Heterodontosauridae - Classification

Classification

South African paleontologist Robert Broom created the name Geranosaurus in 1911 for dinosaur jaw bones missing all of the teeth. In 1924, Lycorhinus was named, and classified as a cynodont, by Sidney Haughton. Heterodontosaurus was named in 1962 and it, Lycorhinus and Geranosaurus were recognized as closely related ornithischian dinosaurs. Alfred Romer named Heterodontosauridae in 1966 as a family of ornithischian dinosaurs including Heterodontosaurus and Lycorhinus. It was defined as a clade in 1998 by Paul Sereno and redefined by him in 2005 as the stem clade consisting of Heterodontosaurus tucki and all species more closely related to Heterodontosaurus than to Parasaurolophus walkeri, Pachycephalosaurus wyomingensis, Triceratops horridus, or Ankylosaurus magniventris.

Heterodontosauridae includes the genera Abrictosaurus, Lycorhinus, and Heterodontosaurus, all from South Africa. While Richard Thulborn once reassigned all three to Lycorhinus, all other authors consider the three genera distinct. Within the family, Heterodontosaurus and Lycorhinus are considered sister taxa, with Abrictosaurus as a basal member. Geranosaurus is also a heterodontosaurid, but is usually considered a nomen dubium because the type specimen is missing all its teeth, making it indistinguishable from any other genus in the family. More recently, the genus Echinodon has been considered a heterodontosaurid in several studies. Lanasaurus was named for an upper jaw in 1975, but more recent discoveries have shown that it belongs to Lycorhinus instead, making Lanasaurus a junior synonym of that genus. Dianchungosaurus was once considered a heterodontosaurid from Asia, but it has since been shown that the remains were a chimera of prosauropod and mesoeucrocodylian remains. José Bonaparte also classified the South American Pisanosaurus as a heterodontosaurid at one time, but this animal is now known to be a more basal ornithischian.

The membership of Heterodontosauridae is well-established in comparison to its uncertain phylogenetic position. Several early studies suggested that heterodontosaurids were very primitive ornithischians. Due to supposed similarities in the morphology of the forelimbs, Robert Bakker proposed a relationship between heterodontosaurids and early sauropodomorphs like Anchisaurus, bridging the orders Saurischia and Ornithischia. The dominant hypothesis over the last several decades has placed heterodontosaurids as basal ornithopods. However, others have suggested that heterodontosaurids instead share a common ancestor with Marginocephalia (ceratopsians and pachycephalosaurs), a hypothesis that has found support in a number of recent studies. The clade containing heterodontosaurids and marginocephalians has been named Heterodontosauriformes. Heterodontosaurids have also been seen as basal to both ornithopods and marginocephalians. In 2007, a cladistic analysis suggested that heterodontosaurids are basal to all known ornithischians except Pisanosaurus, a result that echoes some of the very earliest work on the family.