ANATOMICAL EVALUATION OF A DROMAEOSAURID MAXILLA REVEALS MICRORAPTORINE PRESENCE IN THE LATE CRETACEOUS TWO MEDICINE FORMATION OF MONTANA, USA

The microraptorines represent a clade of small, basal dromaeosaurs best known from the Lower Cretaceous of Asia, specifically the Yixian and Jiufotang Formations of Liaoning Province, China. In 2009, a microraptorine pelvic girdle reported from the Upper Cretaceous (Campanian) Dinosaur Park Formation (DPF) of Alberta greatly extended the temporal and geographical range of the clade. A new genus and species, Hesperonychus elizabethae, was erected based on a pelvic girdle. Some unassociated small material from the DPF was also referred to this new species. In the early 1990s, Museum of the Rockies crews discovered a small dromaeosaurid maxilla (MOR 9753) from the middle Two Medicine Formation (TMF) in Glacier County, Montana. The locality, Bob's Vacation Site, primarily preserves elements less than 10 centimeters, with most specimens being 3 centimeters or less. Other dinosaurian remains include a partial eudromaeosaur skeleton not associated with MOR 9753 and isolated Orodromeus material. Lizard elements, associated and articulated frogs, gastropods, and numerous teeth were also reported. Based on an initial interpretation of its anatomy, MOR 9753 was referred to as a juvenile Saurornitholestes. However, a new anatomical re-evaluation identified features not associated with Saurornitholestes and its relatives, such as a squared-off antorbital fenestra, a straight ventral margin, an antorbital fossa ventrally bounded by a prominent lip, and an extremely large, dorsoventrally extended maxillary fenestra. These features align more closely with the microraptorines, a finding further supported by phylogenetic analysis. This maxilla represents the first microraptorine cranial material known from North America. Given its temporal and geographic separation from the Hesperonychus material, it likely represents a distinct taxon. This discovery significantly contributes to our understanding of microraptorine presence and diversity in the region, shedding light on the clade's distribution during the Late Cretaceous.