Pegomastax
| Pegomastax Temporal range:
Early Jurassic, | |
|---|---|
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| Lower jaws of holotype SAM-PK-K10488 | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Family: | †Heterodontosauridae |
| Subfamily: | †Heterodontosaurinae |
| Genus: | †Pegomastax Sereno, 2012[1] |
| Type species | |
| Pegomastax africana[a] Sereno, 2012[2]
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Pegomastax is a genus of
The only known material of Pegomastax included a partial skull with well-preserved
Along with many other heterodontosaurids, Pegomastax is known from the upper
History of discovery
A partial disarticulated skull of a new taxon of
Pegomastax comes from the Red Beds, or upper Elliot Formation, of South Africa and Lesotho. This part of the formation is considered to be from the earliest Jurassic, the Hettangian and Sinemurian epochs from around 200 to 190 million years ago.[1] The provenance of Pegomastax and the Voisana locality within the upper Elliot Formation is uncertain, but it is likely from the middle deposits of the formation, which places it in the mid-Sinemurian.[4][5] Pegomastax is known from a single partial skull, which includes a postorbital, both dentaries, a predentary and teeth; prepared from the matrix at Harvard University. The specimen is preserved in a small sandstone block of matrix, and further preparation of CT scanning may allow for the identification of more intermediate bone fragments. The lower jaws are preserved close to their position in life, but bones on the inside are shifted or displaced if they are preserved.[1]
Description
Pegomastax is estimated to have a skull length of 73 mm (2.9 in), compared to Heterodontosaurus at 115 mm (4.5 in) and Tianyulong at 65 mm (2.6 in). It can be distinguished from other heterodontosaurids by having a very deep predentary that projects forwards and down, a curved primary ridge on the dentary teeth, and a concave margin to the crown on either side of the middle denticle. Though there are many similarities to Heterodontosaurus, the unusually deep, parrot-like shape of the predentary was suggested by Sereno to indicate different feeding specializations among different heterodontosaurids.[1]
A postorbital is known for Pegomastax, with a deep rear process more similar to Manidens than Heterodontosaurus. The predentary of Pegomastax is a simple wedge-shaped bone. Like Heterodontosaurus there are no prominent processes, and the predentary sits on a loose and possibly mobile saddle-like articulation at the front of the dentary. Several foramina perforate the dental edge of the predentary, which is angled about 45 degrees below horizontal, and would have supported a keratinous bill. Unlike all other heterodontosaurs the predentary is much taller than long, with the length of the dental margin only 70% of the total height of the bone.[1] Pegomastax has a robust dentary, with the tooth row being 27 mm (1.1 in) and a depth at mid-length of 9 mm (0.35 in). For comparison, the dentary of Heterodontosaurus has an equivalent depth of 10 mm (0.39 in) and a length of 42 mm (1.7 in), though the dentary of Manidens is slightly deeper than Pegomastax. The teeth in the dentary are inset from the cheek by a buccal emargination, and a foramina at the front end of this emargination near the predentary exists, but does not contain a tract as in Echinodon and Fruitadens. The anterior end of the dentary is deeper than the middle, as in Heterodontosaurus and Abrictosaurus.[1]
There were probably 11 teeth in the lower jaw of Pegomastax, the first eight preserved in the right dentary and the last five in the left. A large caniniform tooth is present at the front of the jaw as in most other heterodontosaurids, but unlike in Heterodontosaurus, Abrictosaurus and Lycorhinus the caniniform is straight. Serrations are present on the front edge of the tooth, but absent on the rear. Following the caniniform, there is a toothless diastema in the jaw, following which the teeth rapidly increase in size up to the sixth. All dentary crowns are asymmetrical, with the front edge higher and the crown slightly curving to the rear along its length. The inner face of the rear crowns is worn nearly flat by jaw action. The apical margins of the crowns bear denticles, which are present on both sides and at the end of the prominent primary ridge.[1]
Classification
Originally identified as an intermediate undescribed member of Heterodontosauridae, the 2012 description of Pegomastax and its placement within the family was supported by a phylogenetic analysis by Sereno. Heteerodontosauridae has historically been considered a sister group to Ornithopoda and a derived family of ornithischians, but since have been reclassified as one of the most primitive forms of ornithischians, with similar features like large grasping hands to basal saurischians like Saturnalia, Eoraptor, Herrerasaurus and Eodromaeus. Sereno also considered Pisanosaurus to be a heterodontosaurid, but noted that its inclusion in the family was controversial and subject to change. Pegomastax was recovered as a derived heterodontosaurid in the subfamily Heterodontosaurinae, united with all other African members of the family and the South American Manidens, which was identified as its closest relative based on the asymmetrical shape and curvature of their tooth crowns.[1] Following this analysis, Pegomastax was added to the matrix of Marcos Becerra and colleagues in 2016, who also added the South American postcranial specimen MPEF-PV 3826. The sister group relationship of Pegomastax and Manidens was supported as reproduced below, even when the postcranial specimen was scored as additional material of Manidens.[6]
| Heterodontosauridae |
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Alternatively, Pegomastax has been resolved as the sister taxon of a group of Heterodontosaurus and Lycorhinus in the 2017 phylogenetic analysis of Daniel Madzia and colleagues, with Manidens two steps more basal between Tianyulong and Fruitadens,[7] or in an unresolved polytomy with Echinodon and Heterodontosaurus, as was the situation in the 2017 phylogenetic analysis of Fenglu Han and colleagues. Han et al. noted that the internal relationships of Heterodontosauridae were weakly supported and needed more work.[8] While previous analyses have all consistently recovered Heterodontosauridae as a group of basal ornithischians, the analysis by Dieudonne and colleagues in 2020, which did not include Pegomastax, recovered heterodontosaurids within Pachycephalosauria, with Heterodontosaurinae and Fruitadens grouped as the most basal pachycephalosaurs, and Echinodon and Tianyulong closer to true pachycephalosaurs.[9]
Paleobiology
Diet and canine function
Heterodontosaurids have traditionally been assumed to be
Ecology
Pegomastax is known from a specimen found in the
Within the upper Elliot Formation and younger
Among non-dinosaur taxa, members of
References
Footnotes
- ^ While originally introduced as Pegomastax africanus, the species name was corrected to Pegomastax africana in order to comply with regulations of the International Code of Zoological Nomenclature, since the Greek word μάσταξ (mastax) is of feminine gender.[2]