Chainosauria is a large and speciose
clade of
anomodont
Anomodontia is an extinct group of non-mammalian therapsids from the Permian and Triassic periods. By far the most speciose group are the dicynodonts, a clade of beaked, tusked herbivores.Chinsamy-Turan, A. (2011) ''Forerunners of Mammals ...
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 mor ...
that includes the highly diverse
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, typica ...
s and a small number of closely related
basal
Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''.
Science
* Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure
* Basal (medicine), a minimal level that is nec ...
genera
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial ...
(to the exclusion of
Venyukovioidea
Venyukovioidea is an infraorder of anomodont therapsids related to dicynodonts from the Permian of Russia. They have also known as Venjukovioidea, as well as by the similar names Venyukoviamorpha or Venjukoviamorpha in literature. This in part ow ...
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
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring count ...
(''
Galechirus'', ''
Galeops
''Galeops'' is an extinct genus of anomodont therapsids from the Middle-Late Permian of South Africa. It was described by Robert Broom
Robert Broom FRS FRSE (30 November 1866 6 April 1951) was a British- South African doctor and palaeonto ...
'', and ''
Galepus
''Galepus'' is an extinct genus of anomodont therapsids.
See also
* List of therapsids
This list of therapsids is an attempt to create a comprehensive listing of all genera that have ever been included in the Therapsida excluding mammals ...
'') that made up the now defunct group '
Dromasauria
Dromasaurs are a paraphyletic group of anomodont therapsids from the Middle Permian. They were small with slender legs and long tails. Their skulls were short, but the eye sockets were large. Dromasauria was once considered to be a major grou ...
'. The name soon fell into disuse, however, as it was functionally replaced by Anomodontia. Chainosauria was later revived
cladistically
Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups (" clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived cha ...
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
Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around (Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pang ...
venyukovioids, with early chainosaurs appearing to have been a
Gondwanan
Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final stages ...
radiation.
Other basal anomodonts, the
anomocephaloids
Anomocephaloidea is a clade (evolutionary grouping) of anomodont therapsids that existed in Gondwana during the Middle Permian and includes two species, ''Anomocephalus africanus'' from South Africa and ''Tiarajudens eccentricus'' from Brazil, ...
and ''
Patranomodon
''Patranomodon'' (from Greek ''patr-'' “father”, thus “father of anomodonts”) is an extinct genus belonging to the group of Anomodontia. Rubidge and Hopson named this anomodont in 1990 after discovering its skull.Rubidge, B. S., & Hopson ...
'', have also sometimes been recovered as chainosaurs, and so the contents and relationships of early chainosaurs are not resolved. As such,
autapomorphies
In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to t ...
(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 vertebra
The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic i ...
e 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
''Tiarajudens'' (" Tiaraju tooth") is an extinct genus of saber-toothed herbivorous anomodonts which lived during the Middle Permian period ( Capitanian stage) in what is now Rio Grande do Sul, Brazil. It is known from the holotype UFRGS P ...
''). 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
The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity.
A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly sepa ...
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
The nasopalatine nerve (long sphenopalatine nerve) is a nerve of the head. It is a branch of the pterygopalatine ganglion, a continuation from the maxillary nerve (V2). It supplies parts of the palate and nasal septum.
Structure
The nasopal ...
inside the
maxilla
The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The ...
is already large and well-developed in ''Patranomodon''. This nerve is a branch of the
trigeminal nerve
In neuroanatomy, the trigeminal nerve (literal translation, lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for Sense, sensation in the face and motor functions ...
, 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
The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...
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
Order, ORDER or Orders may refer to:
* Categorization, the process in which ideas and objects are recognized, differentiated, and understood
* Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...
of Theromorpha (a group roughly equivalent to modern Therapsida) to contain the dicynodonts and the 'dromasaurs'. Nopcsa also originally included the unrelated
sphenacodont
Sphenacodontia is a stem-based clade of derived synapsids. It was defined by Amson and Laurin (2011) as "the largest clade that includes '' Haptodus baylei'', ''Haptodus garnettensis'' and ''Sphenacodon ferox'', but not '' Edaphosaurus pogonias ...
''
Palaeohatteria
''Palaeohatteria'' is an extinct genus of basal sphenacodonts known from the Early Permian period (Sakmarian stage) of Saxony, Germany. It contains a single species, ''Palaeohatteria longicaudata''.
Discovery
''Palaeohatteria'' is based on ve ...
'' in Chainosauria, placing it in the 'Dromasauria' as its own
family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...
Palaeohatteriidae, while other 'dromasaurs' formed the Dromasauridae.
Nopcsa's concept of Chainosauria to encompass dicynodonts and their close relatives was functionally similar to the modern usage of Anomodontia, which was itself used at the time equivalently to what we recognise as Dicynodontia today. As such, the subsequent redefinition and expansion of Anomodontia by other researchers to encompass dicynodonts and their relatives made Nopcsa's Chainosauria redundant, and so the name fell into disuse for the rest of the 20th century.
Chainosauria was criticised by
American
American(s) may refer to:
* American, something of, from, or related to the United States of America, commonly known as the " United States" or "America"
** Americans, citizens and nationals of the United States of America
** American ancestry, ...
palaeontologist
Alfred Romer
Alfred Sherwood Romer (December 28, 1894 – November 5, 1973) was an American paleontologist and biologist and a specialist in vertebrate evolution.
Biography
Alfred Romer was born in White Plains, New York, the son of Harry Houston Romer an ...
that same year, regarding Nopcsa's choice to group dicynodonts and 'dromasaurs' together as being "on not very obvious grounds".
However, a genuine
phylogenetic
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 o ...
relationship between the 'dromasaurs' and dicynodonts has since been upheld.
Furthermore, as more basal anomodonts were discovered and new cladistic methods of determining phylogenetic relationships were developed, a close association between the 'dromasaurs' and dicynodonts exclusive of other anomodonts has also been recognised.
This led palaeontologists Christian F. Kammerer and Kenneth D. Angielczyk to resurrect the name in 2009 for this phylogenetic group. They cladistically re-defined Chainosauria as the clade of all anomodonts closer 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 ...
'' and ''Galeops'' than to ''
Venyukovia
''Venyukovia'' (named after its discoverer, Pavel N. Venyukov) is an extinct genus of venyukovioid therapsid, a basal anomodont from the Middle Permian of Russia. The type and sole species, ''V. prima'', is known only by a partial lower jaw wi ...
'', in effect defining Chainosauria to act as a counterpart to the Russian anomodont clade Venyukovioidea. As conceived by Kammerer and Angielczyk, Chainosauria also excluded the basal South African anomodonts ''
Anomocephalus
''Anomocephalus'' is an extinct genus of primitive anomodonts and belongs to the clade Anomocephaloidea. The name is said to be derived from the Greek word ''anomos'' meaning lawless and ''cephalos'' meaning head. The proper word for head in Gr ...
'' and ''Patranomodon''. However, recent phylogenetic analyses have variably recovered ''Patranomodon'' as well as the anomocephaloids as closer to dicynodonts than venyukovioids and thus chainosaurs by definition.
Classification
Although intended to be defined as including dicynodonts and 'dromasaurs', the 'dromasaurs' themselves are a
paraphyletic
In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
series (or grade) or even a
polyphyletic
A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...
collection of early chainosaurs outside of dicynodonts.
As a stem- or branch-based clade, Chainosauria includes all anomodonts more closely related to ''Dicynodon'' (representing dicynodonts) and ''Galeops'' (representing 'dromasaurs') than to ''Venyukovia'', and is defined so that the group is always the
sister clade
In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree.
Definition
The expression is most easily illustrated by a cladogram:
Taxon A and t ...
of Venyukovioidea (and vice versa). This means that Chainosauria also includes other anomodonts not regarded as dicynodonts or 'dromasaurs' as long as they are found to be closer to them than to venyukovioids.

Due to the unstable relationships of basal anomodonts, this has lead to varying compositions for Chainosauria. The basal anomodont ''Patranomodon'' has been particularly unstable in this regard. It was at first thought to be the basalmost anomodont upon discovery, an interpretation supported by early
phylogenetic analyses
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 o ...
. However, multiple more recent analyses from 2009 onwards have instead found it as a chainosaur nestled amongst various 'dromasaurs'.
Likewise, ''Anomocephalus'' was thought to be a basal anomodont outside of the Chainosauria + Venyukovioidea clade, but following an analysis in 2017 it and its close relative ''
Tiarajudens
''Tiarajudens'' (" Tiaraju tooth") is an extinct genus of saber-toothed herbivorous anomodonts which lived during the Middle Permian period ( Capitanian stage) in what is now Rio Grande do Sul, Brazil. It is known from the holotype UFRGS P ...
'' (i.e. Anomocephaloidea) have been recovered as a basal chainosaur as well.
The inclusion of both ''Patranomodon'' and anomocephaloids in Chainosauria has been described by Angielczyk ''et al.'' (2017) as "intuitively pleasing". Both taxa have similar deep and short skull proportions to other chainosaurs, and many of their supposed basal features could alternatively be interpreted as
plesiomorphies
In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades.
Plesiomorphy, symplesiomorphy, apomorphy, an ...
—i.e. shared ancestral traits inherited from the common ancestor of chainosaurs and other anomodonts. At the same time, derived traits shared between derived chainosaurs and venyukovioids (namely those of the jaw joint and jaw musculature) are likely convergent.
The relationships of the various 'dromasaurs' are similarly unsettled. ''Galeops'' is consistently recognised as the sister taxon of Dicynodontia, but ''Galepus'' and ''Galechirus'' have been found in varying positions. Initially they were recovered as closely related to ''Galeops'' and dicynodonts, such as by Kammerer ''et al.'' (2011), and sometimes forming a clade together (Galechiridae).
However, in later analyses ''Galechirus'' tended to fall outside of Chainosauria altogether in a
polytomy
An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tr ...
with it and Venyukovioidea (such as in the left cladogram below). Furthermore, in a more recent 2017 analysis with an expanded Chainosauria, ''Galechirus'' returned to a position close to ''Galeops'', while ''Galepus'' was instead found as the most basal chainosaur.
Examples of these varying relationships are shown in the two cladograms below:
Biogeography
The composition of Chainosauria has implications for the origins, evolutionary history and biogeography of early anomodonts. An early biogeographic split amongst basal anomodonts has been proposed by researchers between Chainosauria and Venyukovioidea, with Chainosauria radiating in
Gondwana
Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final st ...
in the southern hemisphere and venyukovioids as a
Laurasia
Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...
n radiation in the northern hemisphere.
''Patranamodon'' and ''Anomocephalus'', having each been considered the most basal anomodont at times, have been used to argue a Gondwanan origin for both chainosaurs and venyukovioids.
The recognition of the
Chinese
Chinese can refer to:
* Something related to China
* Chinese people, people of Chinese nationality, citizenship, and/or ethnicity
**''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation
** List of ethnic groups in China, people of v ...
taxon ''
Biseridens
''Biseridens'' ("two rows of teeth") is an extinct genus of anomodont therapsid, and one of the most basal anomodont genera known. Originally known from a partial skull misidentified as an eotitanosuchian in 1997, another well-preserved skull ...
'' as the basal-most known anomodont alternatively supports a Laurasian origin of anomodonts, from where chainosaurs would have migrated out of and only later radiated in Gondwana.
References
{{Taxonbar, from=Q5067994
Anomodonts
Permian synapsids
Triassic synapsids
Guadalupian first appearances
Late Triassic extinctions
Taxa named by Franz Nopcsa von Felső-Szilvás