Dinocephalia

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Dinocephalia
Fossil range: Permian
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Synapsida
Order: Therapsida


The infraorder Dinocephalia[1] were a group of medium to large, heavily built therapsid mammal-like reptiles that lived during the Mid-Permian.[2][3] Dinocephalians are among the earliest group of therapsids, and have been found in Brazil,[4] China (Li & Cheng 1995),[5][6][7] Russia[8][9] and southern Africa.[10][2][11][12][13][14][3] Despite an early adaptive radiation,[15] the dinocephalians became completely extinct by the end of the Permian, leaving no descendent lineages.[16]

Early taxonomy

Dinocephalia was initially considered to comprise six families, namely the Estemmenosuchidae, Brithopodidae, Anteosauridae, Titanosuchidae, Tapinocephalidae and the Styracocephalidae[10][16] In 1988 King reclassified these as subfamilies within the three families Estemmenosuchidae, Brithopodidae and Titanosuchidae. She considered Styracocephalus, the only genus within the Styracocephalidae as incertia sedis. Later on the basis of the discovery of several additional specimens, the validity of the genus was confirmed, and considered a sister taxon to the Titanosuchidae. Of the three families of Dinocephalia recognised by King (1988), only the Estemmenosuchidae have yet to be found in Africa.[2]

The subfamily Tapinocephalinae (Lydekker 1890) is considered to be the most derived of the infraorder Dinocephalia (Boonstra 1969). The dentition of this taxon is thought to be more specialised than that of other dinocephalians as their canines are reduced in size and resemble the incisors (Boonstra 1962,[10] 1969). Both the incisors and canines are unusual as they have a “talon” and “heel” structure, with the talons of the upper and lower incisors intermeshing to create a crushing surface between the heels (Boonstra 1962,[10] 1969). Boonstra (1962) also thought that the postcanine teeth of the maxilla may have intermeshed, but to a lesser extent. Tapinocephalians thus had crushing surfaces in the front of their mouths, which was a novel adaptation.

Haughton and Brink (1954) compiled a list of the known genera of Tapinocephalia in South Africa, recognising five families. These were; Mormosauridae: including the genera Mormosaurus (Watson 1914), Struthiocephalus (Haughton 1915a), Struthiocephaloides (Boonstra 1952e), Struthionops (Boonstra 1952b) and Taurocephalus (Broom 1928); Moschopidae: including the genera Avenantia (Boonstra 1952f), Delphinognathus,[17] Moschognathus (Broom 1914), Moschcoides (Byrne 1937), Moschops (Broom 1911) and Pnigalion (Watson 1914); Moschosauridae: including the genera Agnosaurus (Boonstra 1952a) and Moschosaurus (Haughton 1915b); Tapinocephalidae: including the genera Keratocephalus (von Huene 1931), Riebeeckosaurus (Boonstra 1952c), Tapinocephalus (Owen 1876) and Taurops (Broom 1912); and Styracocephalidae; including the genus Styracocephalus (Haughton 1929).

Boonstra[10] used the same classifications as Haughton and Brink (1954), but changed their taxonomic levels to that of subfamily. Moschosaurus was viewed as being morphologically the most primitive of the tapinocephalids yet discovered, and proposed that all other sub-families diversified from the Moschosaurinae[10] Boonstra later thought that the genera Moschosaurus and Struthiocephalellus were “growth stages” of Struthiocephalus (1969, p39) and thus considered them as synonyms. As a result Boonstra (1969) synonymised the Moschosaurinae with the Struthiocephalinae. The Struthiocephalinae were described by Boonstra (1969) as having a long and fairly strong snout, as well as having undergone moderate pachyostosis. Other genera of the Struthiocephalinae are Struthiocephaloides, Struthionops and Taurocephalus.

The Moschopinae consisted of the genera Avenantia, Criocephalus (Broom 1928), Delphinognathus and Moschops. Criocephalus was renamed Criocephalosaurus in 2002 by Kammerer and Sidor, as the name had been used previously to describe a species of cerambycid beetle (Mulsant 1839). The genera Agnosaurus, Moschognathus, Moschoides and Pnigalion were synonymised with the genus Moschops (Boonstra 1969). Boonstra (1969) suggested that Delphinognathus may be a skull of a young Moschops, but did not synonymise the two genera. These tapinocephalids were distinguished as having a short, gently curving snout (Boonstra 1969).

Riebeeckosaurus longirostris (Boonstra 1952c), the only species of the Riebeeckosaurinae, is known from two skulls. These specimens have a long, slender snout and very narrow intertemporal region, which forms a sigittal crest at the posterior of the skull (Boonstra 1969).

Boonstra (1969) described the Tapinocephalinae as large to massive tapinocephalids, with a short, weak snout. Members of this taxon have skulls which are usually heavily pachyostosed, and have either prominent naso-frontal boss or a greatly swollen frons (Boonstra 1969).

According to Boonstra’s (1969) classification the Tapinocephalinae consisted of the genera Keratocephalus (with synonym Pelosuchus (Broom 1905b), see Boonstra 1969), Mormosaurus, Phocosaurus (Seeley 1888) and Tapinocephalus (with synonym Taurops, see Boonstra 1969). King (1988) later reclassified Boontsra’s four subfamilies into three tribes, the Struthiocephalini,[10] Tapinocephalini (Gregory 1926) and the Riebeeckosaurini,[10] The Moschopinae, still considered as a valid taxon by Boonstra (1969), was sunk into the Tapinocephalini by King (1988). These groups were based predominantly on the length of the snout and the degree to which the skull had undergone pachyostosis. The genera contained within each tribe did not differ from Boonstra’s 1969 classification. The Struthiocephalini are characterised as having a reasonably long snout, and having a skull that has undergone moderate pachyostosis (King 1988). Tapinocephalini tend to have short, weak snouts, and great to moderate pachyostification of the skull. Some members of this tribe also have a well developed naso-frontal boss or swollen frons (King 1988).

References

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  1. Seeley, H.G. (1892) On Delphinognathus conocephalus (Seeley) from the middle Karroo beds, Cape Colony, preserved in the South African Museum, Cape Town. Quarterly Journal of the Geological Society, London. 48: 469-475.
  2. 2.0 2.1 2.2 Rubidge, B.S. (1991) A new primitive dinocephalian mammal-like reptile from the Permian of southern Africa. Palaeontology. 34: 547-559.
  3. 3.0 3.1 Kemp, T.S. (2005) The origin and evolution of mammals. Oxford University Press, Oxford.
  4. Langer, M.C. (2000) The first record of dinocephalians in South America, Late Permian Rio do Rosta Formation) of the Paraná Basin, Brazil. Neues Jahrbuch für Geologie und Palaontologie, Abhandlungen. 215: 69-95.
  5. Cheng, Z. & Li, J. (1996) First record of a primitive anteosaurid dinocephalian from the Upper Permian of Gansu, China. Vertebrate PalAsiatica. 34: 123-134.
  6. Cheng, Z. & Li, J. (1997) A new genus of primitive dinocephalian: the third report on Late Permian Dashankou lower tetrapod fauna. Vertebrate PalAsiatica. 35: 35-43.
  7. Li, J., Rubidge, B.S. & Cheng, Z. (1996) A primitive anteosaurid dinocephalian from China – implications for the distribution of earliest therapsid faunas. South African Journal of Science. 92: 252-253.
  8. Ivachnenko, M.F. (1995) Primitive Late Permian dinocephalian-titanosuchids of Eastern Europe. Paleontological Journal. 29: 120-129.
  9. Ivachnenko, M.F. (2000) Estemmenosuchus and primitive theriodonts from the Late Permian. Paleontological Journal. 34: 189-197.
  10. 10.0 10.1 10.2 10.3 10.4 10.5 10.6 10.7 Boonstra, L.D. (1963) Diversity within the South African Dinocephalia. South African Journal of Science. 59: 196-206.
  11. Rubidge, B.S. (1994) Australosydon, the first primitive anteosaurid dinocephalian from the Upper Permian of Gondwana. Palaeontology. 37: 579-594.,
  12. Rubidge, B.S. & van den Heever, J.A. (1997) Morphology and systematic position of the dinocephalian Styracocephalus platyrhynchus. Lethaia. 30: 157-168.
  13. Lepper, J., Raath, M.A. & Rubidge, B.S. (2000) A diverse dinocephalian fauna from Zimbabwe. South African Journal of Science. 96: 403-405.
  14. Munyikwa, D. (2001) Cranial morphology of a primitive dinocephalian from the Madumabisa Mudstone Formation, Zimbabwe. Unpublished MSc dissertation, University of the Witwatersrand.
  15. Ruben, J.A. (1986) Therapsids and their environment, a summary. In: Hotton, N., MaClean, P.D., Roth, J.J. & Roth, E.C. (Eds) The ecology and biology of mammal-like reptiles. Smithsonian Institution Press, Washington. pp 307-312.
  16. 16.0 16.1 Kemp, T. S. (1982) Mammal-like reptiles and the origin of mammals. Academic Press, London.
  17. Seeley, H.G. (1892) On Delphinognathus conocephalus (Seeley) from the middle Karroo beds, Cape Colony, preserved in the South African Museum, Cape Town. Quarterly Journal of the Geological Society, London. 48: 469-475.