GEOLOGY AND MICROVERTEBRATE PALEONTOLOGY OF THE J&M SITE (UPPER CRETACEOUS: ?CAMPANIAN), WILLIAMS FORK FORMATION, NORTHWESTERN COLORADO

The J&M Site is a fossiliferous locality near Rangely, Colorado. The site lies in the lower half of the Upper Cretaceous Williams Fork Formation, the uppermost member of the Mesaverde Group of NE Utah and NW Colorado. The paleontology of this unit is understudied, but recent work done at the J&M Site suggests that it represents a fluvio-deltaic deposit near the western edge of the Western Interior Seaway (WIS). Both sedimentary and fossil evidence support a fluvial interpretation. Freshwater fossils include bivalve mollusks, gastropods, lepisosteid teeth and scales, amiid teeth, alligatoroid teeth, and turtle shell fragments. Teeth of the lizard Peneteius, dromaeosaurs, hadrosaurs, and multituberculate mammals have been retrieved from this site, but the hadrosaur teeth are more worn than specimens of aquatic animals, potentially suggesting increased transport distance for the terrestrial elements. These fossils were near the base of a cross-bedded sandstone scour that cuts down ~1 m into the underlying finer-grained strata. The lowest 0.5 meters of this scour hold much of the vertebrate fossils from the site. The scour has been interpreted as an avulsion event, potentially occurring during the progradation of a delta lobe. A layer approximately 1 m below the scour base contains abundant plant fossils, amber, and lignite. The presence of this plant material indicates the existence of a wetland environment. While less common than osteichthyan fossils, there are teeth belonging to at least six chondrichthyan taxa that may affect the depositional interpretation. Among these are three rhinobatoid rays (Myledaphus bipartitus, Pseudomyledaphus madseni, and Cristomylus sp.), the hybodont shark Lonchidion griffisi, and the two orectolobiformes Cantioscyllium markaguntensis and Chiloscyllium sp. Of these, M. bipartitus, Chiloscyllium sp., and C. markaguntensis are generally interpreted as being freshwater in origin. L. griffisi is associated with a brackish water locality, while the guitarfish P. madseni and Cristomylus sp. are known from marine-influenced assemblages. The presence of a few possibly marine taxa indicates either proximity to the WIS or that these chondrichthyans are euryhaline in nature.