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ABSTRACT
A new hadrosaurid dinosaur, Acristavus gagslarsoni, is here named on the basis of several autapomorphic characteristics of the frontal, postorbital, and dentary. Acristavus is a member of the newly erected clade Brachylophosaurini, which along with its other members, Brachylophosaurus and Maiasaura, constitutes the earliest hadrosaurine hadrosaurid clade. The new taxon occurred approximately 79 million years ago and has been recovered from the Two Medicine Formation of western Montana and nearly simultaneously in the Wahweap Formation of southern Utah. Corresponding with its age and relationship to the other members of the Brachylophosaurini, it is not surprising that Acristavus possesses traits seen in both Brachylophosaurus and Maiasaura, but not necessarily shared between them. One of the most interesting morphological features of Acristavus is the lack of cranial osteological ornamentation, which is in stark contrast to every other hadrosaurid dinosaur except Edmontosaurus. Combining stratigraphic and phylogenetic data from Acristavus yields support for the hypothesis that the hadrosaurid ancestor did not possess cranial ornamentation, and that the subfamilies Hadrosaurinae and Lambeosaurinae each independently developed display structures.
ACKNOWLEDGMENTS
Foremost, the authors wish to thank the Larson and Gollehon families of Choteau, Montana, for their continued support of MOR paleontology. For specimen MOR 1155, we thank Old Trail Museum staff and volunteers for discovery and preliminary excavation, MOR staff and volunteers for collection and preparation, and the Larson-Gollehon clan for specimen donation. For specimen UMNHVP 16607, we thank H. Hutchison for assistance relocating the site, and UMNH staff (M. Getty and J. A. Smith) and volunteers (L. and B. J. Nicholls) for collection and preparation, and A. Titus for permits and support. J. Higgins, N. Wilkins, and L. Zanno provided illustrations. L. Zanno, D. Evans, A. Prieto-Márquez, and A. McDonald provided images, publications, and helpful discussions. Comments by P. Barrett, A. Prieto-Márquez, P. Godefroit, and an anonymous reviewer improved the paper. Funding was provided by The Jurassic Foundation (T.A.G. and R.R.H.), Lake Forest College (T.A.G.), Gae A. Weber (R.R.H.), and Discovery Channel (J.R.H.).
Handling editor: Hailu You
