Francais | English | Espanõl

From Wikipedia, the free encyclopedia

iTherocephalia Fossil range: Middle Permian to Middle Triassic
Image:Ericiolacerta.jpg Ericiolacerta, a notable therocephalian.
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Synapsida Order: Therapsida (unranked) Theriodontia Suborder: Therocephalia Broom, 1905
Families
See "Taxonomy"

The Therocephalians are a lineage of therapsids, and therefore sometimes described as mammal-like reptiles. More specifically, the therocephalians were a line of eutheriodonts, closely related to the cynodonts (the eventual ancestors of the mammals) as well as the gorgonopsids.

Most therocephalians went extinct by the great Permian-Triassic extinction event. However, a subgroup called eutherocephalians survived into the Early Triassic. The last therocephalians became extinct by the Middle Triassic, pushed to extinction by climate and competition with carnivorous cynodonts and various groups of reptiles.

Fossils are numerous in Karoo, South Africa, but have also been found in Russia, China, and Antarctica. There remain many unanswered questions about the phylogeny, anatomy, and physiology of this group of animals. Nevertheless, the therocephalians (literally, "beast head") are named after their massive skulls, which, along with their teeth, suggest that they were successful carnivores.

Contents 1 Classification 2 Anatomy and Physiology 3 Taxonomy and Phylogeny 4 In Popular Culture 5 References

[edit] Classification

The therocephalians evolved from an early line of nonmammalian synapsids, and are the sister group to a branch of therapsids called the cynodonts. They are at least as ancient as a third large branch, the gorgonopsids, which they also resemble, but in fact outlasted, persisting into the early-Mid 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.

[edit] Anatomy and Physiology

Like the gorgonopsids and most cynodonts, the therocephalians were carnivores. Some scientists propose 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 bar of bone separating the orbit from the temporal opening, 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 therocephalian bauriamorph stem. Mammalian characteristics such as this one seem to have evolved in parallel among a number of different groups, including the therocephalians.

[edit] Taxonomy and Phylogeny

It is not surprising that some 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.

The aberrant therocephalian family, Lycosuchidae, once identified by the presence of multiple caniniform teeth, was thought to represent an unnatural group (van den Heever, 1980). However, subsequent analysis has revealed a number of 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
      • Family Akidnognathidae (=Annatherapsididae, Euchambersiidae, Moschorhinidae)
      • Family Hofmeyriidae
      • Family Whaitsiidae
      • Superfamily Baurioidea
        • Family Ictidosuchidae
        • Family Ictidosuchopsidae
        • Family Regisauridae
        • Family Lycideopsidae
        • Family Ericiolacertidae
          • Ericiolacerta
        • Family Bauriidae
          • Subfamily Nothogomphodontinae
          • Subfamily Bauriinae

[edit] 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).

[edit] References

• Sigurdsen, T. 2006. “Newfeatures 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.nl:Therocephalia