Skulls of "brachylophosaurin" hadrosaurs. Left,
outlines of skulls of Acristavus gagslarsoni (based on type and referred specimen),
B, Maiasaura peeblesorum, TCMI 2008.89.2,
C, Brachylophosaurus canadensis, MOR 794.
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.
Phylogenies of Hadrosauridae. A, after Gates et al., 2011, based on the analysis of Horner et al., 2004. B, after Prieto-Márquez, 2010.
Gates, T.A.; Horner, J.R.; Hanna, R.R.; and Nelson, C.R. 2011. New unadorned hadrosaurine hadrosaurid (Dinosauria, Ornithopoda) from the Campanian of North America . Journal of Vertebrate Paleontology. 31 (4): 798–811. DOI:10.1080/02724634.2011.577854
Horner, J. R., Weishampel, D. B. & Forster, c. A. 2004. Hadrosauridae. pp.438–463 in Weishampel, Dodson & Osmólska (eds.) The Dinosauria (second edition). University of California Press (Berkeley, California, USA).
Prieto-Márquez, A. 2010. Global phylogeny of Hadrosauridae (Dinosauria: Ornithopoda) using parsimony and Bayesian methods. Zoological Journal of the Linnean Society. 159:435–502.