Evolution of Hadrosaurs in the Campanian of Laramidia: New Information from the Skull Roof and Braincase

Abstract

This project seeks to investigate the morphology, ontogeny, and evolution of skull roof and braincase characters in hadrosaurids from the Campanian of northern Laramidia (Alberta and Montana), a setting in which they have been historically recognized as having a high abundance and diversity. New material is evaluated to clarify the distribution and informativeness of morphological character states, allow further testing of previous hypotheses about how the ontogenetic development of these characters evolved within clades, and enable the identification of previously unrecorded taxa. Five partial skulls of Maiasaura peeblesorum are described from a bone bed in the Two Medicine Formation of Montana, providing the basis for a description of the braincase morphology of this taxon, and an ontogenetic series allowing the development of the crest to be compared to related taxa. Two new partial skulls from the middle unit of the Oldman Formation at the Milk River Ridge Reservoir near Warner, southern Alberta both represent first occurrences of a taxon in that unit. The first is described as the first diagnostic occurrence of Maiasaura in Canada. Maiasaura and Brachylophosaurus in the middle unit of the Oldman Formation are the first example of co-occurring brachylophosaurins. The second is described as a new taxon of Parasaurolophini, and is the oldest diagnostic lambeosaurine in Alberta. A phylogenetic analysis using maximum parsimony and Bayesian methods supports these identifications, and also re-identifies a previously described specimen from the Dinosaur Park Formation as the geologically youngest brachylophosaurin in Alberta.