Influence of sequence heterochrony on hadrosaurid dinosaur postcranial development

A goal of modern dinosaur paleobiology is to synthesize the understanding of dinosaurian development and phylogeny. This study explores the Iguanodontia, one clade that includes several taxa for which growth series are preserved. It is hypothesized that analysis of growth series of iguanodontian taxa will reveal important developmental differences at play during the evolution of the clade. Such differences can reflect the impact of sequence heterochrony on iguanodontian evolution. Data were collected on the growth stages of the postcranial skeleton of a basal iguanodontian, Tenontosaurus; a lambeosaurine hadrosaurid, Hypacrosaurus; and two hadrosaurine hadrosaurids, Brachylophosaurus and Maiasaura to test the hypothesis. The event-pairing method provides a framework for the comparison of ontogenetic sequences among related taxa to detect significant evolutionary changes in developmental sequence. Significant developmental events are identified for the group and then the relative timings of those events in the developmental sequence are compared among the different species. Synapomorphic characters of Hadrosauridae, including the development of a prominent biceps tubercle on the coracoid, appear to be the result of changes in the developmental sequences of hadrosaurid taxa compared to basal iguanodontian taxa. This analysis also recognizes the developmental sequence differences that result in the development of the robust appendicular elements that characterize Lambeosaurinae. This comparative methodology shows that the development of characters such as the well developed supraacetabular process of the ilium results from the early onset of the development of those characters in the lambeosaurine hadrosaurid Hypacrosaurus compared to the hadrosaurines Brachylophosaurus and Maiasaura.