A PRELIMINARY ANALYSIS OF THE MOGAN SITE, A NEW LATE JURASSIC VERTEBRATE MICROFOSSIL SITE IN THE MORRISON FORMATION IN NORTHEASTERN, WYOMING

We report here a new microvertebrate assemblage from the Upper Jurassic (~150 Ma) Morrison Formation of northeastern Wyoming.The Mogan Site assemblage was collected from the lower portion of the Morrison Formation, which primarily consists of bentonitic mudstones and shales. The dinosaurian fossils in the region are principally non-diagnostic bone fragments of larger taxa. The vertebrate microfossils are disarticulated and/or fragmentary long bones, vertebrae, teeth, and possible cranial pieces belonging to osteichthyans, amphibians, lizards, crocodilians, and mammals. Some millimeter scale gastropod shells are also present. It is important to note that the microvertebrate sites found in this area contain a large quantity of relatively diagnostic fossils, and record a greater diversity of taxa at higher taxonomic levels than the more frequently collected large dinosaur specimens. The fossils we describe here represent the result of a modest (~20kg) screen washed sample of matrix, and consist of disarticulated and disassociated vertebrate hard parts of multiple taxa within the millimeter to centimeter size range. These remains include actinopterygian teeth and tooth plates, lissamphibian dentary fragments, a crocodilian tooth, a pleurodont jaw that may represent a lepidosauromorph, and two multituberculate molars. Actinopterygians include ziphodont teeth types as well as crushing tooth plates. The lissamphibian fossil is an incomplete left dentary fragment and preserves at least 8 tooth positions. The pleurodont jaw is also a left and preserves all or part of 6 tooth positions. The crocodilian tooth is striated, from the lower left or upper right side of the jaw, and is not particularly tall. The multituberculate teeth are an M2/ with cusp formula 2:3:4 and a broken M/1 with seven cusps preserved. The Mogan Site is unusual for the Morrison in yielding a significant concentration of small vertebrate remains, and these microvertebrate fossils, though not as prevalent as the larger dinosaur specimens, represent a wide diversity of taxa at a higher taxonomic level that improve our understanding of the small vertebrate component of the Morrison ecosystem in this region.