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link to palaeos com Kingdoms of Life > Wikipedia link Animalia > link to palaeos com Vertebrata > link to palaeos com Tetrapoda > link to palaeos com Theropsida > Therapsida

Author's note: these pages were written some years ago. I am not planning to update them. For a more current coverage, see the link to palaeos com Palaeos website (to which many links on these pages point to anyway. More info here


link to palaeos com Sinemurian of Europe
length 50 cm
illustration by Steve Kirk - Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals, ed.. Barry Cox, p.191; © 1988 Marshall Editions

The Tritylodontidae were small to medium-sized, highly specialized and extremely mammal-like herbivorous cynodont synapsids. The tritylodonts were among the last of the cynodonts to appear, which evolved from the Traversodontidae in latest Triassic times, and persisted well into the link to palaeos com Jurassic period. In fact they were also the longest-lived group of all the therapsids, and, along with the Trithelodontidae (Ictidosaurs) the only mammal-like reptiles to endure into the Jurassic, and the only non-mammalian Theropsida (Synapsida) to make it through to the link to palaeos com late Jurassic (the previous identification of an interesting Paleocene form (Chronoperates paradoxus) as therapsid is very unlikely, and external link HD Sues found it compared more closely with symmetrodont mammals).

Although in the past often placed in a separate infraorder Tritylodontia (e.g., Romer, 1966), the tritylodontids represent the culmination of the herbivorous cynodont radiation. These must have been quite common animals, and a number of fossil remains, including among them complete specimens, have been found in South Africa, western China, Europe, Arizona, and fragmentary but definitive remains from Argentina.

Tritylodon, after which the group is named, was first discovered in the Upper Triassic rocks of South Africa in the late 19th century, and was for decades considered to be a very early mammal. A typical tritylodontid, like the Jurassic Oligokyphus, was like a modern weasel in appearance, with a long, slim hody and tail. Its forelegs, as well as its hind-legs, were placed directly beneath the body, as they are in mammals (in contrast many more earlier and primitive therapsids, like the dicynodonts and the dinocephalians, had sprawling forelimbs).

The tritylodont's skull had a high flat crest and huge zygomatic arches (at the rear of the skull) for the attachment of very large jaw muscles. As in mammals, there was a well-developed secondary palate. The dentition of these animals was quite percular, and very different from that of other cynodonts. They had no canines, and the front pair of incisors were greatly enlarged, like those of a gnawing mammal like a rodent. As in the Traversodonts, a large gap - the diastema - separated the incisors from the square-shaped cheek teeth (seven of which on each side). Each of the cheek teeth in the upper jaw had three rows of cusps, or projections, running along its length (i.e. longitudinally), with grooves in between; the lower teeth had two rows of cusps which fitted into the grooves in the upper teeth. This matching of the cusps allowed the teeth to occlude, or meet, in a precision bite. The advanced nature of the zygomatic arches, the secondary palate, and the specialized teeth, these animals had feeding habits that were close to those of some mammals. The lower jaw apparently moved forward and backwhen the jaws were closed, thus grinding food between the teeth in somewhat the same fashion as some modern rodents their food. The multcuspid cheek teeth, complex occlusion and extensive palinal power stroke, was well suited for shredding fibrous plant material, and the Tritylodonts can very much be seen as Mesozoic rodents. The structure of the shoulder girdle and forelimb suggests capability for digging, and wear on the enlarged incisors is consistent with digging for underground plant parts.

They clearly were active animals, probably burrowers in dirt or leaf litter like modern rodents and rabbits, without doubt warm-blooded animals. Yet, in spite of disadvances, the tritylodonts still retained the reptilian joint between the quadrate bone of the skull and the articular bone of the lower jaw. But these bones were very much reduced, so that the squamosal bone of the skull and the dentary bone of the lower jaw involved in the mammalian jaw articulation were primary and in fact touching each other. In appearance these animals would have without a doubt been indestinguishable from small mammals, and it is only through the retention of the vestigal reptilian jawbones taht they are technically regarded as reptiles.

Family Tritylodontidae Cope 1884

Includes: Bienotheriidae Young 1940.

Ecological niche/Guild: small to medium terrestrial herbivores
Modern equivalents: large rodent
Horizon: Upper Triassic to Upper Jurassic: Red Beds and Cave Sandstone, Stormberg Series, of South Africa and Lesotho; Lufeng Series of Yunnan, China; lower and upper Shaximioa Formation of Sichuan, China; Los Colorados Formation of Argentina; Kayenta Formation of the United States; Rhaetic of Germany; Lower Jurassic fissures of England; Middle Jurassic Stonesfield Slate of England.
Age: link to palaeos com Rhaetian to late link to palaeos com Oxfordian/ link to palaeos com Kimmeridgian
Distribution: Worldwide ( link to palaeos com Pangea) distribution
Ecological community: Anchisaur-Plateosaur empire
preferred food: herbivorous
length: about 50 cm to a meter long
weight: usually several kg
Metabolism: partially or completely endothermic
Lifestyle: apparently lived or sheltered in burrows
Potential Predators: small theropod dinosaurs (mostly Coelophysidae), medium to large link to palaeos com sphenosuchid and link to palaeos com protosuchid crodylomorphs
Ancestor: Traversodontidae
Replaced: small Traversodontidae
Replaced by: Multituberculate mammals
Descendents: none
Taxonomic status - valid Family

List of genera

Genus Tritylodon Owen 1884

Synonyms: Likhoelia Ginsberg, 1961; Tritylodontoideus Fourie 1962.

Tritylodon skull

Remarks: The generic name Tritylodon was in the past applied to isolated teeth from the Rhaeto-Lias of Europe (eg Tritylodon fraasi below), but these are in fact, generically indeterminate. As used now the genus is restricted to southern African forms

Tritylodon longaevus Owen 1884

(syn. Likhoelia ellenbergeri)
Horizon: Red Beds and Cave Sandstone, especially Upper Middle Elliot (Tritylodon-acme zone) Formation, Stormberg Series, Orange Free State of South Africa, and Lesotho.
age: link to palaeos com Hettangian/ link to palaeos com Sinemurian

Tritylodon maximus (Fourie 1963)

(syn. Tritylodontoideus)
Clarens Formation, Stormberg Series, Orange Free State, South Africa
age: link to palaeos com Sinemurian / link to palaeos com Pliensbachian epoch
remarks: This animal is either a large T. longaevus [J.A.Hopson and J.W.Kitching, Palaeont. afr. 14, 71-85, 1972 p.80] or a distinct but closley related species.  If the latter then this is a case of clear ecological succession, where an earlier smaller species dies out or is supplanted by or evolves into a later larger species.

Genus Bienothenum Young 1940

Bienotherium skull

Bienotherium yunnanense Young 1940

Synonyms: Bienotherium elegans Young 1947.
Horizon:Dull Purplish Beds, Lower Lufeng Series of Yunnan, China. Remarks: B. elegans is based on an immature B. yunnanense.
age: probably link to palaeos com Hettangian or early link to palaeos com Sinemurian age

Bienotherium magnum Chow 1962

Horizon:Dark Red Beds, Lower Lufeng Series of Yunnan, China.
age: probably link to palaeos com Sinemurian or early link to palaeos com Pliensbachian)
Place: eastern link to palaeos com Laurasia Remarks: B. magnum is recognized as a distinct species because it comes from a higher horizon than B. yunnanense and is much larger.

Genus Bienotheroides

Horizon lower and upper Shaximioa Formation of Sichuan, China
Age: Middle to late Jurassic ( link to palaeos com Bathonian/ link to palaeos com Callovian and late link to palaeos com Oxfordian/ link to palaeos com Kimmeridgian
Remarks: the youngest known therapsid. There are two species (an earlier and a later one), but unfortuantely I've mislaid my copy of the paper that describes them. It seems that China was geographically isolated at the time, as it hosted a unique fauna of stegosaur and sauropod dinosaurs and labyrinthodont amphibians (the latter also found in Australia during the Cretaceous). Tritylodontids were obviously able to persist here after becoming exitinct elsewhere because potential competitors and/or preditors were not present. If Xenocretosuchus is also a Tritylodont that shows that eastern Laurasia served as the last refuge for these persistant little animals right trhough to the middle Cretaceous. Strangely, this was also a center of evolution of dinosaurs and other animals; it is surprising that these survivors from an earlier period should be able to cope so well in the new environment.

Genus Bocatherium Clark & Hopson, 1985

Bocatherium mexicanum Clark & Hopson, 1985

Horizon: La Boca Formation, Tamaulipas, Mexico
Age: Early Middle Jurassic (Aalanian/Bajocian?)
Place: south-west Laurasia

Comments: external link synopsis of paper

Genus Diazhongia Cui, 1981

Diazhongia longirostrata Cui, 1981

synonym Lufengia delicata Chow & Hu, 1959?? Horizon:Dark Red Beds, Lower Lufeng Series of Yunnan, China.
age: probably link to palaeos com Sinemurian or early link to palaeos com Pliensbachian)
Place: eastern link to palaeos com Laurasia

Genus Dinnebitodon Sues, 1986

Dinnebitodon amarali Sues, 1986a

Genus Kayentotherium Kermack 1982

Nearctylodon broomi type specimen

image courtesy Dr Pamela Gore

Kayentotherium wellesi D.M.Kermack 1982

synonym: Nearctylodon broomi Lewis
horizon: Kayenta Formation, Arizona
age: late link to palaeos com Sinemurian / early link to palaeos com Pliensbachian

Genus Lufengia Chow and Hu 1959

Lufengia minor (Young 1947)

Synonyms: Bienotherium minor Young 1947; Lufengia delicata Chow and Hu 1959
Horizon: Dull Purplish Beds, Lower Lufeng Series of Yunnan, China.
Age: Early Jurassic, probably link to palaeos com Hettangian or early link to palaeos com Sinemurian
Place: eastern link to palaeos com Laurasia

Remarks: relatively small form

Genus Oligokyphus Hennig 1922


Restoration of Oligokyphus skeleton, image from Fraser, after Kühne. Scale bar represents 5 cm

Synonyms: Mucrotherium E. von Heune 1933 Uniserium E. von Huene 1933.

Oligokyphus triserialis Hennig 1922

Synonyms: Oligokyphus biserialis Hennig 1922; Mucrotherium cingulatum E. von Huene 1933; Uniserium enigmaticum E. von Huene 1933.
Horizon: Lower Jurassic: Liassic Bonebed of Germany.

Oligokyphus major Kuhne 1956.

Synonyms: Oligokyphus minor Kuhne 1956.
Horizon: Lower Jurassic: Lower Lias Fissures of Somerset, England.
age: Early link to palaeos com Sinemurian
length: 50 cm long

Remarks: Kuhne described two size groups of Oligokyphus from a single fissure to which he gave separate names. Hopson and Kitching suggest that it more likely that the groups represent male and female of a single species for which the name O. major, being coined first, has priority.

Further Remarks: Oligokyphus also occurs in the Early Jurassic of China - see Luo Zhexi, and Sun Ailin "Oligokyphus (Cynodontia: Tritylodontidae) from the Lower Lufeng Formation (Lower Triassic - sic!) of Yunnan, China", Journal of Vertebrate Paleontology, 1993, 13(4):477-482. The genus has also been recorded from Arizona (Jenkins et al)

Genus Stereognathus Charlesworth 1855

Stereognathus ooliticus Charlesworth 1855

Horizon: Middle Jurassic: Stonesfield slate of Oxfordshire, England
age: link to palaeos com Bajocian

Remarks: Stereognathus sp. has been recorded from the Forest Marble of Dorset. For a long time Stereognathus was the latest known therapsid reptile, but Beinotheroides discovered in China is younger. Xenocretosuchus is even younger again, but the fragmentary nature of the remains (teeth only) means it is not certain if it is a tritylodont, aklthoufgh it is certainly possible.

Genus Xenocretosuchus Tatarinov and Matchenko 1999

Xenocretosuchus sibiricus Tatarinov and Matchenko 1999

Horizon: [formation] of Shestakovo, Kemerovo Region, eastern Russia
Age: Aptian-Albian?
Place: central Laurasia
Remains: five isolated teeth

Comments: ref Tatarinov and Matchenko - This form is assigned to the tritylodonts on teh basis of the two transversal rows of cusps on the cheek teeth. The buccal and labial cusps on the upper teeth form longitudinal crests. The same feature is developed to a lesser extent in the genus Yunnanodon from the Lower Jurassic of southern China. However, I am somewhat sceptical of a secimen based only some isolated teeth. It may very well be a tritylodont (and if so it would be the youngest stratigrphic record of the family), or it may be an aberrant mammal. This little animal lived alongside dinosaurs (Psittacosauridae, Theropoda, and Sauropoda), crocodiles, turtles, fish, and the Triconodont mammal Gobiconodon borissiaki.

Genus Yunnanodon Cui, 1976

Yunnanodon brevirostre Cui, 1976

Horizon:Dark Red Beds, Lower Lufeng Series of Yunnan, China.
age: probably link to palaeos com Sinemurian or early link to palaeos com Pliensbachian)
Place: eastern link to palaeos com Laurasia

Tritylodontidae indet.

species indeterminate

Horizon: Los Colorados Formation, Argentina
Age: latest link to palaeos com Norian/ link to palaeos com Rhaetian to link to palaeos com Hettangian
Place: west link to palaeos com Gondwana/South-west link to palaeos com Pangea
Remains: incomplete limb elements, some vertebrae and a possible occiput.

Comments: ref. Bonaparte. Remains asosciated with link to palaeos com protosuchids (indicating early Jurassic - link to palaeos com Hettangian), ornithosuchids and "pedeticosuchids" (both indicating late Triassic). The proximal fragment of the femur resembles Bienotherium (differeing in that the trochantor major is not sharply seperated from the condyle) but is very different from Oligokyphus. The would seem to be the only known Triassic Tritylodont. The Los Colorados Formation is usually considered latest Triassic (Late Norian/Rhaetian), but the fauna includes early Jurasisc elements. So it is possible that the upper part of the Los Colorados may be Hettangian (earliest Jurassic). However, the ornithosuchids and "pedeticosuchids" indicate a definate late Triassic age for those associated remains.

some printed references some Links and References Web links

web page JURASSIC CYNODONTS; Tritylodontidae and Trithelodontidae, an internet directory - by Trevor Dykes - the most complete listing (along with the present page) on the Web

printed reference J. F. Bonaparte", 1970 "Annoitated List of the South American Traissic Tetrapods", in Second Gondwana Symposium, Proceedings and Papers (Council of Scientific and Industrial Research, Pretoria).

printed reference Carroll, R. L. Vertebrate paleontology and evolution. -W. H. Freeman and company, New York, 1988

printed reference Clark J.M. & Hopson J.A. (1985). "Distinctive mammal-like reptile from Mexico and its bearing on the phylogeny of Tritylodontidae." Nature, 315 (6018). pp 398-400. (this may refer to Bocatherium mexicanum)

printed reference Edwin H. Colbert, Evolution of the Vertebrates, 2nd edition, 1969, John Wiley & Sons

web page Tritylodontidae by Jack Conrad

printed reference Barry Cox, R.J.G. Savage, Brian Gardiner; Dougal Dixon, 1988 Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals

printed reference D.E. Fastovsky, J.M. Clark, N.H. Strater, M. Montellano, R. Hernandez, and J.A. Hopson, (1995) "Depositional environments of a middle Jurassic terrestrial vertebrate assemblage, Huizachal canyon, Mexico " Journal of Vertebrate Paleontology, 15(3): 561-575 - synopsis of paper - mentions Bocatherium mexicanum

printed reference N.C. Fraser, (1985) Vertebrate faunas from Mesozoic fissure deposits of South-West Britain, Modern Geology, vol.9 pp.273-300

printed reference F.A. Jenkins, A.W. Crompton, and W.R. Downs (1983) Mesozoic mammals from Arizona: new evidence of mammalian evolution, Science, 222, 1233-1235 - refers to Oligokyphus

web pagecladogram Mikko K. Haaramo Tritylodontoidea - cladogram and genus list

web page William R. Hammer, Antarctic Dinosaurs. Refers to tritylodont remains from the Early Jurassic of Antarctica.

printed reference James A. Hopson, "The Origin and Adaptive Radiation of Mammal-Like Reptiles and Non-Therian Mammals, Annals of the New York Academy of Sciences 167:199-216, 1969

printed reference James A. Hopson and Herbert R. Barghusen, "An Analysis of Therapsid Relationships", in The Ecology and Biology of Mammal-Like Reptiles ed. by Nocholas Hotton III, Paul D. MacLean, Jan J. Roth and E. Carol Roth, Smithsonian Institute Press, Washington and London, 1986, pp.83-106

printed reference James A. Hopson and James W. Kitching, 1972, "A Revised Classification of the Cynodonts (Reptilia; Therapsida), Paleontologica Africa, 14. 17-85

Kühneprinted reference W.G. Kühne, (1956), The Liassic therapsid Oligokyphus. British Museum (Natural History), London

printed reference A.L.Sun and K.H. Cui, (1986) A brief introduction to the Lower Lufeng saurischian fauna (Lower Jurassic: Lufeng, Yunnan, People's Republic of China) pp.275-78, Kevin Padian (ed.) The Beginning of the Age of Dinosaurs - faunal change across the Triassic-Jurassic boundary, Cambridge University Press, Cambridge

printed reference L. P. Tatarinov and E. N. Matchenko, (1999) A Find of an Aberrant Tritylodont (Reptilia, Cynodontia) in the Lower Cretaceous of the Kemerovo Region, Paleontologicheskii Zhurnal, synopsis

printed reference Terrestrial Ecosystems Through Time, p.357

web page (Untitled) - on Tritylodonts (in Japanese)

web page Ecology - Skye - Nature Conservation - refers to Tritylodont material from the Isle of Skye, mid - late Bathonian (about contemporary in age with Bienotheroides). See 19. Elgol Coast.

web page Vertebrate Fossils - A headless tritylodont specimen from the Navajo Desert, Pliensbachian-Toarcian. Possibly Kayentatherium.

web pagephotos Vertebrate Trace Fossils - Some Navajo ichnofossils. There's a photo of some possibly tritylodont footprints, as well as photos of burrows apparently made by tritylodontids. Also on this page are photos of dinosaur and possible pterosaur tracks

link to palaeos com Palaeos Page (incorporates some of this material, plus a lot of additional material)

Kheper index page
Palaeo index page
Therapsida main page

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page by M.Alan Kazlev
I am indebted to Trevor Dykes for helpful corrections and references.
page created 2 August 1999. Reposted and last modified 1 September 2005, links updated 16 January 2010