Therocephalia

Taxonomy

Therocephalia was first named and conceived of by Robert Broom in 1903 as an order to include what he regarded as primitive theriodonts, based primarily on Scylacosaurus and Ictidosaurus. However, his original concept of Therocephalia differed strongly from the modern classification by also including various genera of gorgonopsians (including Gorgonops) and dinocephalians. From 1903 to 1907 Broom added more therocephalian genera, as well as some non-therocephalians, to this group, including the anomodont Galechirus. The latter's inclusion highlighted Broom's view of therocephalians as 'primitive' and ancestral to other therapsids, believing anomodonts to be descended from a therocephalian-like ancestor such as Galechirus. However, by 1908 he considered its and some other non-therocephalian's inclusions to the group to be doubtful. In 1913, Broom reinstated Gorgonopsia as distinct from Therocephalia, but 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.^[10]

At the same time, the small 'advanced' therocephalians now classified under Baurioidea were often regarded as belonging to their own subgroup of therapsids distinct from therocephalians, the Bauriamorpha.^[11] Bauriamorphs were classified separately from therocephalians for many decades, though were often inferred to have evolved from therocephalians in parallel with cynodonts, each typically from different therocephalian stock.^[10] The inclusion of baurioids under Therocephalia was only firmly established in the 1980s, namely by Kemp (1982) and Hopson and Barghusen (1986).^[12][13]

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.^[10] 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.^[11]

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.^[14] However, subsequent analysis has exposed additional synapomorphies supporting the monophyly of this group (including delayed caniniform replacement), and Lycosuchidae is currently considered a valid basal clade within Therocephalia.^[15] However, most genera included in the group have since been declared dubious, and it now only includes Lycosuchus and Simorhinella.^[16]

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 more derived therocephalians form the clade Eutherocephalia. Some analyses have found scylacosaurids to be closer to eutherocephalians than to lycosuchids, and so have been united as the clade Scylacosauria, while others have suggested they are each other's sister taxa. Within Eutherocephalia, major clades corresponding to the families Akidnognathidae, Chthonosauridae, Hofmeyriidae, Whaitsiidae are recognised, along with various subclades grouped under Baurioidea. However, while individual groups of therocephalians are broadly recognised as valid, the interrelationships between them are often poorly supported.^[17][18][19] 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.^[13] Despite these shortcomings, subsequent discussions of therocephalian relationships relied almost exclusively on these analyses.^[11] Later analyses focused on the relationships of early cynodonts, namely Abdala (2007) and Botha et al. (2007), included some therocephalian taxa and supported the existence of Eutherocephalia, but also found cynodonts to be the sister taxon to the whaitsiid therocephalian Theriognathus and thus rendering Therocephalia paraphyletic.^[20][21]

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.^[11][7]

Below is a cladogram modified from an analysis published by Christian A. Sidor, Zoe. T Kulik and Adam K. Huttenlocker in 2022, simplified to illustrate the relationships of the major recognised therocephalian subclades.^[22] It is based on the data matrix first published by Huttenlocker et al. (2011),^[8] and represents the broad topologies found by other iterations of this dataset, such as Sigurdsen et al. (2012), Huttenlocker et al. (2014), and Liu and Abdala (2022).^[23][24][17] 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.^[25]

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).^[6]

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.^[6]