Dicynodont
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The Dicynodontia are a taxon of Therapsids or mammal-like reptiles. Dicynodonts were small to large herbivorous animals with two tusks, hence their name, which means 'two-tooth dog'. They are also the most successful and diverse of the therapsids, with over 70 genera known, varying from rat to ox-sized.
Characteristics
The Dicynodont skull is highly specialised; light but strong, with the synapsid temporal openings at the rear of the skull are 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 shoulder girdle and the ilium are large and strong. The tail is short.
Evolutionary History
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 including 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), when - 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.
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 have survived into the Cretaceous in southern Gondwana (now Queensland) (Thulborn and Turnmer, 2003). If authentic, this is an astonishing example of a Lazarus taxon.
Taxonomy
- Suborder Anomodontia
- Infraorder Dicynodontia
- *Superfamily Eodicynodontoidea
- **Family Eodicynodontidae
- ***Eodicynodon
- *Diictodontia
- **Robertoidea
- ***Family Robertiidae
- ****Robertia
- ***Family Diictodontidae
- ****Anomodon
- ****Diictodon
- ***Superfamily Emydopoidea
- ****Family Emydopidae
- *****Emydops
- *****Myosauroides
- *****Palemydops
- ****Family Cistecephalidae
- *****Cistecephaloides
- *****Cistecephalus
- *****Kawingasaurus
- *Superfamily Kingorioidea
- **Family Kingoriidae
- ***Kingoria
- ***Kombusia
- *Colobodectes
- *Pristerodontia
- **Family Pristerodontidae
- **Family Oudenodontidae
- **Family Aulacocephalodontidae
- **Family Lystrosauridae
- **Family Dicynodontidae
- **Superfamily Kannemeyeriiformes
- ***Family Shanisiodontidae
- ***Family Stahleckeriidae
- ***Family Kannemeyeriidae
References
- Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, WH Freeman & Co.
- Colbert, E. H., (1969), Evolution of the Vertebrates, John Wiley & Sons Inc (2nd ed.)
- Cox, B., Savage, R.J.G., Gardiner, B., Harrison, C. and Palmer, D. (1988) The Marshall illustrated encyclopedia of dinosaurs & prehistoric animals, 2nd Edition, Marshall Publishing
- Crompton, A. W, and Hotton, N. 1967. Functional morphology of the masticatory apparatus of two dicynodonts (Reptilia, Therapsida). Postilla, 109:1–51.
- King, Gillian M., "Anomodontia" Part 17 C, Encyclopedia of Paleoherpetology, Gutsav Fischer Verlag, Stuttgart and New York, 1988
- -- -- , 1990, the Dicynodonts: A Study in Palaeobiology, Chapman and Hall, Lomdon and New York
- Thulborn, Tony and Turner, Susan, 2003, "The last dicynodont: an Australian Cretaceous relict" Proceedings: Biological Sciences Vol 270, No 1518 / May 07, 2003; pp 985 - 993
External link
- [Therapsida : Neotherapsida : Dicynodontia] - Palaeos
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