Therocephalia is an extinctclade of eutheriodonttherapsids (mammals and their close relatives) from the Permian and Triassic periods. The therocephalians ("beast-heads") are named after their large skulls, which, along with the structure of their teeth, suggest that they were carnivores. Like other non-mammalian synapsids, therocephalians were once described as "mammal-like reptiles". Therocephalia is the group most closely related to the cynodonts, which gave rise to the mammals, and this relationship takes evidence in a variety of skeletal features. Indeed, it had been proposed that cynodonts may have evolved from therocephalians and so that therocephalians as recognised are paraphyletic in relation to cynodonts.
The fossils of therocephalians are numerous in the Karoo of South Africa, but have also been found in Russia, China, Tanzania, Zambia, and Antarctica. Early therocephalian fossils discovered in Middle Permian deposits of South Africa support a Gondwanan origin for the group, which seems to have spread quickly across Earth. Although almost every therocephalian lineage ended during the great Permian–Triassic extinction event, a few representatives of the subgroup called Eutherocephalia survived into the Early Triassic. Some genera belonging to this group are believed to have possessed venom, which would make them the oldest tetrapods known to have such characteristics. However, the last therocephalians became extinct by the early Middle Triassic, possibly due to climate change, along with competition with cynodonts and various groups of reptiles — mostly archosaurs and their close relatives, including archosauromorphs and archosauriforms.
Anatomy and physiology
Head reconstruction of Lycosuchus, an early therocephalian
Like the Gorgonopsia and many cynodonts, most therocephalians were presumably carnivores. The earlier therocephalians were, in many respects, as primitive as the gorgonopsians, but they did show certain advanced features. There is an enlargement of the temporal opening for broader jaw adductor muscle attachment and a reduction of the phalanges (finger and toe bones) to the mammalian phalangeal formula. The presence of an incipient secondary palate in advanced therocephalians is another feature shared with mammals. The discovery of maxilloturbinal ridges in forms such as the primitive therocephalian Glanosuchus, suggests that at least some therocephalians may have been warm-blooded.[1]
The later therocephalians included the advanced Baurioidea, which carried some theriodont characteristics to a high degree of specialization. For instance, small baurioids and the herbivorous Bauria did not have an ossified postorbital bar separating the orbit from the temporal opening—a condition typical of primitive mammals. These and other advanced features led to the long-held opinion, now rejected, that the ictidosaurs and even some early mammals arose from a baurioid therocephalian stem. Mammalian characteristics such as this seem to have evolved in parallel among a number of different therapsid groups, even within Therocephalia.[1]
Several more specialized lifestyles have been suggested for some therocephalians. Many small forms, like ictidosuchids, have been interpreted as aquatic animals. Evidence for aquatic lifestyles includes sclerotic rings that may have stabilized the eye under the pressure of water and strongly developed cranial joints, which may have supported the skull when consuming large fish and aquatic invertebrates. One therocephalian, Nothogomphodon, had large sabre-like canine teeth and may have fed on large animals, including other therocephalians. Other therocephalians such as bauriids and nanictidopids have wide teeth with many ridges similar to those of mammals, and may have been herbivores.[2]
Many small therocephalians have small pits on their snouts that probably supported vibrissae (whiskers). In 1994, the Russian paleontologist Leonid Tatarinov proposed that these pits were part of an electroreception system in aquatic therocephalians.[3] However, it is more likely that these pits are enlarged versions of the ones thought to support whiskers, or holes for blood vessels in a fleshy lip.[2] The generaEuchambersia and Ichibengops, dating from the Lopingian, particularly attract the attention of paleontologists, because the fossil skulls attributed to them have some structures which suggests that these two animals had organs for distributing venom.[4][5]
Classification
Fossilized skull of Gorynychus, one of the most basal therocephalians identified to date
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 cladisticallyparaphyletic 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.[6] The oldest known therocephalians first appear in the fossil record at the same time as other major therapsid groups, including the Gorgonopsia, which they resemble in many primitive features. For example, many early therocephalians possess long canine teeth similar to those of gorgonopsians. The therocephalians, however, outlasted the gorgonopsians, persisting into the early-Middle Triassic period as small weasel-like carnivores and cynodont-like herbivores.[7]
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,[8] going extinct during the late Anisian.[9]
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 anomodontGalechirus. 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 wich genera beloned 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 prevelant 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]
A therapsid is a member of the clade Therapsida which is a major group of eupelycosaurian synapsids that includes mammals and 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 underneath the body, as opposed to the sprawling posture of many reptiles and salamanders.
Cynodonts are eutheriodont therapsids that first appeared in the Late Permian, and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extinct ancestors and close relatives (Mammaliaformes), having evolved from advanced probainognathian cynodonts during the Late Triassic.
Ericiolacerta is an extinct genus of small therocephalian therapsids from the early Triassic of South Africa and Antarctica. Ericiolacerta, meaning "hedgehog lizard", was named by D.M.S. Watson in 1931. The species E. parva is known from the holotype specimen which consists of a nearly complete skeleton found in the Lystrosaurus Assemblage Zone within the Katberg Formation of the Beaufort Group in South Africa, and from a partial jaw found in the Lower Triassic Fremouw Formation in Antarctica. Ericiolacerta was around 20 centimetres (7.9 in) in length, with long limbs and relatively small teeth. It probably ate insects and other small invertebrates. The therocephalians – therapsids with mammal-like heads – were abundant in Permian times, but only a few made it into the Triassic. Ericiolacerta was one of those. It is possible that they gave rise to the cynodonts, the only therapsid group to survive into post-Triassic times. Cynodonts gave rise to mammals.
The theriodonts are a major group of therapsids which appeared during the Middle Permian and which includes the gorgonopsians and the eutheriodonts, itself including the therocephalians and the cynodonts.
Trochosaurus is a dubious genus of therocephalian therapsid from South Africa based upon specimens of Lycosuchidae. Three species of Trochosaurus have appeared in literature, T. intermedius, T. major, and T. dirus. The genus Trochosaurus and all three species are considered nomen dubia today, while the specimens referred to them are regarded as Lycosuchidae incertae sedis.
Eutherocephalia is an extinct clade of advanced therocephalian therapsids. Eutherocephalians are distinguished from the lycosuchids and scylacosaurids, two early therocephalian families. While lycosuchids and scyalosaurids became extinct by the end of the Permian period, eutherocephalians survived the Permian–Triassic extinction event. The group eventually became extinct in the Middle Triassic.
Euchambersia is an extinct genus of therocephalian therapsids that lived during the Late Permian in what is now South Africa and China. The genus contains two species. The type species E. mirabilis was named by paleontologist Robert Broom in 1931 from a skull missing the lower jaw. A second skull, belonging to a probably immature individual, was later described. In 2022, a second species, E. liuyudongi, was named by Jun Liu and Fernando Abdala from a well-preserved skull. It is a member of the family Akidnognathidae, which historically has also been referred by as the synonymous Euchambersiidae.
Theriognathus is an extinct genus of therocephalian therapsid belonging to the family Whaitsiidae, known from fossils from South Africa, Zambia, and Tanzania. Theriognathus has been dated as existing during the Late Permian. Although Theriognathus means mammal jaw, the lower jaw is actually made up of several bones as seen in modern reptiles, in contrast to mammals. Theriognathus displayed many different reptilian and mammalian characteristics. For example, Theriognathus had canine teeth like mammals, and a secondary palate, multiple bones in the mandible, and a typical reptilian jaw joint, all characteristics of reptiles. It is speculated that Theriognathus was either carnivorous or omnivorous based on its teeth, and was suited to hunting small prey in undergrowth. This synapsid adopted a sleek profile of a mammalian predator, with a narrow snout and around 1 meter long. Theriognathus is represented by 56 specimens in the fossil record.
Moschowhaitsia is an extinct genus of therocephalian therapsids from the Late Permian (Guadalupian) of Russia and China. The type species, Moschowhaitsia vjuschkovi, was discovered in the Changxingian-aged Archosaurus Assemblage Zone of Russia and named in 1963 by Russian palaeontologist Leonid Petrovich Tatarinov. A second species was discovered in Jingtai County of Gansu, China in 2020 and named as M. lidaqingi in 2023 by Jun Liu and Fernando Abdala, the first whaitsiid therocephalian to be discovered in China. It was among the larger carnivores in the faunal assemblages it occurred in, with a skull-length of up to 25 centimetres (9.8 in) in M. vjuschkovi and an even larger estimated 35 centimetres (14 in) for M. lidaqingi, one of the largest therocephalian skulls reported. The genus name Moschowhaitsia alludes to two other therocephalians, Moschorhinus and Whaitsia, due to the structure of its palate combining physical features of both these genera.
Tetracynodon is an extinct genus of therocephalian. Fossils of Tetracynodon have been found in the Karoo Basin of South Africa. Two species are known: the type species T. tenuis from the Late Permian and the species T. darti from the Early Triassic. Both species were small-bodied and probably fed on insects and small vertebrates. Although Tetracynodon is more closely related to mammals than to reptiles, its braincase is very primitive and more resembles that of modern amphibians and reptiles than of mammals.
Megawhaitsia is an extinct genus of large therocephalian therapsids who lived during the Late Permian (Wuchiapingian) in what is now Eastern Europe. The only known species is M. patrichae, described in 2008 from several fossils discovered in various oblasts of European Russia. The fossils are representative of a large animal whose skull size is estimated to be 40–50 cm (16–20 in) long.
Glanosuchus is a genus of scylacosaurid therocephalian from the Late Permian of South Africa. The type species G. macrops was named by Robert Broom in 1904. Glanosuchus had a middle ear structure that was intermediate between that of early therapsids and mammals. Ridges in the nasal cavity of Glanosuchus suggest it had an at least partially endothermic metabolism similar to modern mammals.
Promoschorhynchus is a genus of akidnognathid therocephalians from the Late Permian and Early Triassic of South Africa. Unlike many other therapsids, Promoschorhynchus survived the Permian-Triassic extinction event.
Akidnognathidae is an extinct family of therocephalian therapsids from the Late Permian and Early Triassic of South Africa, Russia and China. The family includes many large-bodied therocephalians that were probably carnivorous, including Moschorhinus and Olivierosuchus. One akidnognathid, Euchambersia, may even have been venomous. Akidnognathids have robust skulls with a pair of large caniniform teeth in their upper jaws. The family is morphologically intermediate between the more basal therocephalian group Scylacosauridae and the more derived group Baurioidea.
Baurioidea is a superfamily of therocephalian therapsids. It includes advanced therocephalians such as Regisaurus and Bauria. The superfamily was named by South African paleontologist Robert Broom in 1911. Bauriamorpha, named by D. M. S. Watson and Alfred Romer in 1956, is a junior synonym of Baurioidea.
Bauriidae is an extinct family of therocephalian therapsids. Bauriids were the latest-surviving group of therocephalians after the Permian–Triassic extinction event, going extinct in the Middle Triassic. They are among the most advanced eutherocephalians and possess several mammal-like features such as a secondary palate and wide postcanine teeth at the back of the jaws. Unlike other therocephalians, bauriids were herbivorous. They were also smaller than earlier members of the group. Two subfamilies are classified within Bauriidae: Nothogomphodontinae and Bauriinae.
Choerosaurus is an extinct genus of therocephalian therapsids from the Late Permian of South Africa. The type species Choerosaurus dejageri was named by South African paleontologist Sidney H. Haughton from the Tropidostoma Assemblage Zone in 1929.
Blattoidealestes is an extinct genus of therocephalian therapsid from the Middle Permian of South Africa. The type species Blattoidealestes gracilis was named by South African paleontologist Lieuwe Dirk Boonstra from the Tapinocephalus Assemblage Zone in 1954. Dating back to the Middle Permian, Blattoidealestes is one of the oldest therocephalians. It is similar in appearance to the small therocephalian Perplexisaurus from Russia, and may be closely related.
Eutheriodontia is a clade of therapsids which appear during the Middle Permian and which includes therocephalians and cynodonts, this latter group including mammals and related forms.
Lycideops is an extinct genus of therocephalians from the Late Permian of South Africa. The type species is Lycideops longiceps, named in 1931 by South African paleontologist Robert Broom. Fossils of Lycideops come from the Dicynodon Assemblage Zone of the Beaufort Group. Lycideops is a member of the family Lycideopidae. Like other lycideopids, Lycideops has a long snout.
1 2 Ivakhnenko, M.F. (2011). "Permian and Triassic therocephals (Eutherapsida) of Eastern Europe". Paleontological Journal. 45 (9): 981–1144. doi:10.1134/S0031030111090012. S2CID128958135.
↑ Tatarinov, L.P. (1994). "On the preservation of rudimentary rostral tubular complex of crossopterygians in theriodonts and on possible development of the electroreceptor systems in some members of this group". Doklady Akademii Nauk. 338 (2): 278–281.
1 2 Angielczyk, Kenneth D.; Kammerer, Christian F. (2018). "Non-Mammalian synapsids: the deep roots of the mammalian family tree". In Zachos, Frank E.; Asher, Robert J. (eds.). Mammalian Evolution, Diversity and Systematics. Berlin: De Gruyter. pp.160–162. ISBN9783110275902.
1 2 Huttenlocker, A.K.; Sidor, C.A.; Smith, R.M.H. (2011). "A new specimen of Promoschorhynchus (Therapsida: Therocephalia: Akidnognathidae) from the Lower Triassic of South Africa and its implications for theriodont survivorship across the Permo-Triassic boundary". Journal of Vertebrate Paleontology. 31 (2): 405–421. doi:10.1080/02724634.2011.546720. S2CID129242450.
1 2 3 Van den Heever, J. (1987). The comparative and functional cranial morphology of the early Therocephalia (Amniota: Therapsida) (Ph.D. thesis). University of Stellenbosch.
↑ Kemp, T. S. (1982). Mammal-like reptiles and the origin of mammals. London: Academic Press. ISBN9780124041202.
1 2 Hopson, J. A.; Barghusen, H (1986). "An analysis of therapsid relationships". In Hotton, N.; MacLean, P. D.; Roth, J. J.; Roth, E. C. (eds.). The ecology and biology of mammal-like reptiles. Washington: Smithsonian Institution Press. pp.83–106.
↑ van den Heever, J. A. (1994). "The Cranial Anatomy of the Early Therocephalia (Amniota: Therapsida)". Annals of the University of Stellenbosch. 1. ISBN978-0-7972-0498-0.
↑ Abdala, F.; Kammerer, C. F.; Day, M. O.; Jirah, S.; Rubidge, B. S. (2014). "Adult morphology of the therocephalian Simorhinella baini from the middle Permian of South Africa and the taxonomy, paleobiogeography, and temporal distribution of the Lycosuchidae". Journal of Paleontology. 88 (6): 1139–1153. doi:10.1666/13-186. ISSN0022-3360. S2CID129323281.
↑ Sidor, C. A.; Kulik, Z. T.; Huttenlocker, A. K. (2022). "A new bauriamorph therocephalian adds a novel component to the Lower Triassic tetrapod assemblage of the Fremouw Formation (Transantarctic Basin) of Antarctica". Journal of Vertebrate Paleontology. 41 (6): e2081510. doi:10.1080/02724634.2021.2081510. S2CID250663346.
↑ Sigurdsen, T.; Huttenlocker, A. K.; Modesto, S. P.; Rowe, T. B.; Damiani, R. (2012). "Reassessment of the morphology and paleobiology of the therocephalian Tetracynodon darti (Therapsida), and the phylogenetic relationships of Baurioidea". Journal of Vertebrate Paleontology. 32 (5): 1113–1134. doi:10.1080/02724634.2012.688693. S2CID84457790.
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