Morphology and ontogeny of the postcranial skelton of the Hadrosauridae
Hadrosaurids, the "duck-billed" dinosaurs, were a diverse group of large herbivores that were very successful and widely distributed in Laurasia during the Late Cretaceous. The Hadrosauridae is the last clade of ornithopods to evolve in the Cretaceous and are characterized by their broad, edentulous beaks and highly modified dental batteries.
Hadrosaurids provide one of the best datasets of all dinosaurian groups. The discoveries of eggs, embryos, juvenile specimens, bonebed material, trackways, skin impressions, and more have enabled us to ask more biologically complex questions about this group. Because the group is well sampled, there exists one of the best possibilities for studies of ontogeny, functional morphology, sexual dimorphism, and phylogeny, among others.
The postcranial skeleton of the hadrosaurids does not exhibit the morphological diversity of the skull; however, an understanding of the diversity of the appendicular elements is necessary in order to maximize the use of isolated postcranial elements. Qualitative and quantitative analyses confirmed differences in the appendicular skeleton at the family, subfamily, and in some cases generic level. The scapula, humerus, and pelvic girdle elements proved to be particularly distinctive and contain taxonomically informative characters.
With a clearer understanding of the taxonomic significance of the appendicular elements of mature hadrosaurids, a comparative study of the ontogenies of the postcranial skeleton of several hadrosaurid genera was possible. The qualitative analysis found that heterochrony, a change in developmental timing, was responsible for the differing adult morphologies among the hadrosaurid taxa. The qualitative description and assessment of the ontogenetic changes occurring in the hadrosaurid postcranial skeleton provided the background needed for a quantitative approach to analyzing ontogeny.
The event-pairing methodology, applied here for the first time to an ornithischian dinosaur group, established a quantitative framework with which to analyze the influence of sequence heterochrony on the development of the skeleton. The event-pairing analysis concluded that for some postcranial characters of the scapula and humerus a change in the rate of development occurs; however, for other characters of the pelvic girdle, sequence heterochrony was the mechanism behind the developmental changes in that region of the appendicular skeleton.