Evolution and ontogeny of Hadrosaurs (Dinosauria: Ornithischia) in the Judith River Formation (late Cretaceous: Campanian) of northcentral Montana

Abstract

Hadrosaurs, the 'duck-billed' dinosaurs, were abundant members of Campanian (Late Cretaceous) communities in the Western Interior of North America, and are thus an ideal group for studying high-resolution evolutionary trends. This dissertation describes two new hadrosaurine taxa from the Judith River Formation exposed in Kennedy Coulee, northcentral Montana. The localities are stratigraphically equivalent to Unit 1 of the Oldman Formation, a time interval with no previously described hadrosaurine species. Phylogenetic and geometric morphometric analyses, combined with recalibrated radiometric dates, demonstrate that the new taxa form morphologic and stratigraphic intermediates within the lineages of Gryposaurus and Acristavus-Brachylophosaurus. The new genus of brachylophosaurin has a short posteriorly-oriented nasal crest hypothesized as an intermediate evolutionary state between the stratigraphically lower crestless Acristavus (lower Two Medicine Formation) and the stratigraphically higher Brachylophosaurus (middle Oldman Formation), with its wide posteriorly elongated crest. The nasal crest of Brachylophosaurus elongates posteriorly ontogenetically (as the individual grows). Histologic analysis demonstrates that the holotype of the new genus is relatively more mature than the largest Brachylophosaurus specimen, so its smaller crest size is not due to the ontogenetic status of the holotype. The new species of Gryposaurus is from a monodominant bonebed of at least ten individuals and three size classes: juvenile, subadult, and adult. The taphonomy of the bonebed is interpreted as a mass death assemblage deposited along a lake margin. The abundant postcranial bones are illustrated in a photographic atlas. In the new Gryposaurus species, the shape and position of the nasal crest is morphologically intermediate between the stratigraphically lower G. latidens (lower Two Medicine Formation) and the stratigraphically higher G. notabilis (lower Dinosaur Park Formation). In G. latidens, the nasal crest is low and anterodorsal to the posterior narial fenestra. The nasal crest becomes progressively higher and more posteriorly located in stratigraphically younger species. A similar trend occurs ontogenetically within specimens of the same species. Thus, in Gryposaurus and Acristavus-Brachylophosaurus lineages, directional trends in nasal crest morphology are observed both through ontogeny and between stratigraphically separated non-overlapping taxa, suggesting that the new taxa may be transitional members of anagenetic evolutionary lineages.