Temporal range: Early Cretaceous, 145.5–136.4 Ma
|Reconstruction of the skull, grey material is unknown.|
|Species:||† P. africanus|
Palaeoscincus africanus Broom, 1912
Paranthodon (pə-RAN-thə-don) is a genus of extinct stegosaurian dinosaur that lived in South Africa during the Early Cretaceous, approximately 145.5–136.4 million years ago. Discovered in 1845, it was one of the first stegosaurians found. Its only remains, a partial skull and isolated teeth, were found in the Kirkwood Formation. Although Owen initially identified the fragments as those of the pareiasaur Anthodon, after years of storage in the British Museum of Natural History, Broom identified the partial skull as belonging to a different genus, and named the specimen Palaeoscincus africanus. Several years later, Nopcsa, unaware of Broom's new name, similarly concluded that it represented a new taxon, and named the binomial Paranthodon owenii. However, since the Nopcsa's species name was assigned after Broom's, and Broom did not assign a new genus, both names are now synonyms under the current naming, Paranthodon africanus. The genus name was chosen from the Ancient Greek para, "near" and Anthodon, for the originally proposed similarity of the specimens.
In identifying the remains as those of Palaeoscincus, Broom basically classified Paranthodon as an ankylosaurian, a statement backed by the research of Coombs. Nopcsa however, identified the genus as a stegosaurid, which most modern studies agree with. In 1981, the genus was reviewed, and found to be a valid genus of stegosaurid. Paranthodon is one of a few genera found in the Kirkwood Formation; other such taxa include theropods, like Nqwebasaurus; ornithopods; and sauropods, like Algoasaurus.
Discovery and naming
In 1845, amateur geologists William Guybon Atherstone and Andrew Geddes Bain discovered several fossils near Dassieklip, Cape Province, in the Bushmans River Valley. This was the first dinosaur find in all of the Southern Hemisphere and Africa. In 1849 and 1853, Bain sent some of the fossils to the British paleontologist Richard Owen for identification. Among them was an upper jaw Bain referred to as the "Cape Iguanodon"; as such the site was named "Iguanodonhoek". Atherstone published about the find in 1857, but lamented in 1871 that it had thus far received no attention in London. Only in 1876 did Owen name a series of specimens from the collection as Anthodon serrarius. Anthodon means "flower tooth". The partial holotype skull BMNH 47337, the left jaw BMNH 47338, the matrix BMNH 47338 including bone fragments and impressions of the anterior skull, and the vertebrae BMNH 47337a were all assigned to Anthodon. In 1882, Othniel Charles Marsh assigned Anthodon to Stegosauridae based on BMNH 47338, and in 1890, Richard Lydekker found that although Anthodon was a pareiasaur, its teeth were similar to those of Stegosauridae.
In 1909, the South-African paleontologist Robert Broom visited the collection of the British Museum of Natural History. He concluded that Owen had mixed the partial distorted skull, teeth, and a mandible of a pareiasaur and a partial upper jaw of a dinosaur BMNH 47338, which were actually from two different species. Broom kept the name Anthodon for the pareiasaur, but identified the other fossil as a member of the genus Palaeoscincus, naming the new species Paleoscincus africanus in 1912. He found that the anatomy of the teeth were quite different, even thought they resembled each other, as well as those of Stegosaurus. In 1929, Baron Franz Nopcsa, unaware of Broom's previous publication, provided a new name for Broom's P. africanus, as D.M.S. Watson believed that the jaw should be differentiated from Anthodon. Nopcsa named the species Paranthodon Owenii, with the genus name derived from the Latin para, meaning "similar", "near", or "beside", and Anthodon, and specific name honoring Owen. Due to present conventions, the specific name was later emended to owenii. In 1978, Walter Coombs incorporated both names into the current nomenclature, Paranthodon africanus, as Paranthodon was the first new genus for the fossils and africanus was the first named species. This makes Palaeoscincus africanus and Paranthodon owenii junior synonyms of Paranthodon africanus.
The holotype of Paranthodon, BMNH 47338, was found in a layer of the Kirkwood Formation dated between the Berriasian and early Valanginian ages. It consists of the back of the snout, containing the maxilla with teeth, the posterior caudodorsal ramus of the premaxilla, part of the nasals, and some isolated teeth probably from the lower jaw. One additional specimen can be assigned to it based on the dentition, BMNH 47992, including only isolated teeth sharing the same morphology as those from the holotype. However, the teeth do not bear any autapomorphies of Paranthodon, and were referred to an indeterminate stegosaurid in 2008.
Paranthodon was a small stegosaurid relative to larger relatives 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 fossilized 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. However, Stegosaurus is the only stegosaurid known from adequate cranial material to compare with Paranthodon, and even though their resemblance is great, tooth morphology is very distinguishing among the stegosaurians. For example, cranial material is only known from Stegosaurus, Paranthodon, Kentrosaurus, and Tuojiangosaurus, and in all of them, the tooth morphology differs.
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 amount of overhang of the maxilla. The teeth also 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, possibly cause by rubbing with other teeth. However, wear is absent on most teeth, similar to Huayangosaurus, meaning it is likely that Paranthodon lacked occlusion between teeth. Paranthodon is distinguished from other stegosaurs by a long, wide, posterior process of the premaxilla, teeth in the maxilla with a very large cingulum, and large ridges on the tooth crowns. Not all of these features were considered valid in a 2008 review of Stegosauria, with the only autapomorphy found being the possession of a partial second bony palate on the maxilla.
Currently, Paranthodon is classified as a stegosaurus related to Stegosaurus, Tuojiangosaurus, and Loricatosaurus. However, when Broom assigned the name Palaeoscincus africanus to the Paranthodon fossils, he classified them as an ankylosaurian. This classification was later changed by Nopcsa, who found that Paranthodon best resembled a stegosaurid (before the group was truly defined). Coombs (1978) found Paranthodon to be an ankylosaurian, like Brooms, naming it Ankylosauria incertae sedis. However, in reviews by Galton in 1981, it was found to instead represent a stegosaurid from the Late Cretaceous. Multiple phylogenetic analyses have placed Paranthodon in Stegosauria, and often in Stegosauridae. A 2010 analysis including nearly all species of stegosaurians found that Paranthodon was outside Stegosauridae, and in a polytomy with Tuojiangosaurus, Huayangosaurus, Chungkingosaurus, Jiangjunosaurus, and Gigantspinosaurus. However, when the latter two genera were removed, Paranthodon grouped with Tuojiangosaurus just outside Stegosauridae, and Huayangosaurus grouped with Chungkingosaurus in Huayangosauridae. Below is the analysis with all included genera.
Other analyses have found Paranthodon closely related to Tuojiangosaurus, Loricatosaurus, and Kentrosaurus within Stegosaurinae. Even though phylogenetic analyses recognize Paranthodon as a stegosaurid, the type material actually bears no synapomorphies of Stegosauria. However, the material is likely of stegosaurian nature, and phylogenies by many authors have found it to be within the group.
The Kirkwood Formation is in South Africa, and many fossils of different species and genera have been discovered in it, with Paranthodon being the first uncovered. The formation is of a Late Jurassic to Early Cretaceous age, with the oldest deposits from the Tithonian, about 145.5 million years ago, and the youngest rocks being from the Valanginian, about 136.4 million years ago. A large variety of different animal groups have been found in the formation, including dinosaurs, at least two different sphenodontian lizards, multiple teleost fishes, a few crocodylians, some frog specimens, and also turtles. However, a large amount of the material of the Kirkwood formation only includes isolated teeth or partial and fragmentary pieces of bone. Dinosaurs of the formation include a basal tetanuran and the primitive ornithomimosaurian Nqwebasaurus, the possible brachiosaurid Algoasaurus and a potential titanosaurian, and many ornithischians, Paranthodon, a genus of iguanodontian, and a "hypsilophodontid" (the family Hypsilophodontidae is no longer considered to be a natural grouping).
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