Paranthodon

Paranthodon ( ) 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".
History of discovery.
In 1845, amateur geologists William Guybon Atherstone and Andrew Geddes Bain discovered several fossils near Dassieklip, Cape Province, in the Bushman's River Valley. This was the first dinosaur find in Africa and in the Southern Hemisphere. In 1849 and 1853, Bain sent some of the fossils to palaeontologist Richard Owen for identification. Among them was an upper jaw Bain referred to as the "Cape "Iguanodon"", so the site was named "Iguanodonhoek". Atherstone published a short paper about the discovery in 1857, but lamented in 1871 that it had thus far received no attention in London. In 1876 Owen named a series of specimens from the collection "Anthodon serrarius", basing the generic name on the resemblance of the teeth to a flower. 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 the Stegosauridae. Lydekker in 1890 also corrected a mistake of Owen, who had incorrectly summarised all the material as coming from a single locality, whereas there was separate material from two clearly distinct localities.
In 1909, 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 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 though they resembled each other, as well as those of "Stegosaurus". In 1929, Franz Nopcsa, unaware of Broom's previous publication, provided a second novel name as D. M. S. Watson believed that the jaw should be differentiated from "Anthodon". Nopcsa named the species "Paranthodon Owenii", with the generic name derived from the Latin "para", meaning "similar", "near", or "beside", and "Anthodon", and specific name honouring Owen. Following modern conventions, the specific name was later emended to "owenii". Both names were brought into the current nomenclature by Walter P. Coombs in his 1971 dissertation as the new combination "Paranthodon africanus", as the name "Paranthodon" was the first new generic name for the fossils and "africanus" was the first new specific name. This makes "Paranthodon africanus" the proper name for the taxon previously known as "Palaeoscincus africanus" and "Paranthodon owenii".
Material.
The holotype of "Paranthodon", BMNH 47338, was found in a layer of the Kirkwood Formation that has been 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 was assigned to it based on the dentition, BMNH (now NHMUK) R4992, including only isolated teeth sharing the same morphology as those from the holotype. Some bones that were unidentified by Galton & Coombs (1981) were described as a fragment of a vertebra in 2018 by Raven & Maidment. The teeth do not bear any autapomorphies of "Paranthodon", and were referred to an indeterminate stegosaurid in 2008. The teeth were identified in 2018 as also lacking any distinct stegosaurian features, and were thus designated as Thyreophora indeterminate.
The Mugher Mudstone of Ethiopia was screened in the 1990s by the University of California Museum of Paleontology, and in it were discovered multiple dinosaur teeth, pertaining to many groups of taxa. The locality has been described as "the largest and most complete record of dinosaur fossils from a Late Jurassic African locality outside of Tendaguru". Two of the partial teeth discovered were referred to "Paranthodon" by Lee Hall and Mark Goodwin in 2011. The reasons for the referral to "Paranthodon" were not discussed.
Description.
"Paranthodon" was a small relative of larger stegosaurids such as "Stegosaurus". Thomas R. Holtz Jr. estimated that the animal was long and weighed between . 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.
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.
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.
Classification.
Currently, "Paranthodon" is classified as a stegosaur related to "Stegosaurus", "Tuojiangosaurus", and "Loricatosaurus". Initially, 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) did not follow Nopcsa's classification, keeping "Paranthodon" as an ankylosaurian, like Broom, although he only classified it as Ankylosauria "incertae sedis". A subsequent review by Galton and Coombs in 1981 instead confirmed Nopcsa's interpretation, redescribing "Paranthodon" as a stegosaurid from the Late Cretaceous. "Paranthodon" was 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.
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". When the latter two genera were removed, "Paranthodon" grouped with "Tuojiangosaurus" just outside Stegosauridae, and "Huayangosaurus" grouped with "Chungkingosaurus" in Huayangosauridae. An elaboration upon this analysis was published in 2017 by Susannah Maidment and Thomas Raven, and it resolved relationships within Stegosauria much more. All taxa were remained included, and "Paranthodon" grouped with "Tuojiangosaurus", "Huayangosaurus" and "Chunkingosaurus" as the most basal true stegosaurians. The position of "Alcovasaurus" was uncertain, and further work could change the result. Below is the analysis.
Other analyses have found "Paranthodon" closely related to "Tuojiangosaurus", "Loricatosaurus", and "Kentrosaurus" within Stegosaurinae. Even though phylogenetic analyses recognise "Paranthodon" as a stegosaurid, the type material bears no synapomorphies of Stegosauria. The material is likely of stegosaurian nature, and phylogenies by many authors have found it to be within the group.
Paleoecology.
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 130 million years ago. The specific vertebrate-bearing portion of the formation is approximately level with the upper region of the Sundays River Formation, which has been dated to 139 to 131 mya based on microfossils. 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. 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, the primitive ornithomimosaurian "Nqwebasaurus", the sauropod "Algoasaurus", a potential titanosaurian, many ornithischians including "Paranthodon", a genus of iguanodontian, and a "hypsilophodontid" (the family Hypsilophodontidae is no longer considered to be a natural grouping). Multiple additional sauropod taxa have been discovered, including a basal eusauropod, a brachiosaurid, a dicraeosaurid and a derived diplodocid.
If the referral of teeth from Ethiopia to "Paranthodon" is correct, then the taxon's geographic range is extended significantly. The Mugher locality is approximately 151 million years old, about 14 million older than has previously been suggested for "Paranthodon", as well as across both southern and eastern Africa. The fauna in the Mugher locality differ from elsewhere of the same time and place in Africa. While the Tendaguru has abundant stegosaurs, sauropods, ornithopods and theropods, the Mugher Mudstone preserves the stegosaur "Paranthodon", a hypsilophodontid ornithopod, a probable sauropod, and theropods related to Allosauridae and Dromaeosauridae.