Emydops

History and discovery

Emydops was first discovered and named in 1912 by Robert Broom, in which he described the species E. minor. Although later on it was suggested that the genus included 13 more species, Frobisch and Reisz (2008) suggests that only E. oweni and E. arctatus are the only valid species of Emydops.

With the appearance of E. minor marking the beginning of the stratigraphic range of Emydops in the Late Permian-aged Tropidostoma Assemblage Zone from the Karoo Basin of South Africa, it is argued that a specimen found in the vicinity of the Middle Permian to Late Permian Tapinocephalus Assemblage Zone and Middle Permian Pristerognathus Assemblage Zone of Beauford West belongs to Emydops. Whether the specimen is from the Tapinocephalus Assemblage Zone or Pristerognathus Assemblage Zone, both indicate that Emydops stratigraphic range began earlier than it has been previously accepted.

Description

Skull

The skull of Emydops is small, up to 5 cm (2 in) long.

The premaxillae and maxillae of Emydops are less pronounced than those of more mature dicynodonts, and they are generated downward as a distinct outer rim that is most developed around the canines. Anteriorly, the rim continues slightly downwards and forms the tip of the upper beak.{{cite journal |last1=Crompton |first1=A. W. |last2=Hotton III |first2=Nicholas |title=Functional morphology of the masticatory apparatus of two dicynodonts (Reptilia, Therapsida)

Convergently evolved from cynodonts, dicynodonts also have a secondary palate which consists of a broad, flat, horizontal plate formed anteriorly by the palatines. Additionally, it extends posteriorly by the palatines, which dip gently downward to the rear.

The reflected lamina of the angular bone of Emydops is large and fan-shaped, like other small dicynodonts. However, unlike many dicynodonts, the reflected lamina of the angular bone of Emydops has a broad, thin, unsupported sheet that terminates in a long, free border rather than the sheet folding and converging anteriorly toward a marginal thickening on the angular body. As well, a broadened lateral dentary shelf on the lower jaw serves as another distinguishing feature of the genus.

Dentition

Most skulls of Emydops bear a pair of caniniform tusks, diagnostic of dicynodonts, There are generally three postcanine teeth present in the upper jaw of Emydops. They are arranged in a straight line parallel to the mandibular teeth. As seen in cross-section, the crowns of the maxillary postcanine teeth are unserrated and round to slightly widened. In contrast, the mandible of Emydops consists of seven functional teeth, in which the crowns are described as pear-shaped: they have wide, blunt anterior edges and sharp, strongly serrated posterior edges.

The holotype specimen of E. oweni is unusual in that it has two pairs of tusks. The second pair of tusks is not seen in any other dicynodont, and is a feature unique to the specimen. The extra tusks are considered a pathological feature; they are thought to have been the result of a mutation in the individual and are not considered a defining characteristic of the species.

Systematics

Taxonomy

Robert Broom first came up with the name Emydops in 1912 when describing small dicynodonts with unserrated postcanines. Keyser (1993) suggested that the genus be named Emydoses and only include some species previously described and not all of them. having priority over the other described species of Emydops. All of the species before E. oweni were restudied and their taxonomic status was reevaluated. Frobisch and Reisz (2008) argue that the thirteen species before the discovery of E. oweni all fall under E. arctatus.

Phylogeny

Below is a cladogram from Kammerer et al. (2013). The data matrix of Kammerer et al. (2013), a list of characteristics that was used in the analysis, was based on that of Kammerer et al. (2011), which followed a comprehensive taxonomic revision of Dicynodon. Because of this, many of the relationships found by Kammerer et al. (2013) are the same as those found by Kammerer et al. (2011). However, several taxa were added to the analysis, including Tiarajudens, Eubrachiosaurus, Shaanbeikannemeyeria, Zambiasaurus, and many "outgroup" taxa (positioned outside Anomodontia), while other taxa were re-coded. As in Kammerer et al. (2011), the interrelationships of non-kannemeyeriiform dicynodontoids are weakly supported and thus vary between the analyses.

Paleobiology

Feeding

Dicynodonts were dominant tetrapod herbivores most of the Upper Permian, thanks to their masticatory apparatus.

When Emydops ate, the cutting of material happened at the beak when the external adductor muscles applied a vertical force (jaw elevated). Later on in the masticatory cycle, the dentary teeth also did cutting when the external adductor muscles applied an even stronger horizontal force (jaw retracted).

The bottom teeth of Emydops are described as pear-shaped: they have wide, blunt anterior edges and sharp, strongly serrated posterior edges, suggesting that the teeth were effective in cutting only when the jaw moved backwards.

Paleoenvironment

Although speculated to originate from an earlier assemblage zone, it is well known that Emydops was found from the Late Permian Tropidostoma Assemblage Zone of the Karoo Basin, South Africa. Other herbivorous dicynodonts were found in this assemblage zone such as Pristerodon, Diictodon, Tropidostoma, and Endothiodon.

References

References

1. Ray, S.. (2001). "Small Permian dicynodonts from India". Paleontological Research.
2. Fröbisch, J.. (2008). "A new species of ''Emydops'' (Synapsida, Anomodontia) and a discussion of dental variability and pathology in dicynodonts". Journal of Vertebrate Paleontology.
3. (1932). "The Mammal-like Reptiles of South Africa and the Origin of Mammals". H. F. & G. Witherby.
4. (1993). "A re-evaluation of the smaller Endothiodontidae". Memoirs of the Geological Survey of South Africa.
5. "On the stratigraphic range of the dicynodont taxon ''Emydops'' (Therapsida: Anomodontia) in the Karoo Basin, South Africa". Palaeontologia Africana.
6. (2013). "On the Validity and Phylogenetic Position of ''Eubrachiosaurus browni'', a Kannemeyeriiform Dicynodont (Anomodontia) from Triassic North America". PLOS ONE.
7. Kammerer, C.F.. (2011). "A comprehensive taxonomic revision of ''Dicynodon'' (Therapsida, Anomodontia) and its implications for dicynodont phylogeny, biogeography, and biostratigraphy". Journal of Vertebrate Paleontology.