Chainosauria is a large and speciose clade of anomodont therapsid that includes the highly diverse dicynodonts and a small number of closely related basal genera (to the exclusion of Venyukovioidea and more basal anomodonts)—although the total composition and taxonomic scope of Chainosauria is in flux. Chainosauria was named in 1923 to group together the dicynodonts and their close relatives, namely three small anomodont genera from South Africa (Galechirus, Galeops, and Galepus) that made up the now defunct group 'Dromasauria'. The name soon fell into disuse, however, as it was functionally replaced by Anomodontia. Chainosauria was later revived cladistically in 2009, preserving the association of dicynodonts and the 'dromasaurs' and has since served in effect as both a cladistic and a biogeographic counterpart to the Laurasian venyukovioids, with early chainosaurs appearing to have been a Gondwanan radiation.
Other basal anomodonts, the anomocephaloids and Patranomodon, have also sometimes been recovered as chainosaurs, and so the contents and relationships of early chainosaurs are not resolved. As such, autapomorphies (unique defining traits) that unite chainosaurs are difficult to determine with certainty. Early chainosaurs are much more lightly built and slender than dicynodonts, however, in general their skulls are proportionally more dicynodont-like than more basal anomodonts. Furthermore, while the first chainosaurs lacked the characteristic tusks and beaks of dicynodonts and had complete rows of teeth, the sliding jaw-joint and jaw musculature of dicynodonts had first evolved in earlier chainosaurs (namely the dicynodont-adjacent genus Galeops), and their teeth were simplified and reduced (apart from anomocephaloids).
Morphology
The majority of chainosaur diversity is found in the dicynodonts, which span a broad range of body sizes but are conservative in form, typically with barrel-shaped bodies, stocky limbs and short tails. By contrast, earlier chainosaurs where much more lightly build and gracile, with elongated and slender limb bones. Long tails are also known in at least Galepus and Galechirus, which each had at least approximately 30 caudal vertebrae that were longer than wide (unlike the short and broad caudals of dicynodonts but similar to those of the venyukovioid Suminia).
Early chainosaurs had also not yet evolved the reduced post-canine dentition, beaks and tusks of dicynodonts, and instead possessed complete rows of teeth lacking differentiated canines (with the potential exception of the anomocephaloid Tiarajudens). The dentition of definitive early chainosaurs (i.e. 'dromasaurs'), as well as Patranomodon, is made up of simplified, peg-like teeth. A possible exception are the anomocephaloids, which possess leaf-shaped incisiforms and molar-like palatal teeth (as well as sabre-like caniniforms in Tiarajudens), should they indeed be chainosaurs. Notably, though, the characteristic elongated lower jaw joint of dicynodonts and associated novel jaw musculature (the external lateral adductor) that allows the mandible to slide back and process food is first found outside of Dicynodontia in Galeops. An incipient beak at the tips of the jaws has sometimes been proposed for Galeops and Galepus, but this is likely an artefact of poor preservation.
Internally, the nasopalatine nerve inside the maxilla is already large and well-developed in Patranomodon. This nerve is a branch of the trigeminal nerve, and is associated with the innervation of the keratinous beak in dicynodonts (similar to turtles), and its enlargement in Patranomodon may indicate that greater sensitivity in the premaxilla may have preceded the development of beaks in early chainosaurs.
History of study
Chainosauria was coined by Hungarian palaeontologist Baron Franz Nopcsa in 1923 as an order of Theromorpha (a group roughly equivalent to modern Therapsida) to contain the dicynodonts and the 'dromasaurs'. Nopcsa also originally included the unrelated sphenacodont Palaeohatteria in Chainosauria, placing it in the 'Dromasauria' as its own family Palaeohatteriidae, while other 'dromasaurs' formed the Dromasauridae.