Bidentalia is a group of

dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typic ...

therapsid Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented more ...

s. Bidentalia was one of the first names used to describe dicynodonts; the group was established in 1876, while the name "bidentals" dates back as far as 1845. With the increasing prominence of

phylogenetics In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...

, the group was redefined as a clade in 2009. Bidentalia is now considered a

stem-based taxon Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional approach, in which taxon names are defined by a '' type'', which ...

that includes all taxa more closely related to '' Aulacephalodon bainii'' and ''

Dicynodon lacerticeps ''Dicynodon'' ("two dog-teeth") is a genus of dicynodont therapsid that flourished during the Permian, Upper Permian Period (geology), period. Like all dicynodonts, it was herbivore, herbivorous animal. This reptile was toothless, except for prom ...

'' than '' Emydops arctatus''.

History

In 1845, South African geologist

Andrew Geddes Bain Andrew Geddes Bain (baptised 11 June 1797 – 20 October 1864), was a South African geologist, road engineer, palaeontologist and explorer. Life history The only child of Alexander Bain and Jean Geddes, both of whom died when Bain was still a ...

described the first known dicynodonts as "bidentals" for their two prominent tusks. "Bidental" was the first name ever used for a group of non-mammalian

synapsid Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...

s. The name Dicynodontia, which is more commonly used to refer to these animals, was erected by English paleontologist Richard Owen in 1860. Realizing that Bain's "bidentals" predated his "dicynodonts", Owen named Bidentalia in 1876 as a replacement name for Dicynodontia. Owen described Bidentalia as reptiles with "a long ever-growing tusk in each maxillary; premaxillaries connate, forming with the lower jaw a beak-shaped mouth, probably sheathed with horn. Sacrum of more than two vertebrae; trunk-vertebrae amphicoelian; limbs ambulatory." At this time, Bidentalia included three main species: ''

'', '' Dicynodon bainii'', and '' Ptychognathus declivis''. The two ''Dicynodon'' species were named in 1845, just before Bain described his bidentals. ''Ptychognathus'' was named in 1859, and is now called ''

Lystrosaurus ''Lystrosaurus'' (; 'shovel lizard'; proper Greek is λίστρον ''lístron'' ‘tool for leveling or smoothing, shovel, spade, hoe’) is an extinct genus of herbivorous dicynodont therapsids from the late Permian and Early Triassic epochs ( ...

''. In the following years Dicynodontia became the preferred name for these reptiles and Bidentalia quickly fell out of use. Bidentalia was reinstated as a clade in 2009. It was used to include all therochelonians more closely related to ''

Dicynodon ''Dicynodon'' ("two dog-teeth") is a genus of dicynodont therapsid that flourished during the Upper Permian period. Like all dicynodonts, it was herbivorous animal. This reptile was toothless, except for prominent tusks, hence the name. It proba ...

'' than to emydopoids (a group of more basal dicynodonts). As a clade, Bidentalia forms a more inclusive group than it did under Owen's use. Owen's Bidentalia was equivalent to Dicynodontia, which today is used as a much larger group encompassing all dicynodonts. In its current use, Bidentalia includes two major subgroups,

Cryptodontia Cryptodontia is a group of dicynodont therapsids that includes the families Geikiidae, Oudenodontidae, and Rhachiocephalidae. It was first named in 1860 by English paleontologist Richard Owen. Owen intended Cryptodontia to be a family, and the n ...

and Dicynodontoidea.

Evolution

Bidentalia originated in the

Guadalupian The Guadalupian is the second and middle series/epoch of the Permian. The Guadalupian was preceded by the Cisuralian and followed by the Lopingian. It is named after the Guadalupe Mountains of New Mexico and Texas, and dates between 272.95 ± ...

epoch of the

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last ...

and quickly spread worldwide, with early species known from both Brazil and Russia. However, they were conspicuously absent from Africa despite its well-known Permian fauna until the

late Permian Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effect, ...

. The oldest and most basal known bidentalian is ''

Rastodon ''Rastodon'' is an extinct genus of anomodonts. It is the oldest and most basal known genus of bidentalian dicynodonts. Uniquely among dicynodonts, its tusks curve forward. The type and only species is ''R. procurvidens''. Etymology ''Rasto ...

''.

Phylogeny

Bidentalia is divided into two major groups, the Cryptodontia and the Dicynodontoidea, as well as a small number of basal genera such as '' Elph'' and ''

'' However, the Cryptodontia may be paraphyletic. Below is a

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

showing the phylogeny of Bidentalia from a recent study, Kammerer ''et al.'' (2011):

References

{{Taxonbar, from=Q4904117 Dicynodonts Guadalupian first appearances Early Cretaceous extinctions