Postcranial anatomy of Lesothosaurus diagnosticus (Dinosauria: Ornithischia) from the Lower Jurassic of southern Africa: implications for basal ornithischian taxonomy and systematics

Lesothosaurus diagnosticus from the upper Elliot Formation of South Africa and Lesotho (?Hettangian–Sinemurian) is an important early representative of Ornithischia. In previous studies it has been recovered in several positions on the ornithischian tree including as the earliest known member of Thyreophora, the earliest known member of Neornithischia and as a member of an even a more basal ornithischian lineage, one that diverged before these major groups appeared. Given this taxon's important position as one of the few well-known early ornithischians, it is surprising that details of much of its anatomy remain unpublished. A second non-heterodontosaurid ornithischian taxon, also from the upper Elliot Formation, Stormbergia dangershoeki, has also been recovered as the earliest known member of Neornithischia; however, the validity of this taxon has been challenged. Hence, a reassessment of the taxonomic statuses and systematic positions of L. diagnosticus and S. dangershoeki within Ornithischia is required as an important step in the investigation of the early evolution of this group. A comprehensive redescription of the post-cranial material of L. diagnosticus and a re-evaluation of other ‘fabrosaur’ material (including the hypodigm of S. dangershoeki) was carried out to provide new information on the anatomy of these taxa and to better resolve the relationships at the base of the ornithischian tree. This study has found that the material of L. diagnosticus and S. dangershoeki almost certainly represents the same taxon and that the apparent differences between these genera is probably a product of ontogentic (rather than taxonomic) differences. A revision of the phylogenetic relationships of L. diagnosticus (when synonymized with S. dangershoeki) recovers it as the basal most member of Neornithischia.