Cranial osteology and morphology of the type specimen ofMaiasaura peeblesorum(Ornithischia: Hadrosauridae), with a discussion of its phylogenetic position

ABSTRACT Maiasaum peeblesorum has a number of primitive characteristics, including an elongated facial region, restricted nares, deflected premaxillae, and an apparent remnant of the supraorbital fenestra, all of which are more common to the iguanodonts than to other hadrosaurs. It also has an incipient crest similar to that of Prosaurolophus, and posterior skull elements similar to Brachylophosaurus. The anterior premaxillary foramina may have served the function of vocalization or possibly for passage of olfactory organs. Growth stages of the dentary teeth indicate that the "budding teeth" were small enough to have been introduced into the alveoli through "special foramina." Individual tooth growth, from implantation to occlusion, occurred with initial growth of the apex and continued backward. Disregarding the nasal/frontal crest as a feature warranting separate subfamilial status, and thus following Steinberg's classification of hadrosaurs, Maiasaura is placed into the subfamily Hadrosaurinae. Western North American hadrosaurine phylogeny has been affected by the transgressive/regressive cycles of the western interior seaway. Opportunistic hadrosaur and other dinosaur lineages that radiated into lowland environments during periods of regression were highly specialized, but terminated during transgressive periods. Dinosaurs such as Maiasaura, endemic to "upland" environments and unaffected by the cyclic fluctuations, retained their primitive characters.