Phylogeny of iguanodontian dinosaurs and the evolution of quadrupedality

Iguanodontians are a large and biogeographically widespread group of dinosaurs, known from every modern continent, with a temporal range from the Late Jurassic through the Late Cretaceous. While the nested hadrosauroids have been studied extensively, the phylogeny of non-hadrosauroid iguanodontians remains less clear. This study presents a character matrix with 323 characters, and both parsimony and time-calibrated Bayesian analyses. While these result in different topologies, they both recover a Thescelosauridae outside of Iguanodontia. Within Iguanodontia, they both recover Muttaburrasaurus and Tenontosaurus as sister taxa to Rhabdodontidae, with a larger group of Gondwanan Rhabdodontoidea (nomen cladi novum) in the Bayesian analysis. A small Dryosauridae forms the sister group to Ankylopollexia, which has Uteodon and Camptosaurus as the most basally branching taxa. Within Styracosterna two distinct clades are recovered: Iguanodontidae, and a group of taxa with robust forelimbs. The holotype of Mantellisaurus is sister to “Dollodon”, supporting the hypothesis that these taxa are synonymous. The “hatchet-shaped” sternal thought to be a synapomorphy of Styracosterna occurs in two taxa recovered outside that group: Macrogryphosaurus and the unnamed taxon from the Kirkwood Formation of South Africa. Characters associated with quadrupedality are mapped on the phylogeny, indicating a transition from bipedality to quadrupedality occurred in a stepwise manner at the base of Ankylopollexia. Based on synapomorphies of the groups, the major innovations in Ankylopollexia were postcranial, while those of hadrosauroids were centered on dentition and the dentaries. There is a clear faunal succession from the Late Jurassic to Early Cretaceous non-hadrosauroid ankylopollexians to the Late Cretaceous hadrosauroids. Karen Poole. Department of Basic Sciences, New York Institute of Technology College of Osteopathic Medicine at Arkansas State University, Wilson Hall, 2405 Aggie Rd., Jonesboro Arkansas 72401, USA. kpoole@nyit.edu