THE NORTHERNMOST OCCURRENCE OF A DEINOCHEIRID ORNITHOMIMOSAUR FROM MONTANA (UPPER CRETACEOUS, JUDITH RIVER FORMATION): ITS PALEOBIOGEOGRAPHIC AND BIODIVERSITY SIGNIFICANCE

Y. Kobayashi,  Ts. Chinzorig, R. Takasaki, A. Fiorillo , K. Chiba,, M. Saneyoshi, S. Ishigaki

SVP annual meeting 2023

Society of Vertebrate Paleontology

Ohio, USA

Ornithomimosauria is a clade of maniraptoriforms that are edentulous in derived taxa and flourished during the Late Cretaceous in the Northern Hemisphere. Edentulous taxa form at least two clades: Ornithomimidae and Deinocheiridae. While members of Ornithomimidae are medium to large in body size, highly specialized in cursoriality that were diversified and rich in both Asia and North America, members of Deinocheiridae, exemplified by the gigantic Deinocheirus mirificus from Mongolia, are less cursorial and were recognized only in Asia until the recent discovery of Paraxenisaurus from Mexico. Here we report a pair of nearly complete edentulous dentaries, discovered from a Corythosaurus quarry of the late Campanian Coal Ridge Member of the Judith River Formation, northern Montana, approximately 30 km southeast to the type section of the Judith River Formation. As in other edentulous ornithomimosaurs, it has a dorsolaterally facing, well-developed cutting edge anteriorly, occupies more than one-third of the preserved length, and a mediolaterally thickened dorsal rim posteriorly throughout its length. The dentaries are dorsoventrally high and massive. The maximum height reaches as much as 40% of the length from the anterior tip to the estimated anterior margin of the external mandibular fenestra, similar to Deinocheirus mirificus. Additionally, this specimen shows a significant symphysial expansion, an anteroposteriorly long Meckelian groove, and a convex dorsal outline of the cutting edge with the anteriorly downturned dentaries. The combination of these characters suggests that this material belongs to Deinocheiridae. This is the first record of the family from the northern North American continent, extending the biogeographic range of the group approximately ~3,000 km northward. Along with the occurrence of the Asian ornithomimid Qiupalong from the Belly River Group of Alberta (penecontemporaneous to the Judith River Formation), the new material suggests more active inter-continental faunal exchange of ornithomimosaurs between Asia and North America during the Late Cretaceous than previously suggested. The material discussed here not only adds further details to the biogeography of North American ornithomimosaurs but also illustrates potential for understanding dinosaur diversity even within one of the world’s best sampled dinosaurbearing rock sequences.