Dicynodont

Dicynodonts
Temporal range: Middle Permian to Late Triassic (Possible Early Cretaceous record.)[1]
Skull of the dicynodont Endothiodon angusticeps in the American Museum of Natural History
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Order: Therapsida
Suborder: †Anomodontia
Clade: †Chainosauria
Infraorder: †Dicynodontia
Owen, 1859
Clades & Genera

see "Taxonomy"

Dicynodontia is a taxon of anomodont therapsids or synapsids with beginnings in the mid-Permian, which were dominant in the Late Permian and continued throughout the Triassic, with a few possibly surviving into the Early Cretaceous. Dicynodonts were herbivorous animals with two tusks, hence their name, which means 'two dog tooth'. They are also the most successful and diverse of the non-mammalian therapsids, with over 70 genera known, varying from rat- to ox-sized.

Characteristics

Dicynodont fossils.
Diictodon. Life-sized model.

The dicynodont skull is highly specialised, light but strong, with the synapsid temporal openings at the rear of the skull greatly enlarged to accommodate larger jaw muscles. The front of the skull and the lower jaw are generally narrow and, in all but a number of primitive forms, toothless. Instead, the front of the mouth is equipped with a horny beak, as in turtles and ceratopsian dinosaurs. Food was processed by the retraction of the lower jaw when the mouth closed, producing a powerful shearing action,[2] which would have enabled dicynodonts to cope with tough plant material. Many genera also have a pair of tusks, which it is thought may have been an example of sexual dimorphism.[3]

The body is short, strong and barrel-shaped, with strong limbs. In large genera (such as Dinodontosaurus) the hindlimbs were held erect, but the forelimbs bent at the elbow. Both the pectoral girdle and the ilium are large and strong. The tail is short.

History

An illustration of the skull of Dicynodon lacerticeps, first published in an 1845 description by Richard Owen.

Dicynodonts have been known since the mid-1800s. The South African geologist Andrew Geddes Bain gave the first description of dicynodonts in 1845. At the time, Bain was a supervisor for the construction of military roads under the Corps of Royal Engineers and had found many reptilian fossils during his surveys of South Africa. Bain described these fossils in an 1845 letter published in Transactions of the Geological Society of London, calling them "bidentals" for their two prominent tusks.[4] In that same year, the English paleontologist Richard Owen named two species of dicynodonts from South Africa: Dicynodon lacerticeps and Dicynodon bainii. Since Bain was preoccupied with the Corps of Royal Engineers, he wanted Owen to describe his fossils more extensively. Owen did not publish a description until 1876 in his Descriptive and Illustrated Catalogue of the Fossil Reptilia of South Africa in the Collection of the British Museum.[5] By this time, many more dicynodonts had been described. In 1859, another important species called Ptychognathus declivis was named from South Africa. A year later, in 1860, Owen named the group Dicynodontia.[6] In his Descriptive and Illustrated Catalogue, Owen honored Bain by erecting Bidentalia as a replacement name for his Dicynodontia.[5] The name Bidentalia quickly fell out of use in the following years, replaced by popularity of Owen's Dicynodontia.[7]

Evolutionary history

Eodicynodon, a primitive dicynodont from the middle Permian of South Africa

Dicynodonts first appear during Middle Permian, and underwent a rapid evolutionary radiation, becoming the most successful and abundant land vertebrates of the Late Permian. During this time, they included a large variety of ecotypes, including large, medium-sized, and small herbivores and short-limbed mole-like burrowers.

Unnamed giant dicynodont from Late Triassic of Poland

Only four lineages are known to have survived the end of Permian extinction; the first three represented with a single genus each; Myosaurus, Kombuisia and Lystrosaurus, the latter being the most common and widespread herbivores of the Induan (earliest Triassic). None of these survived long into the Triassic. The fourth group was the Kannemeyeriiformes, the only dicynodonts who diversified during the Triassic.[8] These stocky, pig- to ox-sized animals were the most abundant herbivores worldwide from the Olenekian to the Ladinian age. By the Carnian they had been supplanted by traversodont cynodonts and rhynchosaur reptiles. During the Norian (middle of the Late Triassic), perhaps due to increasing aridity, they drastically declined, and the role of large herbivore was taken over by sauropodomorph dinosaurs.

Fossils discovered in Poland indicate that dicynodonts survived at least until the early Rhaetian (latest Triassic).[9] Six fragments of fossil bone interpreted as remains of a skull discovered in Australia (Queensland) might indicate that dicynodonts survived into the Cretaceous in southern Gondwana.[1] However, Agnolin et al. (2010) considered the affinities of the specimen from Australia to be uncertain, and noted its similarity to the skull bones of some baurusuchian crocodyliforms, such as Baurusuchus pachecoi.[10]

With the decline and extinction of the kannemeyerids, there were to be no more dominant large synapsid herbivores until the middle Paleocene epoch (60 Ma) when mammals, descendants of cynodonts, began to diversify after the extinction of the dinosaurs.

Systematics

Taxonomy

Dicynodontia was originally named by the English paleontologist Richard Owen. It was erected as a family of the order Anomodontia and included the genera Dicynodon and Ptychognathus. Other families of Anomodontia included Gnathodontia, which included Rhynchosaurus (now known to be an archosauromorph) and Cryptodontia, which included Oudenodon. Cryptodonts were distinguished from dicynodonts from their absence of tusks. Although it lacks tusks, Oudenodon is now classified as a dicynodont, and the name Cryptodontia is no longer used. Thomas Henry Huxley revised Owen's Dicynodontia as an order that included Dicynodon and Oudenodon.[11] Dicynodontia was later ranked as a suborder or infraorder with the larger group Anomodontia, which is classified as an order. The ranking of Dicynodontia has varied in recent studies, with Ivakhnenko (2008) considering it a suborder, Ivanchnenko (2008) considering it an infraorder, and Kurkin (2010) considering it an order.[12]

Many higher taxa, including infraorders and families, have been erected as a means of classifying the large number of dicynodont species. Cluver and King (1983) recognized several main groups within Dicynodontia, including Diictodontia, Endothiodontia, Eodicynodontia, Kingoriamorpha, Pristerodontia, and Venyukoviamorpha.[13] Many families have been proposed, including Cistecephalidae, Diictodontidae, Dicynodontidae, Emydopidae, Endothiodontidae, Kannemeyeriidae, Kingoriidae, Lystrosauridae, Myosauridae, Oudenodontidae, Pristerodontidae, and Robertiidae. However, with the rise of phylogenetics, most of these taxa are no longer considered valid. Kammerer and Angielczyk (2009) suggested that the problematic taxonomy and nomenclature of Dicynodontia and other groups results from the large number of conflicting studies and the tendency for invalid names to be mistakenly established.[7]

Wadiasaurus.
Kingoria, a small dicynodont from Africa's Upper Permian
Placerias.
Moghreberia.
Myosaurus.

Unknown placement:

Phylogeny

Below is a cladogram modified from Angielczyk and Rubidge (2010) showing the phylogenetic relationships of Dicynodontia:[14]

Dicynodontia 

Eodicynodon

Colobodectes

Lanthanostegus
 Pylaecephalidae 

Robertia

Diictodon

Prosictodon

Chelydontops

Endothiodon

Pristerodon
 Emydopoidea 

Emydops


Myosaurus


 Kingoriidae 

Dicynodontoides

Kombuisia




 Cistecephalidae 

Cistecephalus

Cistecephaloides

Kawingasaurus






 Pristerodontia 


Interpresosaurus

Elph

Rhachiocephalus
 Cryptodontidae 

Oudenodon

Tropidostoma

Australobarbarus




 Geikiidae 

Odontocyclops

Idelesaurus

Aulacephalodon

Geikia

Pelanomodon









 Dicynodontoidea 

Katumbia

Delectosaurus

Dicynodon

Lystrosauridae

Kannemeyeriiformes

Vivaxosaurus















See also

References

  1. 1 2 Thulborn, T.; Turner, S. (2003). "The last dicynodont: an Australian Cretaceous relict". Proceedings of the Royal Society B: Biological Sciences 270 (1518): 985–993. doi:10.1098/rspb.2002.2296. JSTOR 3558635.
  2. ↑ Crompton, A. W, and Hotton, N. 1967. Functional morphology of the masticatory apparatus of two dicynodonts (Reptilia, Therapsida). Postilla, 109:1–51
  3. ↑ Colbert, E. H., (1969), Evolution of the Vertebrates, John Wiley & Sons Inc (2nd ed.), p. 137
  4. ↑ Bain, A.G. (1845). "On the discovery of fossil remains of bidental and other reptiles in South Africa". Transactions of the Geological Society of London 1: 53–59. doi:10.1144/GSL.JGS.1845.001.01.72.
  5. 1 2 Owen, R. (1876). Descriptive and Illustrated Catalogue of the Fossil Reptilia of South Africa in the Collection of the British Museum. London: British Museum. p. 88.
  6. ↑ Owen, R. (1860). "On the orders of fossil and recent Reptilia, and their distribution in time". Report of the Twenty-Ninth Meeting of the British Association for the Advancement of Science 1859: 153–166.
  7. 1 2 Kammerer, C.F.; Angielczyk, K.D. (2009). "A proposed higher taxonomy of anomodont therapsids" (PDF). Zootaxa 2018: 1–24.
  8. ↑ On the Validity and Phylogenetic Position of Eubrachiosaurus browni, a Kannemeyeriiform Dicynodont (Anomodontia) from Triassic North America
  9. ↑ Jerzy Dzik, Tomasz Sulej, and Grzegorz Niedźwiedzki (2008). "A dicynodont−theropod association in the latest Triassic of Poland". Acta Palaeontologica Polonica 53 (4): 733–738. doi:10.4202/app.2008.0415.
  10. ↑ Agnolin, F. L.; Ezcurra, M. D.; Pais, D. F.; Salisbury, S. W. (2010). "A reappraisal of the Cretaceous non-avian dinosaur faunas from Australia and New Zealand: Evidence for their Gondwanan affinities". Journal of Systematic Palaeontology 8 (2): 257–300. doi:10.1080/14772011003594870.
  11. ↑ Osborn, H.F. (1904). "Reclassification of the Reptilia". The American Naturalist 38 (446): 93–115. doi:10.1086/278383.
  12. ↑ Kurkin, A.A. (2010). "Late Permian dicynodonts of Eastern Europe". Paleontological Journal 44 (6): 72–80. doi:10.1134/S0031030110060092.
  13. ↑ Cluver, M.A.; King, G.M. (1983). "A reassessment of the relationships of Permian Dicynodontia (Reptilia, Therapsida) and a new classification of dicynodont". Annals of the South African Museum 91: 195–273.
  14. ↑ Kenneth D. Angielczyk; Bruce S. Rubidge (2010). "A new pylaecephalid dicynodont (Therapsida, Anomodontia) from the Tapinocephalus Assemblage Zone, Karoo Basin, Middle Permian of South Africa". Journal of Vertebrate Paleontology 30 (5): 1396–1409. doi:10.1080/02724634.2010.501447.

Further reading

External links

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