The theriodonts (

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

Theriodontia) are a major group of

therapsid Therapsida is a major group of eupelycosaurian synapsids that includes mammals, 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 ...

s which appeared during the

Middle Permian The Guadalupian is the second and middle series/ epoch of the Permian. The Guadalupian was preceded by the Cisuralian and followed by the Lopingian. It is named after the Guadalupe Mountains of New Mexico and Texas, and dates between 272.95 ± ...

and which includes the gorgonopsians and the

eutheriodont 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. With the dicynodonts, they form one of two lineages of thera ...

s, itself including the

therocephalia Therocephalia is an extinct suborder of eutheriodont therapsids (mammals and their close relatives) from the Permian and Triassic. The therocephalians ("beast-heads") are named after their large skulls, which, along with the structure of thei ...

ns and the

cynodont The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide varie ...

Naming

In 1876,

Richard Owen Sir Richard Owen (20 July 1804 – 18 December 1892) was an English biologist, comparative anatomist and paleontologist. Owen is generally considered to have been an outstanding naturalist with a remarkable gift for interpreting fossils. Ow ...

named a suborder Theriodontia, which he divided into the Cynodontia and the Gomphodontia. The modern

concept was devised by James Allen Hopson. In his system, Theriodontia fall into two main groups: the

Gorgonopsia Gorgonopsia (from the Greek Gorgon, a mythological beast, and 'aspect') is an extinct clade of sabre-toothed therapsids from the Middle to Upper Permian roughly 265 to 252 million years ago. They are characterised by a long and narrow skull, a ...

and the Eutheriodontia. The latter consist of the

Therocephalia Therocephalia is an extinct suborder of eutheriodont therapsids (mammals and their close relatives) from the Permian and Triassic. The therocephalians ("beast-heads") are named after their large skulls, which, along with the structure of thei ...

and Cynodontia.J.A. Hopson. 1999. "Therapsids". ''Encyclopedia of Paleontology'' 2: 1256-1266

Evolution

Theriodonts appeared at the same time as their

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

within the Neotherapsida, the

Anomodontia Anomodontia is an extinct group of non-mammalian therapsids from the Permian and Triassic periods. By far the most speciose group are the dicynodonts, a clade of beaked, tusked herbivores.Chinsamy-Turan, A. (2011) ''Forerunners of Mammals: Ra ...

, about 270 million years ago, in the

. Even these early theriodonts were more mammal-like than their

anomodont Anomodontia is an extinct group of non-mammalian therapsids from the Permian and Triassic periods. By far the most speciose group are the dicynodonts, a clade of beaked, tusked herbivores.Chinsamy-Turan, A. (2011) ''Forerunners of Mammals: Ra ...

and

dinocephalia Dinocephalians (terrible heads) are a clade of large-bodied early therapsids that flourished in the Early and Middle Permian between 279.5 and 260 million years ago (Ma), but became extinct during the Capitanian mass extinction event. Dinocephal ...

n contemporaries. Early theriodonts may have been

warm-blooded Warm-blooded is an informal term referring to animal species which can maintain a body temperature higher than their environment. In particular, homeothermic species maintain a stable body temperature by regulating metabolic processes. The on ...

. Early forms were carnivorous, but several later groups became herbivorous during the

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...

. Theriodont jaws were more mammal-like than was the case of other therapsids, because their

dentary In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

was larger, which gave them more efficient chewing ability. Furthermore, several other bones that were on the lower jaw (found in

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchocephalia ...

s), 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

synapsid Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...

Eutheriodontia

Eutheriodontia refers to all theriodonts except the gorgonopsians (the most "primitive" group). They included the therocephalians and the cynodonts. The cynodonts include the mammals. The eutheriodonts have larger skulls, accommodating larger

brain A brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as Visual perception, vision. I ...

s and improved jaw muscles. The eutheriodontian theriodonts are one of the two

survivors of the great

Permian–Triassic extinction event The Permian–Triassic (P–T, P–Tr) extinction event, also known as the Latest Permian extinction event, the End-Permian Extinction and colloquially as the Great Dying, formed the boundary between the Permian and Triassic geologic periods, ...

, the other being the

dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typic ...

s. Therocephalians included both carnivorous and herbivorous forms; both died out after the

Early Triassic The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which is ...

. The remaining theriodonts, the cynodonts, also included carnivores, such as '' Cynognathus'', as well as newly evolved herbivores ( Traversodontidae). While traversodontids for the most part remained medium-sized to reasonably large (the length of the largest species was up to two meters), the carnivorous forms became progressively smaller as the Triassic progressed. They "miniaturised". By the Late Triassic, the small cynodonts included the rodent-like

Tritylodontidae Tritylodontidae ("three-knob teeth", named after the shape of their cheek teeth) is an extinct family of small to medium-sized, highly specialized mammal-like cynodonts, bearing several mammalian traits like erect limbs, endothermy and details ...

(possibly related to or descended from traversodontids), and the tiny, shrew-like,

Trithelodontidae Tritheledontidae, the tritheledontids or ictidosaurs, is an extinct family of small to medium-sized (about 10 to 20 cm long) cynodonts. They were highly mammal-like, specialized cynodonts, although they still retained a few reptile-like ana ...

, related to the

Mammaliaformes Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent c ...

. The trithelodontids died out during the

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

, and the tritylodontids survived into the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

, but their relatives, the mammals, continued to evolve. Many mammal groups managed to survive the

Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the ...

, which wiped out the non-

avian Avian may refer to: * Birds or Aves, winged animals *Avian (given name) (russian: Авиа́н, link=no), a male forename Aviation *Avro Avian, a series of light aircraft made by Avro in the 1920s and 1930s *Avian Limited, a hang glider manufactur ...

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

s, allowing the

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...

s to diversify and dominate the Earth.

Taxonomy

Classification

* Order Therapsida ** Theriodont * *** Suborder †

**** Family †

Gorgonopsidae Gorgonopsia (from the Greek Gorgon, a mythological beast, and 'aspect') is an extinct clade of sabre-toothed therapsids from the Middle to Upper Permian roughly 265 to 252 million years ago. They are characterised by a long and narrow skull, ...

*** Eutheriodontia **** Suborder †

***** Family †

Lycosuchidae Lycosuchidae is an extinct family of therocephalian therapsids from the Middle Permian Beaufort Group of South Africa. It currently contains two monotypic genera, '' Lycosuchus'', represented by ''L. vanderrieti'', which was named by paleontologi ...

***** (unranked) †

Scylacosauria Scylacosauria is a clade of therocephalian therapsids. It includes the basal family Scylacosauridae and the infraorder Eutherocephalia. Scylacosauridae and Eutherocephalia form this clade to the exclusion of Lycosuchidae, the most basal theroc ...

****** Family †

Scylacosauridae Scylacosauridae is an extinct family of therocephalian therapsids. Scylacosaurids lived during the Permian period and were among the most basal therocephalians. The family was named by South African paleontologist Robert Broom in 1903. Scylacosa ...

****** Infraorder †

Eutherocephalia Eutherocephalia ("true beast head") is an extinct clade of advanced therocephalian therapsids. Eutherocephalians are distinguished from the lycosuchids and scylacosaurids, two early therocephalian families. While lycosuchids and scyalosaurids ...

******* Family †

Hofmeyriidae Hofmeyriidae is a family of therocephalian therapsids. It includes the genus '' Ictidostoma''. References External links Hofmeyriidaein the Paleobiology Database The Paleobiology Database is an online resource for information on the distrib ...

******* Family †

Moschorhinidae 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 ''Mosch ...

******* Family †

Whaitsiidae Whaitsiidae is an extinct family of therocephalian therapsid Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin ...

******* Superfamily †

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 ...

******** Family †

Bauriidae 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 euthero ...

******** Family † Ericiolacteridae ******** Family †

Ictidosuchidae Ictidosuchidae is an extinct family of therocephalian therapsid Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origi ...

********* Genus †''

Ictidosuchoides ''Ictidosuchoides'' is an extinct genus of ictidosuchid therocephalians. Fossils have been found from the Karoo Basin in South Africa. The genus is known to have been one of the few therocephalians to have survived the Permian-Triassic extinct ...

'' ********* Genus †''

Ictidosuchus ''Ictidosuchus'' is an extinct genus of therocephalian therapsid Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their orig ...

'' ******** Family † Ictidosuchopsidae ******** Family †

Lycideopidae Lycideopidae is an extinct family of therocephalians from the Late Permian and Early Triassic The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between Ma and Ma (million yea ...

**** Suborder Cynodontia

Phylogenetics of Theriodontia

* Therapsida **†

Biarmosuchia Biarmosuchians are an extinct clade of non-mammalian synapsids from the Permian. They are the most basal group of the therapsids. All of them were moderately-sized, lightly-built carnivores, intermediate in form between basal sphenacodont " pelyc ...

**†

Eotitanosuchia Eotitanosuchidae is an extinct family of biarmosuchian therapsids. The Eotitanosuchidae were large predatory therapsids of the Wordian epoch. It was once considered to belong to a separate infraorder of therapsids called Eotitanosuchia. Characte ...

**†

Dinocephalia Dinocephalians (terrible heads) are a clade of large-bodied early therapsids that flourished in the Early and Middle Permian between 279.5 and 260 million years ago (Ma), but became extinct during the Capitanian mass extinction event. Dinocephal ...

**†

**Theriodontia ***†

***†

*** Cynodontia ****† Dviniidae ****

Mammalia Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...

****†

Procynosuchidae Procynosuchidae is an extinct family of therapsids which, along with Dviniidae, were the earliest cynodonts. They appeared around 260 million years ago, and were most abundant during the latest Permian time (251 mya), shortly before the Permia ...

****† Galesauridae ****†

Thrinaxodontidae Thrinaxodontidae is an extinct family of cynodonts that includes the genera ''Thrinaxodon'', '' Nanictosaurus'', and '' Nanocynodon'', and possibly '' Bolotridon'', '' Novocynodon'' and '' Platycraniellus''. All thrinaxodontids share a bony secon ...

****† Cynognathidae ****† Gomphodontia ****† Chiniquodontidae ****† Probainognathidae ****† Tritheledontidae (Ictidosauria)

Notes

References

External links

Theriodontia
- at Paleos {{Taxonbar, from=Q132812 Therapsids Guadalupian first appearances Taxa named by Richard Owen

Naming

Evolution

Eutheriodontia

Taxonomy

Classification

Phylogenetics of Theriodontia

See also

Notes

References

External links