Systematics of hypsilophodontidae and basal Iguanodontia (dinosauria: Ornithopoda)

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Abstract
The phylogenetic relationships of species attributed to the ornithopod family Hypsilophodontidae are evaluated using morphological characters from the skull, dentition, and postcranium. Based on our analyses, Hypsilophodontidae constitutes a monophyletic taxon that comprises the sister taxon to Iguanodontia, together forming Euornithopoda. Three clades within the family are consistently demonstrated: Zephyrosaurus schaff+Orodromeus makelai, Parksosaurus warreni+Hypsilophodon foxii and Yandusaurus hongheensis+Othnielia rex. Thescelosaurus neglectus is the sister taxon to these six genera and constitutes the basal hypsilophodontid. Tenontosaurus tilletti is the basal member of Iguanodontia, with species of Dryosaurus and Camptosaurus as higher taxa within the clade. To understand the effects missing data may have on tree topology, tree length, and consistency indices, poorly represented characters were secondarily removed from the character matrix. In these analyses, all relationships remain stable, but tree length and consistency index decrease with increasingly more complete culled data sets. An average of 42.5 million years is accumulated as minimal divergence time for the hypsilophodontid and basal iguanodontian relationships described here. These figures underscore the large amount of hypsilophodontid evolution yet unaccounted for in the fossil record.