Paranthodon

Paranthodon (/pəˈrænθədɒn/ pə-RAN-thə-don) is a genus of stegosaurian dinosaur that lived in what is now South Africa during the Early Cretaceous, between 139 and 131 million years ago. Discovered in 1845, it was one of the first stegosaurians found. Its only remains, a partial skull, isolated teeth, and fragments of vertebrae, were found in the Kirkwood Formation. British paleontologist Richard Owen initially identified the fragments as those of the pareiasaur Anthodon. After remaining untouched for years in the British Museum of Natural History, the partial skull was identified by South African paleontologist Robert Broom as belonging to a different genus; he named the specimen Palaeoscincus africanus. Several years later, Hungarian paleontologist Franz Nopcsa, unaware of Broom's new name, similarly concluded that it represented a new taxon, and named it Paranthodon owenii. Since Nopcsa's species name was assigned after Broom's, and Broom did not assign a new genus, both names are now synonyms of the current binomial, Paranthodon africanus. The genus name combines the Ancient Greek para (near) with the genus name Anthodon, to represent the initial referral of the remains
In identifying the remains as those of Palaeoscincus, Broom initially classified Paranthodon as an ankylosaurian, a statement backed by the research of Coombs in the 1970s. In 1929, Nopcsa identified the taxon as a stegosaurid, with which most modern studies agree. In 1981, the genus was reviewed with modern taxonomic techniques, and found to be a valid genus of stegosaurid. A 2018 review of Paranthodon could only identify one distinguishing feature, and while that study still referred it to Stegosauria based on similarity and multiple phylogenetic analyses, no diagnostic features of the group could be identified in Paranthodon.
Paranthodon was a small relative of larger stegosaurids such as Stegosaurus . Thomas R. Holtz Jr. estimated that the animal was 5.0 m (16.4 ft) long and weighed between 454 and 907 kg (1,001 and 2,000 lb).The snout is elongated, though not extremely so, and convex on top. The back of the premaxilla is long and broad, and the external nares are large. The teeth have a prominent primary ridge. The fossilised nasal and maxillary bones are relatively complete, and an incomplete premaxilla is also preserved. The partial snout resembles Stegosaurus in its large posterior premaxillary process and the extension of the palate. Stegosaurus was the only stegosaurid known from adequate cranial material to compare with Paranthodon during the 1981 review of the taxon, and even though their resemblance is great, tooth morphology is very distinguishing among the stegosaurians. For example, cranial material is known from Stegosaurus, Paranthodon, Kentrosaurus , and Tuojiangosaurus , and the tooth morphology differs in all of them.

Maxilla and premaxilla in multiple views
The premaxilla of Paranthodon is incomplete, but the anterior process is sinuous and curves ventrally. This is similar to in Miragaia, Huayangosaurus, the ankylosaur Silvisaurus, and Heterodontosaurus, but unlike in Chungkingosaurus , Stegosaurus, Edmontonia and Lesothosaurus. The premaxilla also lacks any teeth, like in every stegosaur except Huayangosaurus where the premaxilla is preserved. Like in Huayangosaurus but not Stegosaurus or Hesperosaurus , the nasal fenestra faces anterolaterally, being visible from the front and sides. The naris is longer than wide like in other stegosaurs, and also has a smooth internal surface, so it was most likely a simple passage. The maxilla is roughly triangular, as in most other thyreophorans. The tooth row is horizontal in lateral view, and in ventral view it is sinuous. Stegosaurus and Huayangosaurus possess a straight tooth row in ventral view, although Scelidosaurus and Jiangjunosaurus do not. The maxilla of Paranthodon preserves the tooth row, and shows that there is little to no overhang. This differs from ankylosaurians, where there is a large overhang of the maxilla. As with Stegosaurus and Silvisaurus, there is a diastema (gap in the tooth row) on the maxilla in front of the tooth row. The posterior maxilla is incomplete so no information is known about the jugal or lacrimal contact. Paranthodon has an elongate, dorsally convex nasal, like in most other stegosaurs. There are thickened ridges along the sides of the nasals. The preserved portion of the nasal does not contact the premaxilla or maxilla.
Known vertebra fragments, compared to a complete Stegosaurus dorsal vertebra (F)
Thirteen teeth are preserved in Paranthodon, but as they extend to the back of the maxilla there were possibly more in life. The teeth are symmetrical as in stegosaurs except Chungkingosaurus. Along the base of the tooth crown there is a swelling (cingulum), which is seen in all other known stegosaurid teeth except Huayangosaurus. The teeth have a middle ridge, with five fewer prominent ridges on either side. This is similar to the size ridges seen on Kentrosaurus. Like all stegosaurians, the denticles on the teeth are rounded at the tips, in contrast to ankylosaurians. Also, like Huayangosaurus, but unlike Kentrosaurus and Stegosaurus, Paranthodon possesses a prominent buccal margination (a ridge beside the tooth row). Paranthodon teeth preserve wear, but wear is absent on most teeth, similar to Huayangosaurus, meaning it is likely that Paranthodon lacked occlusion between teeth.

As only two fragments of a vertebra are known, few anatomical details can be observed. The right transverse process and prezygapophysis are preserved. The vertebra is possibly a middle dorsal, based on the angle of the transverse process and the orientation of the prezygapophysis. Similar to Stegosaurus and Chungkingosaurus mid-dorsals, the transverse process is angled about 60º dorsally. Unlike in all other stegosaurs except Stegosaurus, the prezygapophysis faces dorsally.