Regnosaurus

Discovery and species

The fossil, a portion of the right lower jaw, was found near Cuckfield in Sussex and became part of the British Museum of Natural History collection. Gideon Mantell noticed the specimen during an 1839 visit and initially believed it to be the lower jaw of a juvenile Iguanodon, presenting it as such to the Royal Society on 8 February 1841. Richard Owen immediately challenged this interpretation, citing a lack of proof. After several actual Iguanodon jaws were discovered by 1848, Mantell revised his view, concluding it belonged to a related but different genus or subgenus, and coined the name Regnosaurus Northamptoni.

The generic name derives from the Regni tribe of Sussex; the specific name honors Spencer Compton, 2nd Marquess of Northampton, then president of the Royal Society. The type species is now written Regnosaurus northamptoni. The holotype, BMNH 2422, is a right mandibular fragment from the Tunbridge Wells Sand Formation, six inches long with fifteen tooth sockets and some replacement teeth. Other bone fragments, such as a pubis from the Isle of Wight and dermal spikes reported by William Blows, have been referred to Regnosaurus but cannot be proven to belong to it.

A possible synonymy with Craterosaurus has been raised but is considered equivocal because the remains cannot be directly compared.

Phylogeny

Initially regarded as an iguanodontid, Regnosaurus was later linked to armored dinosaurs. Richard Lydekker assigned it to Scelidosauridae in 1888, and Friedrich von Huene placed it in the stegosaurian Omosauridae in 1909, while Alfred von Zittel assigned it to Stegosauridae in 1911.

Alfred Romer synonymised it with Hylaeosaurus in 1956, and John Ostrom suggested it might be a sauropod. George Olshevsky in 1993 was the first to state it was a stegosaur in the modern sense, a view supported by Paul Barrett and Paul Upchurch in 1995 based on shared jaw characters with Huayangosaurus. However, subsequent research, including a 2008 study by Susannah Maidment, David Norman, Barrett, and Upchurch, found those characters invalid, shared by non-stegosaurine stegosaurs or thyreophorans more broadly, and concluded Regnosaurus should be regarded as a dubious indeterminate thyreophoran.