Trirachodon
Encyclopedia
Trirachodon is an extinct genus
of tritylodontoid cynodont. Fossil
s have been found from the Cynognathus Assemblage Zone
of the Beaufort Group
in South Africa
, dating back to the Early
and Middle Triassic
.
es were relatively slender. Trirachodon was quite small for a cynodont, growing no larger than 50cm in length. It had noticeably less molariform
teeth than its closely related contemporary Diademodon
. These teeth tended to be tranversely broader than Diademodon as well. A bony secondary palate and precise postcanine tooth occlusion are seen as derived characteristics in Trirachodon that are similar to those of mammal
s.
is T. berryi, named in 1895 on the basis of a single cranial skeleton. Three other specimens were later referred to T. kannemeyeri, which was distinguished from the type on the basis of snout length and number of postcanine teeth. These differences have since been considered to small to assign them to two different species, and thus the T. kannemeyeri has fallen out of use due to this possible synonymy.
A new species, T. minor, was named by Robert Broom
in 1905 to describe a poorly preserved snout. Broom later named T. browni in 1915, in which he distinguished it from all other species on the basis of the length of the molars. In 1932, Broom proposed that T. berryi be reassigned to a new genus, Trirachodontoides. Another species of Trirachodon called T. angustifrons was named in 1946 from a narrow skull found in Tanzania
, but this material was later proven to be from the traversodontid
Scalenodon
. All species of Trirachodon were suggested to by synonymous with the type species in 1972 except T. browni, which was reassigned to Diademodon tetragonus.
, South Africa as well as the Omingonde Formation in Namibia
have been attributed to the genus. At least 20 individuals have been found in one of the complexes. The entrance shafts slope down at shallow angles and have bilobate floors and vaulted roofs. The floors of the lower levels are less noticeably bilobate. The burrows typically terminate quite narrow. The tunnels tend to tightly curve as they progress deeper, with chambers branching off at right angles to the main tunnel. A semi-erect posture of the hindlimbs of Trirachodon is seen as an adaptation for sustained efficiency in locomotion in the tunnels. The relatively thick walls seen in these bones may also have provided extra rigidity to the limbs while digging. The burrows where the occupants were preserved inside are thought to have been filled with sediment in a flash flood; if it were a gradual filling, the occupants would have had time to evacuate.
Many features of the burrows suggest that they were used as colonial dwelling structures. The wide entrance would have been useful for a burrow inhabited by many individuals, and branching tunnels and terminating chambers would unlikely have been made by one animal. The worn, bilobate floors suggest that the tunnels were used rather frequently by numerous inhabitants as they passed one another while moving through them.
A colonial lifestyle for Trirachodon suggests complex social behaviors previously thought to be unique to Cenozoic
mammals, and is one of the earliest signs of cohabitation in a burrow complex by tetrapods (a partial burrow cast associated with Thrinaxodon liorhinus
, also from the Beaufort Group, has recently been found that predates these burrows by several million years). There have been many suggested reasons for this behavior in Trirachodon, including protection from predation, sites for reproduction and or rearing young, and thermoregulation.
Recent studies in the bone histology of many specimens of Trirachodon have led to an increased understanding of the ontogeny and lifestyle of these animals. There is evidence in the growth rings of bones that growth rates in these animals was strongly influenced by the fluctuation in seasonal conditions in their environment.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of tritylodontoid cynodont. Fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s have been found from the Cynognathus Assemblage Zone
Cynognathus Assemblage Zone
The Cynognathus Assemblage Zone is a geological stratum and a faunal zone of the Beaufort Group, of the South African Karoo. The name refers to Cynognathus, a genus of eucynodontian mammal-like reptile, whose fossils have been found in that structure....
of the Beaufort Group
Beaufort Group
The Beaufort Group is the third of the main subdivisions of the Karoo Supergroup of geological strata in Southern Africa. It follows conformably after the Ecca Group and consists essentially of sandstones and shales, deposited in the Karoo Basin from the Middle Permian to the early part of the...
in 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...
, dating back to the Early
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...
and 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...
.
Description
The skull of Trirachodon had a short, narrow snout with a wide orbital region. The zygomatic archZygomatic arch
The zygomatic arch or cheek bone is formed by the zygomatic process of temporal bone and the temporal process of the zygomatic bone , the two being united by an oblique suture; the tendon of the Temporalis passes medial to the arch to gain insertion into the coronoid process...
es were relatively slender. Trirachodon was quite small for a cynodont, growing no larger than 50cm in length. It had noticeably less molariform
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
teeth than its closely related contemporary Diademodon
Diademodon
Diademodon is an vegetarian extinct genus of therapsid. It was about the size of a small cow. Fossils of the Diademodon tetragonus were found in levels of the Rio Seco de la Quebrada geological formation in Mendoza Province, Argentina....
. These teeth tended to be tranversely broader than Diademodon as well. A bony secondary palate and precise postcanine tooth occlusion are seen as derived characteristics in Trirachodon that are similar to those of 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.
Species
The type speciesType species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
is T. berryi, named in 1895 on the basis of a single cranial skeleton. Three other specimens were later referred to T. kannemeyeri, which was distinguished from the type on the basis of snout length and number of postcanine teeth. These differences have since been considered to small to assign them to two different species, and thus the T. kannemeyeri has fallen out of use due to this possible synonymy.
A new species, T. minor, was named by Robert Broom
Robert Broom
Professor Robert Broom was a Scottish South African doctor and paleontologist. He qualified as a medical practitioner in 1895 and received his DSc in 1905 from the University of Glasgow...
in 1905 to describe a poorly preserved snout. Broom later named T. browni in 1915, in which he distinguished it from all other species on the basis of the length of the molars. In 1932, Broom proposed that T. berryi be reassigned to a new genus, Trirachodontoides. Another species of Trirachodon called T. angustifrons was named in 1946 from a narrow skull found in Tanzania
Tanzania
The United Republic of Tanzania is a country in East Africa bordered by Kenya and Uganda to the north, Rwanda, Burundi, and the Democratic Republic of the Congo to the west, and Zambia, Malawi, and Mozambique to the south. The country's eastern borders lie on the Indian Ocean.Tanzania is a state...
, but this material was later proven to be from the traversodontid
Traversodontidae
Traversodontidae is a 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. Traversodonts first appeared in the Middle Triassic, diversified...
Scalenodon
Scalenodon
Scalenodon is a genus of traversodontid cynodont from the Middle Triassic of Africa and possibly Russia. The type species S. angustifrons was named in 1946 and several other species were named in the following years...
. All species of Trirachodon were suggested to by synonymous with the type species in 1972 except T. browni, which was reassigned to Diademodon tetragonus.
Paleobiology
Trirachodon is thought to have had a fossorial lifestyle. Scratch-marked burrow complexes found from the Driekoppen Formation in norhteastern Free StateFree State
The Free State is a province of South Africa. Its capital is Bloemfontein, which is also South Africa's judicial capital. Its historical origins lie in the Orange Free State Boer republic and later Orange Free State Province. The current borders of the province date from 1994 when the Bantustans...
, South Africa as well as the Omingonde Formation in Namibia
Namibia
Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...
have been attributed to the genus. At least 20 individuals have been found in one of the complexes. The entrance shafts slope down at shallow angles and have bilobate floors and vaulted roofs. The floors of the lower levels are less noticeably bilobate. The burrows typically terminate quite narrow. The tunnels tend to tightly curve as they progress deeper, with chambers branching off at right angles to the main tunnel. A semi-erect posture of the hindlimbs of Trirachodon is seen as an adaptation for sustained efficiency in locomotion in the tunnels. The relatively thick walls seen in these bones may also have provided extra rigidity to the limbs while digging. The burrows where the occupants were preserved inside are thought to have been filled with sediment in a flash flood; if it were a gradual filling, the occupants would have had time to evacuate.
Many features of the burrows suggest that they were used as colonial dwelling structures. The wide entrance would have been useful for a burrow inhabited by many individuals, and branching tunnels and terminating chambers would unlikely have been made by one animal. The worn, bilobate floors suggest that the tunnels were used rather frequently by numerous inhabitants as they passed one another while moving through them.
A colonial lifestyle for Trirachodon suggests complex social behaviors previously thought to be unique to Cenozoic
Cenozoic
The Cenozoic era is the current and most recent of the three Phanerozoic geological eras and covers the period from 65.5 mya to the present. The era began in the wake of the Cretaceous–Tertiary extinction event at the end of the Cretaceous that saw the demise of the last non-avian dinosaurs and...
mammals, and is one of the earliest signs of cohabitation in a burrow complex by tetrapods (a partial burrow cast associated with Thrinaxodon liorhinus
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...
, also from the Beaufort Group, has recently been found that predates these burrows by several million years). There have been many suggested reasons for this behavior in Trirachodon, including protection from predation, sites for reproduction and or rearing young, and thermoregulation.
Recent studies in the bone histology of many specimens of Trirachodon have led to an increased understanding of the ontogeny and lifestyle of these animals. There is evidence in the growth rings of bones that growth rates in these animals was strongly influenced by the fluctuation in seasonal conditions in their environment.
External links
- Trirachodon in the Paleobiology DatabasePaleobiology Database' is an online resource for information on the distribution and classification of fossil animals, plants, and microorganisms.-History:The Paleobiology Database was founded in 2000. It has been funded by the National Science Foundation and the Australian Research Council...