| Dicynodontia | |
|---|---|
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| Skeleton ofDiictodon | |
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| Skeleton ofPlacerias | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Therapsida |
| Clade: | †Anomodontia |
| Clade: | †Chainosauria |
| Clade: | †Dicynodontia Owen, 1859 |
| Clades & genera | |
Seetext | |
| Synonyms | |
Dicynodonta[1] | |
Dicynodontia is an extinctclade ofanomodonts, an extinct type of non-mammaliantherapsid. Dicynodonts wereherbivores that typically bore a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typically toothless beak, unique amongst allsynapsids. Dicynodonts first appeared in Southern Pangaea during themid-Permian, ca. 270–260 million years ago, and became globally distributed and the dominant herbivorous animals in theLate Permian, ca. 260–252 Mya. They were devastated by theend-Permian mass extinction that wiped out most other therapsids ca. 252 Mya. They rebounded at beginning of the followingTriassic, but subsequently declined and died out towards the end of that period. They were the most successful and diverse of the non-mammalian therapsids, with over 80-90genera known, varying from rat-sizedburrowers to elephant-sizedbrowsers.
The dicynodontskull is highly specialised, light but strong, with thesynapsid 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 inturtles andceratopsiandinosaurs. 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. Dicynodonts typically had a pair of enlarged maxillary caniniform teeth, analogous to thetusks present in some living mammals. In the earliest genera, they were merely enlarged teeth, but in later forms they independently evolved into ever-growing teeth like mammal tusks multiple times.[3] In some dicynodonts, the presence of tusks has been suggested to besexually dimorphic.[4] Some dicynodonts such asStahleckeria lacked true tusks and instead bore tusk-like extensions on the side of the beak.[5][6]: 139
The body is short, strong and barrel-shaped, with strong limbs. In large genera (such asDinodontosaurus) the hindlimbs were held erect, but the forelimbs bent at the elbow. Both thepectoral girdle and theilium are large and strong. The tail is short.[citation needed]
Pentasauropus dicynodont tracks suggest that dicynodonts had fleshy pads on their feet.[7] Mummified skin from specimens ofLystrosaurus in South Africa have numerous raised bumps.[8]
Dicynodonts have long been suspected of beingwarm-blooded animals. Their bones are highly vascularised and possessHaversian canals, and their bodily proportions are conducive to heat preservation.[9] In young specimens, the bones are so highly vascularised that they exhibit higher channel densities than most other therapsids.[10] Yet, studies onLate Triassic dicynodontcoprolites paradoxically showcase digestive patterns more typical of animals with slow metabolisms.[11]
More recently, the discovery ofhair remnants inPermiancoprolites possibly vindicates the status of dicynodonts as endothermic animals. As these coprolites come from carnivorous species and digested dicynodont bones are abundant, it has been suggested that at least some of these hair remnants come from dicynodont prey.[12] A new study using chemical analysis seemed to suggest that cynodonts and dicynodonts both developed warm blood independently before the Permian extinction.[13]
A 2024 paper posited thatrock art of a superficially walrus-like imaginary creature with downcurved tusks created by theSan people ofSouth Africa prior to 1835 may have been partly inspired by fossil dicynodont skulls which erode out of rocks in the area.[14]
Dicynodonts have been known to science since the mid-1800s. The South African geologistAndrew Geddes Bain gave the first description of dicynodonts in 1845. At the time, Bain was a supervisor for the construction of military roads under theCorps of Royal Engineers and had found many reptilian fossils during his surveys of South Africa. Bain described these fossils in an 1845 letter published inTransactions of the Geological Society of London, calling them "bidentals" for their two prominent tusks.[15] In that same year, the English paleontologistRichard Owen named two species of dicynodonts from South Africa:Dicynodon lacerticeps andDicynodon 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 hisDescriptive and Illustrated Catalogue of the Fossil Reptilia of South Africa in the Collection of the British Museum.[16] By this time, many more dicynodonts had been described. In 1859, another important species calledPtychognathus declivis was named from South Africa. In the same year, Owen named the group Dicynodontia.[17] In hisDescriptive and Illustrated Catalogue, Owen honored Bain by erectingBidentalia as a replacement name for his Dicynodontia.[16] The name Bidentalia quickly fell out of use in the following years, replaced by popularity of Owen's Dicynodontia.[18]
Dicynodonts first appeared during theMiddle Permian in the Southern Hemisphere, with South Africa being the centre of their known diversity, and underwent a rapidevolutionary radiation, becoming globally distributed and amongst the most successful and abundant land vertebrates during theLate Permian.[19][20] During this time, they included a large variety of ecotypes, including large, medium-sized, and small herbivores and short-limbed mole-like burrowers.[21]
Only four lineages are known to have survived theGreat Dying; the first three represented with a single genus each:Myosaurus,Kombuisia, andLystrosaurus, the latter being the most common and widespread herbivores of theInduan (earliestTriassic). None of these survived long into the Triassic. The fourth group was theKannemeyeriiformes, the only dicynodonts who diversified during the Triassic.[22] These stocky, pig- to ox-sized animals were the most abundant herbivores worldwide from theOlenekian to theLadinian age. By theCarnian they had been supplanted bytraversodont cynodonts andrhynchosaur reptiles. During theNorian (middle of the Late Triassic), perhaps due to increasing aridity, they drastically declined, and the role of large herbivores was taken over bysauropodomorph dinosaurs.[citation needed]
Fossils of anAsian elephant-sized dicynodontLisowicia bojani discovered inPoland indicate that dicynodonts survived at least until the late Norian or earliestRhaetian (latest Triassic, at earliest 211 million years ago,[23] and probably no later than around 205 million years ago[24]); this animal was also the largest known dicynodont species.[25][26] Six fragments of fossil bone discovered inQueensland, Australia, were interpreted as remains of a skull in 2003. This suggested by some author to indicate that dicynodonts survived into theCretaceous in southernGondwana.[27] The dicynodont affinity of these specimens was questioned (including a proposal that they belonged to abaurusuchian crocodyliform by Agnolin et al. in 2010),[28] and in 2019 Knutsen and Oerlemans considered this fossil to be ofPlio-Pleistocene age, and reinterpreted it as a fossil of a large mammal, probably adiprotodontid marsupial.[29]
With the decline and extinction of the kannemeyerids, there were to be no more dominant large synapsid herbivores until the middlePaleocene epoch (60 Ma) whenmammals, distant descendants ofcynodonts, began to diversify after the extinction of the non-avian dinosaurs.
Dicynodontia was originally named by the English paleontologistRichard Owen. It was erected as a family of the order Anomodontia and included the generaDicynodon andPtychognathus. Other groups of Anomodontia includedGnathodontia, which includedRhynchosaurus (now known to be anarchosauromorph) andCryptodontia, which includedOudenodon. 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 includedDicynodon andOudenodon.[30] 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.[31]
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) recognised several main groups within Dicynodontia, including Eodicynodontidae (containing onlyEodicynodon),Endothiodontia, Pristerodontia (Pristerodontidae,Cryptodontidae,Geikiidae,Dicynodontidae,Lystrosauridae, andKannemeyeriidae), Kingoriamorpha (containing onlyKingoriidae), Diictodontia (Pylaecephalidae,Robertiidae,Cistecephalidae,Emydopidae, andMyosauridae), andVenyukoviamorpha.[32] 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.[18]
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Below is acladogram modified from Angielczyk et al. (2021):[33]
| Dicynodontia | |
