Theriodontia

Theriodontia Fossil range: Middle Permian–Early Cretaceous, Descendant taxon Mammalia survives to present.
Scientific classification
Class	Synapsida
Order	Therapsida
(Unranked)	Theriodontia
Groups	Gorgonopsia Eutheriodontia Therocephalia Cynodontia Mammaliformes Mammalia

Theriodonts ("Beast Tooth", referring to more mammal-like teeth), are the third main group of therapsids. They can be defined in traditional, Linnaean terms, in which case they are a suborder of synapsids that lived from the Middle Permian to the Middle Cretaceous, or in cladistic terms, in which case they include not only the traditional theriodonts but also their descendants the mammals as well (in the same way that, cladistically speaking, the theropod dinosaurs include the birds as a sub-clade).

Theriodonts appeared almost the same time as the anomodonts, about 265 million years ago, in the Middle Permian. Even these early theriodonts were more mammal-like than their Anomodont and Dinocephalian contemporaries.

Scylacosaurus.

Theriodonts fall into three main groups: Gorgonopsia, Therocephalia and Cynodontia. Early Theriodonts may have been warm-blooded. Early forms were carnivorous, but several later groups became herbivorous during the Triassic.

Theriodont jaws were more mammal-like than was the case of other therapsids, because their dentary was larger, which gave them a more efficient bite. Furthermore, several other bones that were on the lower jaw (found in reptiles), moved into the ears, allowing the theriodonts to hear better and their mouths to open wider. This made the theriodonts the most successful group of synapsids.

Eutheriodonts refer to all theriodonts except the Gorgonopsians (the most primitive group). They included the therocephalians, cynodonts and their descendants - the mammals . The name means "true beast tooth". The eutheriodonts have larger skulls, accommodating larger brains and improved jaw muscles.

The theriodonts (Eutheriodonts) are one of the two synapsid survivors of the great Permian–Triassic extinction event, the other being the dicynodonts. Therocephalians included both carnivorous and herbivorous forms; both died out after the Early Triassic. The remaining theriodonts, the cynodonts, also included carrnivores such as Cynognathus, as well as newly evolved herbivorous (Traversodonts). While Traversodonts for the most part remained medium-sized to reasonably large (length of largest species up to 2 meters), the carnivorous forms became progressively smaller as the Triassic progressed. By the Late Triassic the small cynodonts included the rodent-like tritylodonts (possibly related to or descended from travsersodonts), and the tiny, shrew-like, trithelodonts, which evolved into the first mammals. The trithelodonts died out during the Jurassic, and the tritylodonts in the Cretaceous, but the mammals continued to evolve. Many mammal groups managed to survive the Cretaceous–Tertiary extinction event, which wiped out the non-avian dinosaurs, allowing the mammals to diversify and dominate the Earth.

Phylogeny

Plylogenetics of Theriodontia from Tree of Life ([www.tolweb.org Tree of Life])

• Therapsida
  • Mammals and extinct relatives

• • • Biarmosuchia
    • Eotitanosuchia
    • Dinocephalia
    • Anomodontia
    • Theriodontia
      • Gorgonopsia
      • Therocephalia
      • Cynodontia
        • Diviniidae
        • Mammalia
        • Procynosuchidae
        • Galesauridae
        • Thrinaxodontidae
        • Cynognathidae
        • Gomphodonts
        • Chiniquodontidae
        • Probainognathidae
        • Tritheledontidae (Ictidosauria)

See also

• Evolution of mammals
• Anomodonts
• Timeline of evolution

External links

• Theriodontia - at Paleos