''Trirachodon'' (Greek: "three ridge tooth") is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

cynodonts Cynodontia () is a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extinct ances ...

Fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

s have been found in the ''Cynognathus'' Assemblage Zone of the

Beaufort Group The Beaufort Group is the third of the main subdivisions of the Karoo Supergroup in South Africa. It is composed of a lower Adelaide Subgroup and an upper Tarkastad Subgroup. It follows conformably after the Ecca Group and unconformably under ...

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

and the

Omingonde Formation The Omingonde Formation is an Early Triassic, Early to Middle Triassic (Anisian to Ladinian) geologic Formation (geology), formation, part of the Karoo Supergroup, in the western Otjozondjupa Region and northeastern Erongo Region of north-central N ...

Namibia Namibia, officially the Republic of Namibia, is a country on the west coast of Southern Africa. Its borders include the Atlantic Ocean to the west, Angola and Zambia to the north, Botswana to the east and South Africa to the south; in the no ...

, dating back to the

Early Early may refer to: Places in the United States * Early, Iowa, a city * Early, Texas, a city * Early Branch, a stream in Missouri * Early County, Georgia * Fort Early, Georgia, an early 19th century fort Music * Early B, stage name of Jamaican d ...

and

Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geology), period or the middle of three series (stratigraphy), series in which the Triassic system (stratigraphy), system is di ...

Description

The skull of ''Trirachodon'' had a short, narrow snout with a wide orbital region. The

zygomatic arch In anatomy, the zygomatic arch (colloquially known as the cheek bone), is a part of the skull formed by the zygomatic process of temporal bone, zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the ...

es were relatively slender. ''Trirachodon'' was quite small for a cynodont, growing no larger than 50 cm in length. It had noticeably less molariform teeth than its closely related contemporary ''

Diademodon ''Diademodon'' is an extinct genus of cynodonts. It was about long. Discovery Harry Seeley had found his fossil in the Burgersdorp Formation of the Beaufort Group in the Karoo Basin of South Africa. As late as 1988, ''Diademodon'' had ...

''. These teeth tended to be transversely 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 A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

Species

The

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

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 too 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 Fellow of the Royal Society, FRS FRSE (30 November 1866 6 April 1951) was a British- South African medical doctor and palaeontologist. He qualified as a medical practitioner in 1895 and received his DSc in 1905 from the University ...

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, officially the United Republic of Tanzania, is a country in East Africa within the African Great Lakes region. It is bordered by Uganda to the northwest; Kenya to the northeast; the Indian Ocean to the east; Mozambique and Malawi to t ...

, but this material was later proven to be from the

traversodontid 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 North America. Traversodon ...

Scalenodon ''Scalenodon'' is an extinct genus of traversodontid cynodonts 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. Most of th ...

''. All species of ''Trirachodon'' were suggested to by synonymous with the type species in 1972 except ''T. browni'', which was synonymous with ''Diademodon tetragonus''.

Paleobiology

''Trirachodon'' is thought to have had a fossorial lifestyle. Scratch-marked burrow complexes found from the Driekoppen Formation in northeastern Free State, South Africa as well as the

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 The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...

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'' is an extinct genus (biology), genus of cynodonts which lived in what are now South Africa and Antarctica during the Late Permian - Early Triassic. ''Thrinaxodon'' lived just before, during, and right after the Permian–Triassic ...

'', 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.Botha, J. and Chinsamy, A. (2004). Growth and life habits of the Triassic cynodont ''Trirachodon'', inferred from bone histology. ''Acta Palaeontologica Polonica'' 49(4):619-627.

References

External links

''Trirachodon''
in the

Paleobiology Database The Paleobiology Database (PBDB) is an online resource for information on the distribution and classification of fossil animals, plants, and microorganisms. History The Paleobiology Database originated in the NCEAS-funded Phanerozoic Marine Pale ...

{{Taxonbar, from=Q956058 Cynognathia Early Triassic synapsids of Africa Middle Triassic synapsids of Africa Triassic South Africa Fossils of Namibia Omingonde Formation Fossils of South Africa Fossil taxa described in 1895 Taxa named by Harry Seeley