Classification of
Moschorhinus, an akidnognathid Therocephalia was named by
Robert Broom in 1903 as a new
order to divide Theriodontia (then essentially containing all known carnivorous Permian and Triassic "mammal-like reptiles") into the "primitive" Permian forms, Therocephalia, and more mammal-like Triassic forms, Cynodontia, based on the anatomy of their palates and the
occipital condyle. Broom's Therocephalia was based primarily on
Scylacosaurus (effectively the
type genus of Therocephalia) From 1903 to 1907 Broom named and recognised more therocephalian genera, including several genera that are now non-therocephalian, mostly gorgonopsians, and even the
anomodont Galechirus. The latter's inclusion highlights Broom's view at the time of therocephalians as a 'primitive' order of therapsids and ancestral to the others, with anomodonts suggested to be descended from a therocephalian-like ancestor such as
Galechirus. Although his classification pre-dates cladistic terminology, Broom effectively conceived Therocephalia to be inherently paraphyletic from the beginning, giving rise to more 'advanced' therapsid groups. By 1908 he considered
Galechirus and some other non-therocephalians inclusion in the group to be doubtful, including members of Gorgonopsia which he reinstated as a valid group in 1913. Nonetheless, for many decades after there was still confusion from him and other researchers over which genera belonged to which group. The group's rank also varied from order, suborder and infraorder depending on authors' preferred therapsid systematics. of
Moschowhaitsia, a whaitsiid Various therocephalian subgroups and clades have been proposed since the group was named, although their contents and nomenclature have often been highly unstable and some previously recognized therocephalian clades have turned out to be artificial or based upon
dubious taxa. This has led to some prevalent names in therocephalian literature, sometimes in use for decades, being replaced by lesser-known names that hold priority. For example, the
Scaloposauridae was based on fossils with mostly juvenile characteristics and is likely represented by immature specimens from other disparate therocephalian families. In another example, the name "Pristerognathidae" was extensively used for a group of basal therocephalians for much of the 20th century, but it has since been recognised that the name
Scylacosauridae holds precedent for this group. Furthermore, the scope of "Pristerognathidae" was unstable and variably was limited to an individual subgroup of early therocephalians (alongside others such as Lycosuchidae, Alopecodontidae, and Ictidosauridae) to encompassing the entirety of early therocephalians. Similarly, various names have been used for therocephalians corresponding to the family Adkidnognathidae in 20th century literature, including Annatherapsididae, Euchambersiidae (the oldest available name) and Moschorhinidae, and members have often had a confused relationship to whaitsiids. Consensus on the name and contents of Akidnognathidae was only achieved in the 21st century, asserting that a family-level group is established on the oldest referable genus and thus Akidnognathidae takes precedent for this group of non-whaitsioid eutherocephalians. On the other hand, some groups previously thought to be artificial have turned out to be valid. The aberrant therocephalian family Lycosuchidae, once identified by the presence of multiple functional
caniniform teeth, was proposed to represent an unnatural group based on a study of canine replacement in early therocephalians by van den Heever in 1980 and its members referred to "Pristerognathidae". However, subsequent examination by van den Heever and later analyses exposed additional
synapomorphies supporting the
monophyly of this group (including delayed caniniform replacement), and Lycosuchidae is currently considered a valid basal clade within Therocephalia. However, most genera included in the group have since been declared dubious, and it now only includes
Lycosuchus and
Simorhinella. of
Regisaurus, a baurioid Modern therocephalian taxonomy is instead based upon
phylogenetic analyses of therocephalian species, which consistently recognises two groups of early therocephalians (the Lycosuchidae and Scylacosauridae) while all other more derived therocephalians form the clade Eutherocephalia. Most phylogenetic analyses have found scylacosaurids to be closer to eutherocephalians than to lycosuchids, and so have been united in the clade
Scylacosauria, but alternatively the two early families could be each other's sister taxa. Such a grouping has been referred to as the
Pristerosauria, originally defined to include "pristerognathids" and various other therocephalian families by
Lieuwe Dirk Boonstra in 1953 and redefined by Hopson and Barghusen in 1986 as the parent taxon to "Pristerognathidae", effectively uniting all primitive therocephalians (including lycosuchids). As such, there are few higher-level named clades uniting the multiple subclades, with the exceptions of Whaitsiioidea (uniting Hofmeyriidae and Whaitsiidae) and Baurioidea.
Phylogeny Early phylogenetic analyses of therocephalians, such as that of Hopson and Barghusen (1986) and van den Heever (1994), recovered and validated many of the therocephalian subtaxa mentioned above in a phylogenetic context. However, the higher-level relationships were difficult to resolve, particularly between the subclades of Eutherocephalia (i.e. Hofmeyriidae, Akidnognathidae, Whaitsiidae and Baurioidea). For example, Hopson and Barghusen (1986) could only recover Eutherocephalia as an unresolved
polytomy. Later phylogenetic analyses of therocephalians, initiated by Huttenlocker (2009), emphasise using a broader selection of therocephalian taxa and characters. Such analyses have reinforced Therocephalia as a sister clade to cynodonts, and the
monophyly of Therocephalia has been supported by subsequent researchers. It is based on the data matrix first published by Huttenlocker et al. (2011), An example of the lability of these relationships is demonstrated by Liu and Abdala (2023), who recovered an alternative topology with Chthonosauridae nested deeply within Akidnognathidae. Relationships are not shown within bolded terminal clades on the cladogram below. }} Below is a cladogram modified from Pusch et al. (2024) analysing the relationships of therocephalians and early cynodonts. Their analysis focused on including endocranial characteristics to help resolve the relations of therocephalians and cynodonts to supplement previous analyses that relied almost entirely on superficial cranial and dental characteristics that are subject to convergent evolution, and as such only includes taxa with available applicable data. Of these, only four therocephalians could be included. However, they each represent four major groups within therocephalian phylogeny: the two 'basal therocephalians'
Lycosuchus (Lycosuchidae) and
Alopecognathus (Scylacosauridae) and two derived members of Eutherocephalia,
Olivierosuchus (Akidnognathidae) and
Theriognathus (Whaitsiidae). Notably, their analyses consistently found cynodonts and eutherocephalians to be sister taxa, with the basal therocephalians
Lycosuchus and scylacosaurids in a more basal position, rendering therocephalians as they are traditionally conceived paraphyletic. This differs from previous proposals of a paraphyletic Therocephalia which typically regarded cynodonts as being closest to derived whaitsiid therocephalians. }}
Evolution '', one the most
basal therocephalians known The therocephalians evolved as one of several lines of non-mammalian
therapsids, and have a close relationship to the cynodonts, which includes mammals and their ancestors. They are broadly regarded as the
sister group to cynodonts by most modern researchers, united together as the clade
Eutheriodontia. However, some researchers have proposed that therocephalians are themselves ancestral to cynodonts, which would render therocephalians
cladistically paraphyletic relative to cynodonts. Historically, cynodonts are often proposed to descend from (or are closest to) the therocephalian family
Whaitsiidae under this hypothesis, however a 2024 study instead found support for a sister relationship between cynodonts and Eutherocephalia. While common ancestry with cynodonts (and, thus, mammals) accounts for many similarities between these groups, some scientists believe that other similarities may be better attributed to
convergent evolution, such as the loss of the
postorbital bar in some forms, a mammalian
phalangeal formula, and some form of a secondary palate in most taxa. Therocephalians and cynodonts both survived the
Permian-Triassic mass extinction; but, while therocephalians soon became extinct, cynodonts underwent rapid diversification. Therocephalians experienced a decreased rate of
cladogenesis, meaning that few new groups appeared after the extinction. Most Triassic therocephalian lineages originated in the Late Permian, and lasted for only a short period of time in the Triassic, going extinct during the late
Anisian. ==See also==