Description of the first definitive Corythosaurus (Dinosauria, Hadrosauridae) specimens from the Judith River Formation in Montana, USA and their paleobiogeographical significance

Ryuji Takasaki;Kentaro Chiba;Anthony R. Fiorillo;Kirstin S. Brink;David C;Evans, Federico Fanti;Mototaka Saneyoshi;Anthony Maltese;Shinobu Ishigaki

The late Campanian Judith River Formation in northern Montana, USA has long been recognized as a dinosaur-bearing rock unit. Despite the long study history in this formation, most of the vertebrate fossils are represented by fragmentary remains, making precise taxonomic identifications difficult. Contrary to this, the partially contemporaneous Dinosaur Park Formation, Alberta, Canada is known for its tremendous fossil preservation, permitting rigorous studies of dinosaur diversity, evolution, and biostratigraphy. Hadrosaurids comprise one of the most abundant dinosaur clades in the Dinosaur Park Formation, but taxonomic affinities of hadrosaurid specimens remain poorly understood in the Judith River Formation. Corythosaurus is the most common hadrosaurid in the Dinosaur Park Formation, and has been restricted to this formation to date. This study reports the first definitive Corythosaurus specimens from the Judith River Formation, which were discovered on two private ranches in northern Montana. The attribution of the most complete skeleton to Corythosaurus is indicated by the combination of the following characters: wide crest-snout angle, presence of premaxilla-nasal fontanelle, dorsoventrally expanded nasal, laterally exposed ophthalmic canal of the laterosphenoid, and tall neural spines. A second specimen preserves a largely ilium that can positively identified as Corythosaurus due to its association with a largely complete articulated skull. The specimens were recovered from the Coal Ridge Member of the Judith River Formation, which is approximately time equivalent to the Dinosaur Park Formation. Thus, the discovery of Corythosaurus in the Judith River Formation extends the biogeographic range of this genus and establishes a framework for future interformational biostratigraphic studies of Late Cretaceous dinosaur faunas in North America. Further discoveries of the shared dinosaur taxa between these two formations are expected to construct the interformational biostratigraphic framework.

Anatomical Record

doi.org/10.1002/ar.25097