Therocephalia
Encyclopedia
Therocephalians are an extinct suborder of carnivorous eutheriodont therapsids that lived from the middle and late Permian
into the Triassic
265.0—245.0 Ma existing for approximately .
The therocephalians ("beast-heads") are named after their large skulls, which, along with the structure of their teeth, suggest that they were successful carnivores. Like other non-mammalian synapsids, therocephalians are described as mammal-like reptiles, although in fact, Therocephalia is the group most closely related to the cynodont
s, which gave rise to the mammal
s. This relationship takes evidence in a variety of skeletal features. The phylogeny of therocephalians has been disputed, as the monophyly
of the group and the relationships of its members are unclear.
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 Gondwana
n origin for the group, which seems to have spread quickly throughout the world. Although almost every therocephalian lineage ended during the great Permian–Triassic extinction event, a few representatives of the subgroup called Eutherocephalia
survived into the Early Triassic
. However, the last therocephalians became extinct by the early Middle Triassic
, possibly due to climate change
and competition with cynodont
s and various groups of reptile
s.
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. There is an enlargement of the temporal opening for broader jaw adductor muscle attachment and a reduction of the phalanges (finger and toe bones) to the mammal
ian phalangeal formula. The presence of an incipient secondary palate
in advanced therocephalians is another feature shared with mammals. The discovery of maxilloturbinal ridges in forms 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.
The therocephalians evolved from an early line of pre-mammalian eutheriodont therapsids, and are the sister group to the cynodonts which include mammals and their ancestors. Therocephalians are at least as ancient as a third large branch of therapsids, the gorgonopsids
, which they resemble in many primitive features. For example, many early therocephalians possess long canine teeth similar to those of gorgonopsids. The therocephalians, however, outlasted the gorgonopsids, 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). Therocephalians and cynodonts both survived the Permian-Triassic mass extinction, but while therocephalians soon went extinct, cynodonts underwent rapid diversification. Therocephalians experienced a decreased rate of cladogenesis
, meaning that few new groups appeared after the extinction. Most Triassic therocephalian lineages originated in the Late Permian, and lasted for only a short period of time in the Triassic.
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).
Below is a cladogram
modified from Sidor (2001) and Huttenlocker (2009):
Below is a cladogram
modified from Huttenlocker et al. (2011):
series, Walking with Monsters
(which look similar to the Thrinaxodon
s from Walking with Dinosaurs
). 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 (e.g., Euchambersia
). Also, a pack of therocephalians appeared in Episode 4.4 of the ITV science-fiction drama Primeval
.
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
into the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
265.0—245.0 Ma existing for approximately .
The therocephalians ("beast-heads") are named after their large skulls, which, along with the structure of their teeth, suggest that they were successful carnivores. Like other non-mammalian synapsids, therocephalians are described as mammal-like reptiles, although in fact, Therocephalia is the group most closely related to the cynodont
Cynodont
Cynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...
s, which gave rise to the mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s. This relationship takes evidence in a variety of skeletal features. The phylogeny of therocephalians has been disputed, as the monophyly
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
of the group and the relationships of its members are unclear.
The fossils of therocephalians are numerous in the Karoo
Karoo
The Karoo is a semi-desert region of South Africa. It has two main sub-regions - the Great Karoo in the north and the Little Karoo in the south. The 'High' Karoo is one of the distinct physiographic provinces of the larger South African Platform division.-Great Karoo:The Great Karoo has an area of...
of South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
, but have also been found in Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
, China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
, and Antarctica. Early therocephalian fossils discovered in Middle Permian deposits of South Africa support a Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
n origin for the group, which seems to have spread quickly throughout the world. Although almost every therocephalian lineage ended during the great Permian–Triassic extinction event, a few representatives of the subgroup called Eutherocephalia
Eutherocephalia
Eutherocephalia is an infraorder of therocephalian therapsids. Eutherocephalians are distinguished from the lycosuchids and scylacosaurids, two early therocephalian families. While lycosuchids and scyalosaurids went extinct by the end of the Permian period, eutherocephalians survived the...
survived into the Early Triassic
Early Triassic
The Early Triassic is the first of three epochs of the Triassic period of the geologic timescale. It spans the time between 251 ± 0.4 Ma and 245 ± 1.5 Ma . Rocks from this epoch are collectively known as the Lower Triassic, which is a unit in chronostratigraphy...
. However, the last therocephalians became extinct by the early Middle Triassic
Middle Triassic
In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided. It spans the time between 245 ± 1.5 Ma and 228 ± 2 Ma...
, possibly due to climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...
and competition with cynodont
Cynodont
Cynodontia or cynodonts are a taxon of therapsids which first appeared in the Late Permian and were eventually distributed throughout all seven continents by the Early Triassic . This clade includes modern mammals and their extinct close relatives. They were one of the most diverse groups of...
s and various groups of reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s.
Anatomy and physiology
Gorgonopsia
Gorgonopsia is a suborder of therapsid synapsids. Their name is a reference to the Gorgons of Greek mythology. Like other therapsids, gorgonopsians were at one time called "mammal-like reptiles"...
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. There is an enlargement of the temporal opening for broader jaw adductor muscle attachment and a reduction of the phalanges (finger and toe bones) to the mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
ian phalangeal formula. The presence of an incipient secondary palate
Secondary palate
The secondary palate is an anatomical structure that divides the nasal cavity from the oral cavity in many vertebrates.In human embryology, it refers to that portion of the hard palate that is formed by the growth of the two palatine shelves medially and their mutual fusion in the midline...
in advanced therocephalians is another feature shared with mammals. The discovery of maxilloturbinal ridges in forms such as the primitive therocephalian Glanosuchus
Glanosuchus
Glanosuchus is an genus of scylacosaurid therocephalian from the Late Permian of South Africa. The type species G. macrops was named by Robert Broom in 1904. Glanosuchus had a middle ear structure that was intermediate between that of early therapsids and mammals...
, suggests that at least some therocephalians may have been warm-blooded.
The later therocephalians included the advanced Baurioidea
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. S. Watson and Alfred Romer in 1956, is a junior synonym of...
, which carried some theriodont
Theriodont
Theriodonts , are a major group of therapsids. They can be defined in traditional, Linnaean terms, in which case they are a suborder of mammal-like reptiles that lived from the Middle Permian to the Middle Cretaceous, or in Cladistic terms, in which case they include not only the traditional...
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
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
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.
Classification
Gorgonopsia
Gorgonopsia is a suborder of therapsid synapsids. Their name is a reference to the Gorgons of Greek mythology. Like other therapsids, gorgonopsians were at one time called "mammal-like reptiles"...
, which they resemble in many primitive features. For example, many early therocephalians possess long canine teeth similar to those of gorgonopsids. The therocephalians, however, outlasted the gorgonopsids, persisting into the early-Middle Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
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
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
, such as the loss of the postorbital bar
Postorbital bar
The postorbital bar is a bone which runs around the eyesocket of strepsirrhine primates. This is in contrast to the higher primates, haplorrhine, which have evolved fully enclosed sockets to protect their eyes....
in some forms, a mammalian phalangeal formula, and some form of a secondary palate in most taxa (see below). Therocephalians and cynodonts both survived the Permian-Triassic mass extinction, but while therocephalians soon went extinct, cynodonts underwent rapid diversification. Therocephalians experienced a decreased rate of cladogenesis
Cladogenesis
Cladogenesis is an evolutionary splitting event in a species in which each branch and its smaller branches forms a "clade", an evolutionary mechanism and a process of adaptive evolution that leads to the development of a greater variety of sister species...
, meaning that few new groups appeared after the extinction. Most Triassic therocephalian lineages originated in the Late Permian, and lasted for only a short period of time in the Triassic.
Taxonomy and phylogeny
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
Synapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
supporting the monophyly
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
of this group, and Lycosuchidae is currently considered the most basal clade within a monophyletic Therocephalia (van den Heever, 1994).
- Order TherapsidaTherapsidaTherapsida is a group of the most advanced synapsids, and include the ancestors of mammals. Many of the traits today seen as unique to mammals had their origin within early therapsids, including hair, lactation, and an erect posture. The earliest fossil attributed to Therapsida is believed to be...
- Suborder Therocephalia
- Family Lycosuchidae
- (unranked) ScylacosauriaScylacosauriaScylacosauria 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 therocephalian family. Thus, Scylacosauria includes all...
van den Heever, 1994- Family ScylacosauridaeScylacosauridaeScylacosauridae is a 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. Scylacosaurids have long snouts and unusual saber-like canine...
- Infraorder EutherocephaliaEutherocephaliaEutherocephalia is an infraorder of therocephalian therapsids. Eutherocephalians are distinguished from the lycosuchids and scylacosaurids, two early therocephalian families. While lycosuchids and scyalosaurids went extinct by the end of the Permian period, eutherocephalians survived the...
Hopson & Barghusen, 1986- Family HofmeyriidaeHofmeyriidaeHofmeyriidae is a family of therocephalian therapsids. It includes the genera Hofmeyria and Ictidostoma.-External links:* in the Paleobiology Database...
- Family Akidnognathidae (=MoschorhinidaeMoschorhinidaeAkidnognathidae is a family of small therocephalian therapsids.-Ecology:Akidnognathids were carnivores, hunting prey such as Dicynodonts....
, Annatherapsididae, Euchambersiidae) - Family Whaitsiidae
- Family Hofmeyriidae
- Superfamily BaurioideaBaurioideaBaurioidea 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. S. Watson and Alfred Romer in 1956, is a junior synonym of...
- Family BauriidaeBauriidaeBauriidae is a family of therocephalian therapsids. Bauriids are among the most advanced eutherocephalians and possess several mammal-like features such as a secondary palate. Unlike other therocephalians, bauriids were herbivorous. Two subfamilies are classified within Bauriidae:...
- Subfamily Nothogomphodontinae
- Subfamily BauriinaeBauriinaeBauriinae is a subfamily of therocephalian therapsids. It is one of two subdivisions of the family Bauriinae, the other being Nothogomphodontinae....
- Family Ericiolacertidae
- Family Ictidosuchidae
- Family Ictidosuchopsidae
- Family Lycideopsidae
- Family Regisauridae
- Family Bauriidae
- Family Scylacosauridae
- Suborder Therocephalia
Below is a cladogram
Cladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...
modified from Sidor (2001) and Huttenlocker (2009):
Below is a cladogram
Cladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...
modified from Huttenlocker et al. (2011):
In popular culture
A pack of unspecified therocephalians appeared in the third episode of the BBCBBC
The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
series, Walking with Monsters
Walking with Monsters
Walking with Monsters is a three-part British documentary film series about life in the Paleozoic, bringing to life extinct arthropods, fish, amphibians, synapsids, and reptiles...
(which look similar to the Thrinaxodon
Thrinaxodon
Thrinaxodon was a cynodont, an ermine-sized therapsid. Pits on the skull suggest that Thrinaxodon may have had whiskers, and by extension a protective covering of fur. There are suggestions that it was warm-blooded...
s from Walking with Dinosaurs
Walking with Dinosaurs
Walking with Dinosaurs is a six-part documentary television miniseries that was produced by BBC, narrated by Kenneth Branagh, and first aired in the United Kingdom, in 1999. The series was subsequently aired in North America on the Discovery Channel in 2000, with Branagh's voice replaced with that...
). One of them attacks a Lystrosaurus
Lystrosaurus
Lystrosaurus was a genus of Late Permian and Early Triassic Period dicynodont therapsids, which lived around 250 million years ago in what is now Antarctica, India, and South Africa...
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 (e.g., Euchambersia
Euchambersia
Euchambersia was a therocephalian therapsid that lived during the Late Permian epoch, approximately 250 million years ago, in what is now South Africa....
). Also, a pack of therocephalians appeared in Episode 4.4 of the ITV science-fiction drama Primeval
Primeval
Primeval or primæval may refer to:* Primeval, a British science fiction television series.* Primeval , a 2007 film* Primeval , a score of music from the BBC TV series Doctor Who...
.