Information about Therocephalia

Therocephalia Fossil range: Middle Permian to Middle Triassic
Bauria, an advanced therocephalian from the Early Triassic of South Africa Bauria, an advanced therocephalian from the Early Triassic of South Africa
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Synapsida Order: Therapsida (unranked) Theriodontia Suborder: Therocephalia Broom, 1905
Families
See "Taxonomy"

Therocephalians are an extinct lineage of eutheriodont therapsids that lived throughout the middle and late Permian and into the Triassic. The therocephalians (literally, "beast-heads") are named after their large skulls, which, along with their teeth, suggest that most were successful carnivores.

While synapsids (therocephalians included) are not part of the reptile clade, they are often called "mammal-like reptiles." They are closely related to the cynodonts, which gave rise to the mammals; this relationship takes evidence in a variety of anatomical features, possibly including whiskers and hair.

Many therocephalian lineages ended during the great Permo-Triassic extinction event. However, a few representatives of the subgroup called Eutherocephalia survived into the Early Triassic and continued to diversify. The last therocephalians became extinct by the early Middle Triassic, possibly due to climatic changes and competition with cynodonts and various groups of reptiles.

Their fossils are numerous in the Karoo Basin, South Africa, but have also been found in Russia, China, and Antarctica. There remain many unanswered questions about the phylogeny, anatomy, and physiology of therocephalians. Early therocephalian fossils discovered in Middle Permian deposits of South Africa support a Gondwanan origin for the group, but like the dicynodonts, therocephalians spread quickly throughout the world.

Classification

The therocephalians evolved from an early line of pre-mammalian therapsids called 'theriodonts', and are a sister group to the cynodonts which include mammals and their ancestors. Therocephalians are at least as ancient as a third large branch of theraspids, the gorgonopsids (also 'theriodonts'), which they resemble in many primitive features. The therocephalians, however, outlasted the gorgonopsians, persisting into the early-Middle Triassic period.

While common ancestry with cynodonts (and, thus, mammals) accounts for many similarities among 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 (see below). A current consensus of the taxonomic framework of therocephalians is provided at the bottom of the page.

Anatomy and Physiology

Like the gorgonopsids and many cynodonts, many therocephalians were presumably carnivores. It has been proposed that at least some therocephalians (such as the primitive therocephalian Glanosuchus) were warm-blooded due to the discovery of maxilloturbinal ridges. The earlier therocephalians were in many respects as primitive as the gorgonopsids, but they did show certain advanced features such as

• enlargement of the temporal opening for broader jaw adductor muscle attachment
• reduction of the phalanges (finger and toe bones) to the mammalian phalangeal formula.
• the presence of an incipient secondary palate

The later therocephalians included the advanced Baurioidea, which carried some theriodont characteristics to a high degree of specialization. For instance, there was no ossified postorbital bar separating the orbit from the temporal opening in small baurioids and the herbivorous Bauria, a condition typical for 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 and among the Therocephalia.

Taxonomy and Phylogeny

It is not surprising that some previously recognized therocephalian clades have turned out to be artificial. For example, the Scaloposauridae were classified based on fossils with mostly juvenile characteristics, but probably represent immature specimens from other known therocephalian families. On the other hand, the aberrant therocephalian family, Lycosuchidae, once identified by the presence of multiple caniniform teeth, was thought to represent an unnatural group based on a study of canine replacement in that group (van den Heever, 1980). However, subsequent analysis has exposed additional synapomorphies supporting the monophyly of this group, and Lycosuchidae is currently considered the most basal clade within a monophyletic Therocephalia (van den Heever, 1994). Order Therapsida

• Suborder Therocephalia
• Family Lycosuchidae
• Scylacosauria van den Heever, 1994
• Family Scylacosauridae
• Infraorder Eutherocephalia Hopson & Barghusen, 1986
• Family Euchambersiidae (=Annatherapsididae, Akidnognathidae, Moschorhinidae)
• Family Hofmeyriidae
• Family Whaitsiidae
• Superfamily Baurioidea
• Family Ictidosuchidae
• Family Ictidosuchopsidae
• Family Regisauridae
• Family Lycideopsidae
• Family Ericiolacertidae
• Ericiolacerta
• Family Bauriidae
• Subfamily Nothogomphodontinae
• Subfamily Bauriinae

In popular culture

A pack of unspecified therocephalians appeared in the BBC series, Walking with Monsters. One of them attacks a Lystrosaurus during the night, injecting the prey with a neurotoxin delivered by a poison gland in the cheek. Although the existence of such a gland is controversial, the absence of postcanine teeth in association with a maxillary pit and grooved caniniform teeth indicate that a venom delivery apparatus may have existed in some therocephalian therapsids (i.e., Euchambersia).

References

• Sigurdsen, T. 2006. “New features of the snout and orbit of a therocephalian therapsid from South Africa.” Acta Palaeontologica Polonica 51 (1) 63-75.
• van den Heever JA. 1980. On the validity of the therocephalian family Lycosuchidae (Reptilia, Therapsida). Annals of the South African Museum 81: 111-125.
• van den Heever JA. 1994. The cranial anatomy of the early Therocephalia (Amniota: Therapsida). Annals of the University of Stellenbosch 1994: 1-59.