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 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 late Jurassic (the previous identification of an interesting Paleocene form (Chronoperates paradoxus) as therapsid is very unlikely, and 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.
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: Rhaetian to late Oxfordian/ Kimmeridgian
Distribution: Worldwide ( 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 sphenosuchid and protosuchid crodylomorphs
Replaced: small Traversodontidae
Replaced by: Multituberculate mammals
Taxonomic status - valid Family
Synonyms: Likhoelia Ginsberg, 1961; Tritylodontoideus Fourie 1962.
Tritylodon longaevus Owen 1884(syn. Likhoelia ellenbergeri)
Tritylodon maximus (Fourie 1963)(syn. Tritylodontoideus)
Bienotherium yunnanense Young 1940Synonyms: Bienotherium elegans Young 1947.
Bienotherium magnum Chow 1962Horizon:Dark Red Beds, Lower Lufeng Series of Yunnan, China.
Bocatherium mexicanum Clark & Hopson, 1985Horizon: La Boca Formation, Tamaulipas, Mexico
Comments: synopsis of paper
Diazhongia longirostrata Cui, 1981synonym Lufengia delicata Chow & Hu, 1959?? Horizon:Dark Red Beds, Lower Lufeng Series of Yunnan, China.
Dinnebitodon amarali Sues, 1986a
Kayentotherium wellesi D.M.Kermack 1982synonym: Nearctylodon broomi Lewis
Lufengia minor (Young 1947)Synonyms: Bienotherium minor Young 1947; Lufengia delicata Chow and Hu 1959
Remarks: relatively small form
Oligokyphus triserialis Hennig 1922Synonyms: Oligokyphus biserialis Hennig 1922; Mucrotherium cingulatum E. von Huene 1933; Uniserium enigmaticum E. von Huene 1933.
Oligokyphus major Kuhne 1956.Synonyms: Oligokyphus minor Kuhne 1956.
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)
Stereognathus ooliticus Charlesworth 1855
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.
Xenocretosuchus sibiricus Tatarinov and Matchenko 1999Horizon: [formation] of Shestakovo, Kemerovo Region, eastern Russia
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.
Yunnanodon brevirostre Cui, 1976Horizon:Dark Red Beds, Lower Lufeng Series of Yunnan, China.
species indeterminateHorizon: Los Colorados Formation, Argentina
Comments: ref. Bonaparte. Remains asosciated with protosuchids (indicating early Jurassic - 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 Links and References|
JURASSIC CYNODONTS; Tritylodontidae and Trithelodontidae, an internet directory - by Trevor Dykes - the most complete listing (along with the present page) on the Web
Carroll, R. L. Vertebrate paleontology and evolution. -W. H. Freeman and company, New York, 1988
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)
Edwin H. Colbert, Evolution of the Vertebrates, 2nd edition, 1969, John Wiley & Sons
Tritylodontidae by Jack Conrad
Barry Cox, R.J.G. Savage, Brian Gardiner; Dougal Dixon, 1988 Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals
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
Mikko K. Haaramo Tritylodontoidea - cladogram and genus list
William R. Hammer, Antarctic Dinosaurs. Refers to tritylodont remains from the Early Jurassic of Antarctica.
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
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
Kühne W.G. Kühne, (1956), The Liassic therapsid Oligokyphus. British Museum (Natural History), London
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
(Untitled) - on Tritylodonts (in Japanese)
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.
Vertebrate Fossils - A headless tritylodont specimen from the Navajo Desert, Pliensbachian-Toarcian. Possibly Kayentatherium.
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
Palaeos Page (incorporates some of this material, plus a lot of additional material)