryhaline elasmobranchs may indicate that the J&M site preserves what was once a riverine environment in close proximity to marine waters.