Trirachodon
| Trirachodon Temporal range:Early-Middle Triassic
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Scientific classification
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Therapsida |
| Clade: | Cynodontia |
| Clade: | †Neogomphodontia |
| Genus: | †Trirachodon Seeley, 1895 |
| Species | |
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Trirachodon(Greek: "three ridge tooth" ) is an extinctgenusofcynodonts.Fossilshave been found in theCynognathusAssemblage Zoneof theBeaufort GroupinSouth Africaand theOmingonde FormationofNamibia,dating back to theEarlyandMiddle Triassic.[1]
Description
[edit]The skull ofTrirachodonhad a short, narrow snout with a wide orbital region. Thezygomatic archeswere relatively slender.[2][3]Trirachodonwas quite small for a cynodont, growing no larger than 50 cm in length. It had noticeably lessmolariformteeth than its closely related contemporaryDiademodon.These teeth tended to be transversely broader thanDiademodonas well.[4][5]A bony secondary palate and precise postcanine tooth occlusion are seen as derived characteristics inTrirachodonthat are similar to those ofmammals.
Species
[edit]Thetype speciesisT. berryi,named in 1895 on the basis of a single cranial skeleton. Three other specimens were later referred toT. 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 theT. kannemeyerihas fallen out of use due to this possible synonymy.
A new species,T. minor,was named byRobert Broomin 1905 to describe a poorly preserved snout. Broom later namedT. browniin 1915, in which he distinguished it from all other species on the basis of the length of the molars. In 1932, Broom proposed thatT. berryibe reassigned to a new genus,Trirachodontoides.Another species ofTrirachodoncalledT. angustifronswas named in 1946 from a narrow skull found inTanzania,but this material was later proven to be from thetraversodontidScalenodon.All species ofTrirachodonwere suggested to by synonymous with the type species in 1972 exceptT. browni,which was synonymous withDiademodon tetragonus.[6]
Paleobiology
[edit]Trirachodonis thought to have had a fossorial lifestyle. Scratch-marked burrow complexes found from theDriekoppen Formationin northeasternFree State,South Africa as well as theOmingonde FormationinNamibiahave 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 ofTrirachodonis 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 forTrirachodonsuggests complex social behaviors previously thought to be unique toCenozoicmammals, and is one of the earliest signs of cohabitation in a burrow complex by tetrapods (a partial burrow cast associated withThrinaxodon 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 inTrirachodon,including protection from predation, sites for reproduction and or rearing young, and thermoregulation.
Recent studies in the bone histology of many specimens ofTrirachodonhave 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]
References
[edit]- ^Rubidge, B.S. (ed.) 1995.Biostratigraphy of the Beaufort Group (Karoo Supergroup).46 pp. Council for Geoscience, Pretoria.
- ^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.
- ^Kemp, T. S. (1982).Mammal−like Reptiles and the Origin of Mammals.363 pp. Academic Press, London.
- ^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.
- ^Crompton, A. W. and Ellenberger, F. (1957). On a new cynodont from Molteno Beds and origin of tritylodontids.Annals of the South African Museum44:1–14.
- ^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.
- ^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.
- ^Carrier, D. R. (1987). The evolution of locomotor stamina in tetrapods: Circumventing a mechanical constraint.Paleobiology13:326–341.
- ^Pough, F. H., Heiser, J. B., and McFarland, W. N. (1996).Vertebrate Life.798 pp. Prentice−Hall, New Jersey.
- ^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.
- ^Damiani, R., Modesto, S., Yates, A., and Neveling, J. (2003). Earliest evidence of cynodont burrowing.Proceedings of the Royal Society B270:1747–1751
- ^Botha, J. and Chinsamy, A. (2004). Growth and life habits of the Triassic cynodontTrirachodon,inferred from bone histology.Acta Palaeontologica Polonica49(4):619-627.
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