Temporal range: Early Triassic, 248–245Ma
Thrinaxodon is a genus of cynodont that lived during the early part of the Triassic period (248-245 million years ago) in what is now South Africa and Antarctica. Because it is closely related to the lineage leading to mammals, Thrinaxodon is often considered a transitional fossil. The two known species ranged in size from 30 to 50 centimetres (12 to 20 in) in length.
Thrinaxodon probably lived in shallow burrows dug into hillsides or riverbanks. Lack of an harderian gland indicate it did not have a full pelage, but it likely had wiskers allowing it to navigate the borrows efficiently.
A low-slung, sharp-toothed carnivore, Thrinaxodon lived in burrows, and its well-differentiated teeth suggest it ate small creatures like insects, reptiles, and other small animals. Clues to its remains show that this creature was more mammal-like than its synapsid ancestors. It had a fairly large head/skull with pits in the snout area which have suggested to some that it had whiskers, but the modern lizard Tupinambis has pits in the same area that are almost identical. An enlarged dentary bone strengthened either side of the lower jaw and contained sockets for its teeth. Along with other cynodonts, Thrinaxodon could chew and breathe at the same time, due to the evolutionary development of the secondary palate. Its chest and lower back regions were probably separated by a diaphragm - a muscular sheet that contracted to fill lungs, and would have enabled Thrinaxodon to breathe more efficiently than its ancestors.
In response to the wide daily temperature swings of the early Triassic, it may have been eurythermic, able to function at a broad range of temperatures; this could have laid the groundwork for the development of homeothermic endothermy. Like its predecessors, Thrinaxodon laid eggs, and there were many reptilian features in its skeleton.
Its name was taken from the Greek θρῖναξ, thrīnax, "trident, three-pronged fork", and ὀδούς/ὀδόντ-, odoús/odónt-, "tooth", in reference to its cheek teeth, albeit with a grammatical error: "Thrinacodon" (Thrinakódōn) would have been the grammatically correct form.
Thrinaxodon belongs to a clade of cynodonts called Epicynodontia. It is a basal member of that clade, lying outside the more derived epicynodont group Eucynodontia. Below is a cladogram from Abdala (2007) showing the phylogenetic position of Thrinaxodon:
In popular culture
- In the television series Walking with Dinosaurs, cynodonts were featured in the first episode. The episode was set in Arizona, where an unnamed species of cynodont is known only from teeth. The reconstruction of this cynodont was therefore based primarily on the South African Thrinaxodon.
- Thrinaxodon is referenced in the trading card game Magic: The Gathering through creatures known as Thrinaxes.
- Kazlev, M. Alan (April 19, 2011). Glossary entry for "Transitional fossil" in palaeos.com. Retrieved November 10, 2012.
- Savage, R.J.G., and Long, M.R. (1986). Mammal Evolution: an illustrated guide. Facts On File Inc. p. 39. ISBN 0-8160-1194-X.
- Fernandez, V.; Abdala, F.; Carlson, K. J.; Cook, D. C.; Rubidge, B. S.; Yates, A.; Tafforeau, P. (2013). Butler, Richard J, ed. "Synchrotron Reveals Early Triassic Odd Couple: Injured Amphibian and Aestivating Therapsid Share Burrow". PLoS ONE 8 (6): e64978. doi:10.1371/journal.pone.0064978.
- Damiani R, Modesto S, Yates A, Neveling J (August 2003). "Earliest evidence of cynodont burrowing". Proc. Biol. Sci. 270 (1525): 1747–51. doi:10.1098/rspb.2003.2427. PMC 1691433. PMID 12965004.
- Ruben, J. A. (1 August 2000). "Selective Factors Associated with the Origin of Fur and Feathers". Integrative and Comparative Biology 40 (4): 585–596. doi:10.1093/icb/40.4.585.
- Cowen, Richard (2005). History of Life (Fourth ed.). Oxford: Blackwell Publishing.
- Chinsamy-Turan, edited by Anusuya (2012). Forerunners of mammals : radiation, histology, biology. Bloomington: Indiana University Press. ISBN 978-0-253-35697-0.
- Kazlev, A. "Thrinaxodon". Palaeos website. Retrieved 22 February 2012.
- Abdala, F. (2007). "Redescription of Platycraniellus elegans (Therapsida, Cynodontia) from the Lower Triassic of South Africa, and the Cladistic Relationships of Eutheriodonts". Palaeontology 50 (3): 591–618. doi:10.1111/j.1475-4983.2007.00646.x.
- ABC Online Services (1999). "Walking with Dinosaurs - Fact File: Cynodont". Dino Fact File. Retrieved August 9, 2013.
- "Real- Thinaxdon appear in animal armegaddon. Life Thrinaxes". 2009-05-28. Retrieved 2009-05-28.
- Tim Haines and Paul Chambers (2006). The Complete Guide to Prehistoric Life. Firefly Books Ltd., Canada. 69.
- David Lambert (2003). Dinosaur Encyclopedia. DK Publishing, New York. 202-203.