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Therocephalia

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Therocephalia
Fossil range: Middle Permian - Middle Triassic
Bauria BW
Bauria, a derived therocephalian from the Early Triassic of South Africa
Scientific classification

Class:

Synapsida

Order:

Therapsida

(Unranked) :

Theriodontia

Suborder:

Therocephalia Broom, 1905

Families:

See "Taxonomy" section.

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.

Like other non-mammalian synapsids, therocephalians are often described as "mammal-like reptiles", although they are not reptiles in the cladistic sense. In fact, Therocephalia is the group most 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. There remain many unanswered questions about the phylogeny, anatomy, and physiology of therocephalians.

The fossils of therocephalians are numerous in the Karoo of South Africa, but have also been found in Russia, China, 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 throughout the world. Although therocephalian lineages ended during the great Permian–Triassic extinction event, a few representatives of the subgroup called Eutherocephalia survived into the Early Triassic and continued to diversify. However, the last therocephalians became extinct by the early Middle Triassic, possibly due to climate change and competition with cynodonts and various groups of reptiles.

ClassificationEdit

Moschorhinus DB

Moschorhinus.

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 PhysiologyEdit

Pristeroognathus DB

Pristerognathus

Like the gorgonopsids and many cynodonts, many therocephalians were presumably carnivores. 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 discovery of maxilloturbinal ridges in some specimens, such as the primitive therocephalian Glanosuchus, suggests that at least some therocephalians may have been warm-blooded.

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.

Taxonomy and PhylogenyEdit

ScaloposaurusDB

Scaloposaurus

Lycosuchus1

Lycosuchus head reconstruction

Aloposaurus

Aloposaurus.

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

ReferencesEdit

  • 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.

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