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Paranthodon Iain Reid* et al.

Abstract Paranthodon (/pəˈrænθədɒn/) is a of extinct stegosaurian that lived in South during the Early , between 139 and 131 million ago. Discovered in 1845, it was one of the first stegosaurians found. Its only remains, a partial and isolated teeth, were found in the . British paleon- tologist initially identified the fragments as those of the . After remaining un- touched for years in the British Museum of , the partial skull was identified by South African pale- ontologist as belonging to a different genus; he named the specimen africanus. Sev- eral 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 name was assigned after Broom's, and Broom did not assign a new genus, both names are now synonyms of the current binomial, Parantho- don africanus. The genus name combines the Ancient Greek para (near) with the genus name Anthodon, to rep- resent 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. However, in 1929, Nopcsa identified the taxon as a stegosaurid, with which most modern studies agree. In 1981, the genus was reviewed with modern , and found to be a valid genus of stegosaurid. However, a 2018 review of Paranthodon could only identify one distin- guishing feature, and while that study still referred it to based on similarity and multiple phylogenetic analyses, no diagnostic features of the group could be identified in Paranthodon.

History of discovery assigned Anthodon to based on BMNH , and in , Richard Lydekker found that alt- In , amateur geologists William Guybon Ather- hough Anthodon was a pareiasaur, its teeth were similar stone and Andrew Geddes Bain discovered several fos- to those of the Stegosauridae.[] Lydekker in also sils near Dassieklip, Cape Province, in the Bushmans corrected a mistake of Owen, who had incorrectly sum- River Valley.[][] This was the first dinosaur find in all of marized all the material as coming from a single local- the Southern Hemisphere and Africa.[] In and ity, whereas there was separate material from two , Bain sent some of the to Richard Owen for clearly distinct localities.[] identification. Among them was an upper Bain re- In , Robert Broom visited the collection of the Brit- ferred to as the "Cape ", so the site was ish Museum of Natural History. He concluded that named "Iguanodonhoek". Atherstone published a short Owen had mixed the partial distorted skull, teeth, and a paper about the discovery in ,[] but lamented in of a pareiasaur and a partial upper jaw of a di- that it had thus far received no attention in Lon- nosaur, BMNH , which were actually from two dif- don.[][] Only in were a series of specimens from ferent species.[][] Broom kept the name Anthodon for the collection named by Owen as Anthodon serrarius, basing the generic name on the resemblance of the teeth to a flower.[] The partial skull BMNH , the left jaw BMNH , the matrix BMNH including fragments and impressions of the anterior skull, and the vertebrae BMNH a were all assigned to Anthodon.[] In ,

*Author correspondence: [email protected] ORCID: 0000-0002-9326-5133 Figure | Reconstruction of the skull of Paranthodon; grey Licensed under: CC-BY-SA material is unknown. Scale bar = cm. Received 18-05-2018; accepted 23-01-2020

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the pareiasaur, but identi- fied the other as a member of the genus Pal- aeoscincus, naming the new species Paleoscincus africanus in . He found that the anatomy of the teeth were quite different, even though they resem- bled each other, as well as those of .[][] In , Franz Nopcsa, una- ware of Broom's previous publication, provided a sec- ond novel name as D.M.S. Watson believed that the jaw should be differenti- ated from Anthodon.[][] Nopcsa named the species Paranthodon Owenii, with the generic name derived from the Latin para, mean- ing "similar", "near", or "be- side", and Anthodon, and specific name honoring Owen.[][][] Per to present conventions, the specific name was later emended to owenii.[] Both names were brought into the cur- rent nomenclature by Wal- ter P. Coombs in his dissertation as the new combination Paranthodon africanus, as the name Paranthodon was the first new generic name for the Figure | Holotype of Anthodon serrarius; the taxon Paranthodon was originally assigned. fossils and africanus was C.L. Griesbach, public domain the first new specific name.[][] This makes Paranthodon africanus the proper name for the taxon previously known as Pal- aeoscincus africanus and Paranthodon owenii.[][]

Material The holotype of Paranthodon, BMNH , was found in a layer of the Kirkwood Formation that has been dated between the and early Val- anginian ages. It consists of the back of the snout, containing the with teeth, the posterior cau- dodorsal ramus of the , part of the nasals, and some isolated teeth probably from the lower jaw. Figure | Previously referred tooth (NHMUK R) in multi- One additional specimen was assigned to it based on ple views. the dentition, BMNH (now NHMUK) R, including Thomas Raven & , CC-BY .

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only isolated teeth sharing the same morphol- ogy as those from the holotype.[] Some that were unidentified by the review of Galton & Coombs () were described as a fragment of a in by Raven & Maidment.[] However, the teeth do not bear any autapo- morphies of Paranthodon and were referred to an indeterminate stegosaurid in .[] The teeth were identified in as also lacking any distinct stegosaurian features, and were thus designated as indeterminate Thyre- Figure | Size comparison between Paranthodon and a human. ophora.[] PaleoGeekSquared, CC-BY-SA . The Mugher Mudstone Formation of Ethiopia stegosaur except where the premaxilla was screened in the s by the University of Califor- is preserved. Like in Huayangosaurus but not Stego- nia Museum of , and in it were discovered saurus or , the nasal fenestra faces anter- multiple dinosaur teeth, pertaining to many groups of olaterally, being visible from the front and sides. The taxa. The locality has been described as "the largest and naris is longer than wide like in other stegosaurs, and also has a smooth internal surface, so it was most likely most complete record of dinosaur fossils from a Late Ju- [] rassic African locality outside of Tendaguru" by Lee a simple passage. The maxilla is roughly triangular, as Hall.[] Two of the partial teeth discovered were re- in most other thyreophorans. The tooth row is horizon- ferred to Paranthodon by Hall and Mark Goodwin in tal in lateral view, and in ventral view it is sinuous. Steg- . However, the reasons for the referral to Parantho- osaurus and Huayangosaurus possess a straight tooth don were not discussed.[] row in ventral view, although and Jiangju- nosaurus do not.[] The maxilla of Paranthodon pre- serves the tooth row, and shows that there is little to no Description overhang. This differs from ankylosaurians, where there is a large amount of overhang of the maxilla.[] Paranthodon was a small relative of larger stegosaurids Like in Stegosaurus and Silvisaurus there is a diastema such as Stegosaurus. Thomas R. Holtz Jr. estimated that the was . m (. ft) long and weighed be- tween and kg (, and , lb).[][] The snout is elongated, though not extremely so, and con- vex on top. The back of the premaxilla is long and broad, and the external nares are large. The teeth have a prom- inent primary ridge. The fossilized nasal and maxillary bones are relatively complete, and an incomplete pre- maxilla is also preserved. The partial snout resembles Stegosaurus in its large posterior premaxillary process and the extension of the palate. However, Stegosaurus was the only stegosaurid known from adequate cranial material to compare with Paranthodon during the 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, , and , 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 , but unlike in Chung- kingosaurus, Stegosaurus, and Lesotho- Figure | Holotype maxilla and premaxilla in multiple view. saurus. The premaxilla also lacks any teeth, like in every Thomas Raven & Susannah Maidment, CC-BY .

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transverse process is angled about º dorsally. Unlike in all other stegosaurs except Stegosaurus, the prezyg- apophysis faces dorsally.[]

Classification Currently, Paranthodon is classified as a stegosaur re- lated to Stegosaurus, Tuojiangosaurus, and Loricatosau- rus. Initially, when Broom assigned the name Pal- aeoscincus africanus to the Paranthodon fossils, he clas- sified 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 () did not follow Nopcsa's classification, keeping Paranthodon as an an- Figure | Known vertebra fragments; compared to a complete kylosaurian, like Broom, although he only classified it as Stegosaurus dorsal. incertae sedis.[] A subsequent review by Thomas Raven & Susannah Maidment, CC-BY . Galton and Coombs in instead confirmed Nopcsa's interpretation, redescribing Paranthodon as a stego- (gap in the tooth row) on the maxilla in front of the [][] tooth row. The posterior maxilla is incomplete so there saurid from the . Paranthodon was is no information known about the jugal or lacrimal con- distinguished from other stegosaurs by a long, wide, tact.[] Paranthodon has an elongate, dorsally convex posterior process of the premaxilla, teeth in the maxilla with a very large cingulum, and large ridges on the nasal, like in most other stegosaurs. There are thick- [] ened ridges along the sides of the nasals. The preserved tooth crowns. Not all of these features were consid- portion of the nasal does not contact the premaxilla or ered valid in a review of Stegosauria, with the only maxilla.[] found being the possession of a partial second bony palate on the maxilla.[] teeth are preserved in Paranthodon, but as they ex- tend to the back of the maxilla there were possibly more Multiple phylogenetic analyses have placed Parantho- don in Stegosauria, and often in Stegosauridae. A in life. The teeth are symmetrical as in stegosaurs ex- cept Chungkingosaurus. Along the base of the tooth analysis including nearly all species of stegosaurians crown there is a swelling (cingulum), which is seen found that Paranthodon was outside Stegosauridae, in all other stegosaurs from teeth are known, 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 promi- nent 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 occlu- sion between teeth.[] As only two fragments of a vertebra are known, lit- tle anatomical details can be observed. The right transverse process and prezygapophysis are pre- served. The vertebra is possibly a middle dorsal, based on the angle of the transverse process and the orientation of the prezygapophysis. Similar to Steg- Figure | Hypothetical restoration based on related stegosaurians. osaurus and Chungkingosaurus mid-dorsals, the Michael Bech, CC-BY-SA .

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Other analyses have found Paranthodon closely related to Tuojiangosaurus, Loricatosaurus, and Kentrosaurus within Stegosaurinae.[][] Even though phylogenetic analyses recognize Paranthodon as a stegosaurid, the material actually bears no synapomorphies of Stegosauria. However, the material is very likely repre- sentative of the group, with phylogenetic analyses per- formed in separate studies by separate authors having resolved it as a stegosaurian.[][]

Paleoecology As a stegosaur, Paranthodon would have been a slow- moving, mid-level quadrupedal browser that defended itself using a tail with spines on it.[] The Kirkwood Formation is in , and many fossils of different species and genera have been discov- ered in it, with Paranthodon being the first uncov- ered.[] The formation is of a Late to Early Cre- taceous age, with the oldest deposits from the Titho- nian, about . million years ago, and the youngest rocks being from the , about million years ago.[][][] The specific vertebrate- bearing por- tion of the formation is approximately level with the up- per region of the Sundays River Formation, which has been dated to to mya based on microfossils.[] Figure | Holotype nasal in multiple views. A large variety of different animal groups have been Thomas Raven & Susannah Maidment, CC-BY . found in the formation, including , at least two different sphenodontian , multiple teleost fishes, and in a polytomy with Tuojiangosaurus, Huayango- a few crocodylians, some frog specimens, and also tur- saurus, Chungkingosaurus, Jiangjunosaurus, and Gi- tles. However, a large amount of the material of the gantspinosaurus. However, when the latter two genera Kirkwood formation only includes isolated teeth or par- were removed, Paranthodon grouped with Tuojiango- tial and fragmentary pieces of bone. Dinosaurs of the saurus just outside Stegosauridae, and Huayangosaurus formation include a tetanuran, the primitive orni- grouped with Chungkingosaurus in Huayangosauri- thomimosaurian , the sauropod Algo- dae.[] An elaboration upon this analysis was published asaurus, a potential titanosaur, many ornithischians, in by Susannah Maidment and Thomas Raven, Paranthodon, a genus of iguanodontian, and a which achieved much greater resolution of the relation- "hypsilophodontid" (the family Hypsilophodontidae is ships inside Stegosauria. Nearly all described stegosaur taxa were included, and Parantho- don grouped with Tuojiangosaurus, Huayangosaurus and Chunkingo- saurus as the most basal true stego- saurians. However, the position of Alcovasaurus was uncertain, and further work could yield different results, with the phylogeny vulnera- ble to changes in taxon sampling. Below is the single most parsmoni- ous tree recovered from analysis of a combined dataset of discrete and continuous characters in that [] study. Figure | The Kirkwood Formation, the location of the fossil discovery

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no longer considered to be a natural grouping[]).[][] References Multiple additional sauropod taxa have been discov- ered in the Kirkwood deposits, including a basal eusau- . Atherstone, W.G. (). "Geology of Uitenhage". The Eastern Province Monthly Magazine (): –. ropod, a brachiosaurid, a dicraeosaurid and a derived . de Klerk, W.J. (). "South Africa's first dinosaur revisited - history of the diplodocid.[] discovery of the stegosaur Paranthodon africanus (Broom)". Annals of the Eastern Cape Museums : –. ISSN -. If the referral of teeth from Ethiopia to Paranthodon is . Durand, J.F. (). "Major African contributions to Palaeozoic and vertebrate palaeontology". Journal of African Earth Sciences correct, then the taxons geographic range is extended (): . doi:./j.jafrearsci.... significantly. The Mugher locality is approximately . Atherstone, W.G. (). "From Graham's Town to the Gouph". Selected articles from the Cape Monthly Magazine (New Series -). Cape Town: million years old, about million older than has previ- Van Riebeeck Society. ously been suggested for Paranthodon, as well as across . Owen, R. (). Descriptive and illustrated catalogue of the fossil Reptilia both southern and eastern Africa. The fauna in the of South Africa in the collection of the British Museum. Order of the Trustees. pp. –. Mugher locality differ from elsewhere of the same time . Galton, P.M.; Coombs, W.P. Jr. (). "Paranthodon africanus (broom) a and place in Africa. While the Tendaguru has abundant stegosaurian dinosaur from the Lower Cretaceous of South Africa". Geobios (): –. doi:./S-()-. stegosaurs, sauropods, ornithopods and theropods, the . Broom, R. (). "Observations on some specimens of South African fossil Mugher Mudstone preserves the stegosaur Parantho- preserved in the British Museum". Transactions of the Royal Society of South Africa : –. doi:./. don, a hypsilophodontid ornithopod, a probably sauro- . Nopsca, F. (). 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