| Dicynodonts Temporal range: 268–105 Ma Middle Permian to Early Cretaceous[1] |
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| Skull of the dicynodont Endothiodon angusticeps in the American Museum of Natural History | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Synapsida |
| Order: | Therapsida |
| clade: | †Chainosauria |
| Infraorder: | †Dicynodontia Owen, 1859 |
| Clades & Genera | |
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see "Taxonomy" |
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Dicynodontia is a taxon of anomodont therapsids or mammal-like reptiles with beginnings in the mid-Permian, which were dominant from Late Permian through the Triassic, with a few surviving into the Early Cretaceous. Dicynodonts were small to large 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.
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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 through retraction of the lower jaw when the mouth closed, producing a powerful shearing action (Crompton and Hotton 1967), 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 (Colbert 1969 p.137)
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.
Dicynodonts have been known since the mid 1800s. 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.[2] In that same year, 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.[3] 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.[4] In his Descriptive and Illustrated Catalogue, Owen honored Bain by erecting Bidentalia as a replacement name for his Dicynodontia.[3] The name Bidentalia quickly fell out of use in the following years, replaced by popularity of Owen's Dicynodontia.[5]
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.
Only two families survived the end Permian extinction, one of which, the Lystrosauridae, were the most common and widespread herbivores of the Induan (earliest Triassic). These medium-sized animals evolved into and were replaced by the Kannemeyeridae, stocky, pig- to ox-sized animals that 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.
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.
It used to be thought that dicynodonts died out completely before the end of the Triassic. Recently however, evidence has come to light showing the dicynodonts survived into the Cretaceous in southern Gondwana[1] (now Queensland) (Thulborn and Turner, 2003); recent studies, however, suggest that this fossil belongs to a crocodylomorph like Baurusuchus.
Dicynodontia was originally named by 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.[6] 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.[7]
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 Venjukoviamorpha.[8] 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.[5]
Unknown placement:
Below is a cladogram modified from Angielczyk and Rubidge (2010) showing the phylogenetic relationships of Dicynodontia:[9]
| Dicynodontia |
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