Trirachodon

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Trirachodon
Temporal range: Early-Middle Triassic
Trirachodon.jpg
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade: Therapsida
Clade: Cynodontia
Clade:† Neogomphodontia
Genus:† Trirachodon
Seeley, 1895
Species
  • T. berryiSeeley, 1895 (type)
Synonyms
  • T. kannemeyeriSeely, 1895
  • T. minorBroom, 1905

Trirachodon (Greek: "three ridge tooth") is an extinct genus of cynodonts. Fossils have been found in the Cynognathus Assemblage Zone of the Beaufort Group in South Africa and the Omingonde Formation of Namibia, dating back to the Early and Middle Triassic. [1]

Contents

Description

The skull of Trirachodon had a short, narrow snout with a wide orbital region. The zygomatic arches were relatively slender. [2] [3] 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 . These teeth tended to be transversely broader than Diademodon as well. [4] [5] A bony secondary palate and precise postcanine tooth occlusion are seen as derived characteristics in Trirachodon that are similar to those of mammals.

Species

The type 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 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 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 synonymous with Diademodon tetragonus. [6]

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 Omingonde Formation in Namibia have been attributed to the genus. [7] 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. [8] [9] 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. [10]

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 [11] ). 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. [12]

Related Research Articles

<i>Cynognathus</i> Extinct genus of cynodonts

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 predator closely related to mammals and had a southern hemispheric distribution. Fossils have so far been recovered from South Africa, Argentina, Antarctica, and Namibia.

<i>Thrinaxodon</i> Extinct genus of cynodonts of Early Triassic South Africa

Thrinaxodon is an extinct genus of cynodonts, including the species T. liorhinus which lived in what are now South Africa and Antarctica during the Early Triassic. Thrinaxodon lived just after the Permian–Triassic mass extinction event, its survival during the extinction may have been due to its burrowing habits.

<span class="mw-page-title-main">Cynognathia</span> Clade of cynodonts

Cynognathia is one of two major clades of cynodonts, the other being Probainognathia. Cynognathians included the large carnivorous genus Cynognathus and the herbivorous or omnivorous gomphodonts such as traversodontids. Cynognathians can be identified by several synapomorphies including a very deep zygomatic arch that extends above the middle of the orbit.

<i>Erythrosuchus</i> Extinct genus of reptiles

Erythrosuchus is an extinct genus of archosauriform reptiles from the Triassic of South Africa. Remains have been found from the Cynognathus Assemblage Zone of the Beaufort Group in the Karoo of South Africa.

<i>Cynognathus</i> Assemblage Zone Biozone which correlates to the Burgersdorp Formation of the Beaufort Group

The Cynognathus Assemblage Zone is a tetrapod biozone utilized in the Karoo Basin of South Africa. It is equivalent to the Burgersdorp Formation, the youngest lithostratigraphic formation in the Beaufort Group, which is part of the fossiliferous and geologically important Karoo Supergroup. The Cynognathus Assemblage Zone is the youngest of the eight biozones found in the Beaufort Group, and is considered to be late Early Triassic (Olenekian) to early Middle Triassic (Anisian) in age. The name of the biozone refers to Cynognathus crateronotus, a large and carnivorous cynodont therapsid which occurs throughout the entire biozone.

<i>Prozostrodon</i> Extinct genus of cynodonts

Prozostrodon is an extinct genus of probainognathian cynodonts that was closely related to mammals. The remains were found in Brazil and are dated to the Carnian age of the Late Triassic. The holotype has an estimated skull length of 6.7 centimetres (2.6 in), indicating that the whole animal may have been the size of a cat. The teeth were typical of advanced cynodonts, and the animal was probably a carnivore hunting reptiles and other small prey.

<i>Moschorhinus</i> Genus of synapsid from late Permian and early Triassic South Africa

Moschorhinus is an extinct genus of therocephalian synapsid in the family Akidnognathidae with only one species: M. kitchingi, which has been found in the Late Permian to Early Triassic of the South African Karoo Supergroup. It was a large carnivorous therapsid, reaching 1.5 m (4.9 ft) in total body length with the largest skull comparable to that of a lion in size, and had a broad, blunt snout which bore long, straight canines.

<i>Microgomphodon</i> Genus of therapsid from the Middle Triassic of southern Africa

Microgomphodon is an extinct genus of therocephalian therapsid from the Middle Triassic of South Africa and Namibia. Currently only one species of Microgomphodon, M. oligocynus, is recognized. With fossils present in the Cynognathus Assemblage Zone (CAZ) of the Burgersdorp Formation in South Africa and Omingonde Formation of Namibia and ranging in age from late Olenekian to Anisian, it is one of the most geographically and temporally widespread therocephalian species. Moreover, its occurrence in the upper Omigonde Formation of Namibia makes Microgomphodon the latest-surviving therocephalian. Microgomphodon is a member of the family Bauriidae and a close relative of Bauria, another South African bauriid from the CAZ. Like other bauriids, it possesses several mammal-like features such as a secondary palate and broad, molar-like postcanine teeth, all of which evolved independently from mammals.

Platycraniellus is an extinct genus of carnivorous cynodonts from the Early Triassic. It is known from the Lystrosaurus Assemblage Zone of the Normandien Formation in South Africa. P. elegans is the only species in this genus based on the holotype specimen from the Ditsong National Museum of Natural History in Pretoria, South Africa. Due to limited fossil records for study, Platycraniellus has only been briefly described a handful of times.

<i>Lumkuia</i> Extinct genus of cynodonts

Lumkuia is an extinct genus of cynodont, fossils of which have been found in the Cynognathus Assemblage Zone of the Beaufort Group in the South African Karoo Basin that date back to the early Middle Triassic. It contains a single species, Lumkuia fuzzi, which was named in 2001 on the basis of the holotype specimen BP/1/2669, which can now be found at the Bernard Price Institute in Johannesburg, South Africa. The genus has been placed in its own family, Lumkuiidae. Lumkuia is not as common as other cynodonts from the same locality such as Diademodon and Trirachodon.

<i>Diademodon</i> Extinct genus of cynodonts

Diademodon is an extinct genus of cynodonts. It was about 2 metres (6.6 ft) long.

<i>Langbergia</i> Extinct genus of cynodonts

Langbergia is an extinct genus of trirachodontid cynodont from the Early Triassic of South Africa. The type and only species L. modisei was named in 2006 after the farm where the holotype was found, Langberg 566. Langbergia was found in the Burgersdorp Formation in the Beaufort Group, a part of the Cynognathus Assemblage Zone. The closely related trirachodontids Trirachodon and Cricodon were found in the same area.

Pascualgnathus is an extinct genus of traversodontid cynodonts from the Middle Triassic of Argentina. Fossils have been found from the Río Seco de la Quebrada Formation of the Puesto Viejo Group. The type species P. polanskii was named in 1966.

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<i>Cricodon</i> Extinct genus of cynodonts

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<i>Aleodon</i> Extinct genus of cynodonts

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<i>Abdalodon</i> Extinct genus of cynodonts

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<i>Ufudocyclops</i> Extinct genus of dicynodonts

Ufudocyclops is an extinct genus of stahleckeriid dicynodont from the Middle Triassic of South Africa. It was found in the Burgersdorp Formation, part of the uppermost Cynognathus Assemblage Zone of the Beaufort Group in the Karoo Basin. The type and only known species is U. mukanelai. It was a large, beaked herbivore like other Triassic dicynodonts, lacking tusks, and is mostly characterised by unique features of the skull. It is known from three specimens, two of which were previously referred to the Tanzanian dicynodont Angonisaurus. The separation of Ufudocyclops from Angonisaurus indicates that the Middle Triassic fauna of the Beaufort Group in South Africa was not part of a larger shared fauna with those of the Manda Beds in Tanzania, as was previously supposed, and suggests that they were separated as more localised faunas, possibly by geographic barriers or in time. Ufudocyclops then would have been a unique part of the uppermost Cynognathus Assemblage Zone in South Africa. It is also the oldest known member of the family Stahleckeriidae, and implies that the family was already diversifying in the Middle Triassic alongside other kannemeyeriiforms, not just in the Late Triassic after other families died out.

<i>Etjoia</i> Extinct genus of cynodonts

Etjoia is an extinct genus of traversodontid cynodonts that lived during the Middle Triassic or Late Triassic period in southern Africa. This medium-sized omnivorous cynognathian provides important information on the dental evolution of early diverging gomphodonts and traversodontids.

<i>Impidens</i> Extinct genus of cynodonts

Impidens is an extinct genus of large omnivorous cynodont from the Triassic of South Africa and Antarctica. Its type and only species is Impidens hancoxi. Impidens inhabited high-latitude environments of southern Gondwana during the Middle Triassic, where it was probably the apex predator.

References

  1. ↑ Rubidge, B.S. (ed.) 1995. Biostratigraphy of the Beaufort Group (Karoo Supergroup). 46 pp. Council for Geoscience, Pretoria.
  2. ↑ Seeley, H. G. (1895). Researches on the structure, organization and classification of the fossil Reptilia. On Diademdon. Philosophical Transactions of the Royal Society of London B185:1029–1041.
  3. ↑ Kemp, T. S. (1982). Mammal−like Reptiles and the Origin of Mammals. 363 pp. Academic Press, London.
  4. ↑ Seeley, H. G. (1895). On the structure, organization and classification of the fossil Reptilia III. On Trirachodon. Philosophical Transactions of the Royal Society of London B186:48–57.
  5. ↑ Crompton, A. W. and Ellenberger, F. (1957). On a new cynodont from Molteno Beds and origin of tritylodontids. Annals of the South African Museum 44:1–14.
  6. ↑ Abdala, F., Neveling, J. and Welman, J. (2006). A new trirachodontid cynodont from the lower levels of the Burgersdorp Formation (Lower Triassic) of the Beaufort Group, South Africa and the cladistic relationships of Gondwanan gomphodonts. Zoological Journal of the Linnean Society147:383-413.
  7. ↑ Smith, R. and Swart, R. (2002). Changing fluvial environments and vertebrate taphonomy in response to climatic drying in a Mid−Triassic rift valley fill: The Omingonde Formation (Karoo Supergroup) of Central Namibia. Palaios17(3):249–267.
  8. ↑ Carrier, D. R. (1987). The evolution of locomotor stamina in tetrapods: Circumventing a mechanical constraint. Paleobiology13:326–341.
  9. ↑ Pough, F. H., Heiser, J. B., and McFarland, W. N. (1996). Vertebrate Life. 798 pp. Prentice−Hall, New Jersey.
  10. ↑ Groenewald, G. H., Welman, J., and Maceachern, J. A. (2001). Vertebrate Burrow Complexes from the Early Triassic Cynognathus Zone (Driekoppen Formation, Beaufort Group) of the Karoo Basin, South Africa. Palaios16(2):148-160.
  11. ↑ Damiani, R., Modesto, S., Yates, A., and Neveling, J. (2003). Earliest evidence of cynodont burrowing. Proceedings of the Royal Society B270:1747–1751
  12. ↑ Botha, J. and Chinsamy, A. (2004). Growth and life habits of the Triassic cynodont Trirachodon, inferred from bone histology. Acta Palaeontologica Polonica49(4):619-627.
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