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 term, ...

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 ± 0. ...

and which includes the

gorgonopsians 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

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 the ...

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 their ...

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 variety ...

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. Owe ...

named a suborder Theriodontia, which he divided into the Cynodontia and the

Gomphodontia Gomphodontia is a clade of cynognathian cynodonts that includes the families Diademodontidae, Trirachodontidae, and Traversodontidae. Gomphodonts are distinguished by wide and closely spaced molar-like postcanine teeth, which are convergent wi ...

. 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 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 therap ...

. 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 their ...

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

Evolution

Theriodonts appeared at the same time as their sister group within the

Neotherapsida 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 mor ...

, the Anomodontia, 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 onl ...

. 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 period ...

. Theriodont

jaw The jaw is any opposable articulated structure at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth and serv ...

s 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 tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...

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 rhynchocephalians ( ...

s), moved into the

ear An ear is the organ that enables hearing and, in mammals, body balance using the vestibular system. In mammals, the ear is usually described as having three parts—the outer ear, the middle ear and the inner ear. The outer ear consists of ...

s, 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 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 vision. It is the most complex organ in a v ...

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, as ...

, 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 a un ...

. The remaining theriodonts, the cynodonts, also included carnivores, such as ''

Cynognathus ''Cynognathus'' is an extinct genus of large-bodied cynodontian therapsids that lived in the Middle Triassic. It is known from a single species, ''Cynognathus crateronotus''. ''Cynognathus'' was a long predator closely related to mammals and ha ...

'', as well as newly evolved herbivores (

Traversodontidae Traversodontidae is an extinct family of herbivorous cynodonts. Traversodonts were primarily Gondwanan, with many species known from Africa and South America. Recently, traversodonts have also been found from Europe and eastern North America. Tra ...

). 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 detail ...

(possibly related to or descended from traversodontids), and the tiny, shrew-like, Trithelodontidae, related to the Mammaliaformes. The trithelodontids died out during the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), 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 J ...

, 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 th ...

, 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 manufacture ...

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 or ...

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, ...

***

**** Suborder †

***** Family † Lycosuchidae ***** (unranked) † Scylacosauria ****** 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. Scylacos ...

****** Infraorder † Eutherocephalia ******* 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 ******* Family † Whaitsiidae ******* 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 ******** Family † Ericiolacteridae ******** Family † Ictidosuchidae ********* Genus †'' Ictidosuchoides'' ********* Genus †'' Ictidosuchus'' ******** Family † Ictidosuchopsidae ******** Family † Lycideopidae **** 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 **†

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 ...

**† Anomodontia **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 o ...

****† Procynosuchidae ****†

Galesauridae Galesauridae is an extinct family of cynodonts. Along with the family Thrinaxodontidae and the extensive clade Eucynodontia (which includes mammals), it makes up the unranked taxon called Epicynodontia. Galesaurids first appeared in the very la ...

****† Thrinaxodontidae ****† Cynognathidae ****†

****†

Chiniquodontidae Chiniquodontidae is an extinct family of basal probainognathian cynodonts that lived in what is now Africa and South America during the Middle and Late Triassic. It is currently thought to include two valid genera: '' Aleodon'' and ''Chiniquod ...

****†

Probainognathidae Probainognathidae is an extinct family of insectivorous cynodonts which lived in what is now South America during the Middle to Late Triassic. The family was established by Alfred Romer in 1973 and includes two genera, ''Probainognathus'' from ...

****†

Tritheledontidae 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 ...

(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