Therapsida │ └─Cynodontia ├─Procynosuchidae └─┬─Galesauridae └─Eucynodontia ├─Cynognathia │ ├─Cynognathidae │ └─Tritylodontidae │ ├─Oligokyphus │ └─┬─Tritylodon │ └─┬─Bienotherium │ └─┬─Kayentatherium │ └─┬─┬─Lufengia │ │ └─Dianzhongia │ └─┬─Bocatherium │ └─┬─Yunnanodon │ └─┬─Stereognathus │ └─Xenocretosuchus └─Probainognathia ├─Tritheledontidae └─Mammaliformes
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 more basal Cynognathia 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 Tritheledontidae (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). [Of course, it is always possible that the allotherians diverged from the mammalian stem earlier than is generally supposed. If so, the two statements are entirely consistent ... ATW020531]
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, was like a modern weasel in appearance, with a long, slim body 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 peculiar, 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 cheek teeth (seven 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. Given the advanced nature of the zygomatic arches, the secondary palate, and the specialized teeth, it is believed that these animals had feeding habits that were close to those of some mammals. The lower jaw apparently moved forward and back when the jaws were closed, thus grinding food between the teeth in somewhat the same fashion as some modern rodents chew their food. The multi-cusped cheek teeth, complex occlusion and extensive palinal power stroke were well suited for shredding fibrous plant material. Thus 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.
Tritylodonts 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 disadvantages, 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 been almost indistinguishable from small mammals. MAK010421.
Image: Left Bienotherium yunnaense from china2001/0405.html (former site).
Range: Late Triassic to middle Cretaceous of South Africa, South America, North America, Europe, Russia, & China.
Phylogeny: Cynognathia: Cynognathidae + *: Oligokyphus + (Tritylodon + (Bienotherium + (Kayentatherium + ((Lufengia + Dianzhongia) + Bocatherium + (Yunnanodon + (Stereognathus + Xenocretosuchus)))))).
Characters: 50-100 cm long herbivores; prefrontal & postorbital absent; high, flat sagittal crest; temporal fenestra very large and confluent with orbit; zygomatic arch strongly enlarged; well-developed secondary palate; dentaries not fused at symphysis; mandible making strong bilateral retraction (as multituberculates) [RS01]; post-dentary bones reduced to slender rod located in deep groove on medial surface of the dentary; enlarged lower incisors, fitting between 2 enlarged upper incisors; canines absent; large diastema; $ lower cheek teeth with 3 longitudinal rows of cusps which occlude against 2 rows in uppers [RS01]; long-bodied; postcranial skeleton generally mammal-like [RS01]; ilium rod-like [RS01]; pubes reduced and posteroventral to acetabulum [RS01]; greater trochanter separated from femoral head by notch as in mammals [RS01].
Links: Vertebrate Fossils; JURASSIC CYNODONTS; Tritylodontidae and Tritheledontidae, an internet directory; Tritylodontidae; Tritylodontoidea; PANGEA; Biology 356; Untitled Document; 4eern.pdf.
References: Luo et al. (2002) [L+02], Rubidge & Sidor (2001) [RS01].
Note:  This phylogenetic position is controversial, with many workers placing the tritylodonts as the sister of mammaliforms. Given the similarity of the dentition to allotherians, this seems possible. [L+02] regard linkage with allotherians as a conclusion from discredited notions of mammaliform paraphyly. But this need not be the case.  Possibly longest-lived therapsid group. ATW 020221.
Oligokyphus: = Chalepotherium = Mucrotherium = Uniserium) O. triserialis Hennig 1922; O. major Kόhne 1956; O. minor Kόhne 1956; O. lufengensis Luo & Sun 1993.
Range: Early Jurassic (possibly Late Triassic) of Europe (Germany, England, Wales?), China, North America.
Phylogeny: Tritylodontidae: (Tritylodon + (Bienotherium + (Kayentatherium + ((Lufengia + Dianzhongia) + Bocatherium + (Yunnanodon + (Stereognathus + Xenocretosuchus)))))) + *.
Characters: ~50 cm; double jaw joint; incisors enlarged; canines absent; alternate tooth replacement with double-rooted cheek teeth, but without mammalian-style tooth occlusion; More flexible neck, with mammalian atlas & axis and double occipital condyle; postcranial skeleton strikingly like monotremes; tail vertebrae simpler, like mammals; scapula substantially mammalian; forelimb is carried directly under the body; limb musculature and locomotion virtually fully mammalian.
Links: Transitional Vertebrate Fossils FAQ: Part 1B; JURASSIC CYNODONTS; Tritylodontidae and Tritheledontidae, an internet directory; PANGEA; Oligokyphus Korean); FUR AND FANGS; Era Mesozoica Triassico - Epoca dei Dinosauri.
Notes: O. minor & O. major found in close association and were male & female morphs per Hopson & Kitching (1972). ATW020531.
Tritylodon: = Likhoelia = Tritylodontoideus) T. longaevus Owen 1884 (= Likhoelia ellenbergeri Ginsberg 1961); T. maximus Fourie, 1962 (1963?).
Range: Early Jurassic (possibly Late Triassic) of South Africa, Antarctica. T. longaevus from the Hettangian - Sinemurian Red Beds and Cave Sandstone, especially Upper Middle Elliot (Tritylodon-acme zone) Formation, Stormberg Series, Orange Free State of South Africa, and Lesotho. T. maximus from the Sinemurian - Pliensbachian Clarens Formation, Stormberg Series, Orange Free State, South Africa.
Phylogeny: Tritylodontidae:: (Bienotherium + (Kayentatherium + ((Lufengia + Dianzhongia) + Bocatherium + (Yunnanodon + (Stereognathus + Xenocretosuchus))))) + *.
Links: South African Museum - Fossil Reptiles of the South African Karoo; JURASSIC CYNODONTS; Tritylodontidae and Tritheledontidae, an internet directory; UMGS Newsletter; exn.ca's Dinosaurs! Mammal-like reptiles: preying on baby dinosaurs; Encyclopedia of Dinosaurs: Hammer.
Image: Tritylodon skull from South African Museum - Fossil Reptiles of the South African Karoo.
Notes:  The generic name Tritylodon was in the past applied to isolated teeth from the Rhaeto-Lias of Europe (e.g. Tritylodon fraasi below), but these are in fact, generically indeterminate. As used now the genus is restricted to southern African forms.  T. maximus is either a large T. longaevus (Hopson & Kitching, 1972) or a distinct but closely 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. MAK010421. ATW020601.
Bienotherium: B. yunnanense Young 1940 (= B. elegans Young 1947); B. magnum Chow 1962.
Range: Early Jurassic of China. B. yunnanense from the Dull Purplish Beds, Lower Lu-feng Series of Yunnan, of Hettangian? age. B. magnum from the Dark Red Beds, Lower Lu-feng Series of Yunnan, of Sinemurian or early Pliensbachian age.
Phylogeny: Tritylodontidae::: (Kayentatherium + ((Lufengia + Dianzhongia) + Bocatherium + (Yunnanodon + (Stereognathus + Xenocretosuchus)))) + *.
Links: JURASSIC CYNODONTS; Tritylodontidae and Tritheledontidae, an internet directory; JURÁSSICO Répteis; Dinosaur museum (Chinese).
Note: B. magnum is recognized as a distinct species because it comes from a higher horizon than B. yunnanense and is much larger. MAK010421. ATW020601.
Kayentatherium: K. wellesi Kermack 1982 (= Nearctylodon broomi Lewis 1986)
Range: Early Jurassic of North America, Kayenta Formation, Arizona of late Sinemurian / early Pliensbachian age.
Phylogeny: Tritylodontidae:::: ((Lufengia + Dianzhongia) + Bocatherium + (Yunnanodon + (Stereognathus + Xenocretosuchus))) + *.
Characters: highly specialized teeth with multiple rows of cusps, high mandibular condyle, teeth and jaw indicative of herbivorous diet.
Links: JURASSIC CYNODONTS; Tritylodontidae and Tritheledontidae, an internet directory; Mesozoic; Vertebrate trace fossils; Vertebrate Fossils; Evolution of the mammalian cortex: past, present and future; MODERN CONCEPTS OF EVOLUTION; Tritylodontidae.
Image: image courtesy Dr Pamela Gore. MAK010421. ATW020601.
Lufengia: Chow & Hu 1959. l. minor Young 1947.
Range: Early Jurassic of China Dull Purplish Beds, Lower Lufeng Series of Yunnan.
Phylogeny: Tritylodontidae:::::: Dianzhongia + *.
Links: URASSIC CYNODONTS; Tritylodontidae and Tritheledontidae, an internet directory. MAK010421. ATW020601.
Dianzhongia: = Diazhongia) D. longirostrata Cui 1981 (= Lufengia delicata? Chow & Hu 1959)
Range: Early Jurassic of China, Dark Red Beds, Lower Lu-feng Series of Yunnan, probably of Sinemurian or early Pliensbachian age.
Phylogeny: Tritylodontidae:::::: Lufengia + *.
Links: JURASSIC CYNODONTS; Tritylodontidae and Tritheledontidae, an internet directory; bienoyokou.html Japanese). ATW020601.
Bocatherium: B. mexicanum Clark & Hopson 1985.
Range: Middle Jurassic of North America, La Boca Formation of Huizachal Canyon, Tamaulipas, Mexico.
Phylogeny: Tritylodontidae::::: (Yunnanodon + (Stereognathus + Xenocretosuchus)) + *.
Links: The Journal of Vertebrate Paleontology; JURASSIC CYNODONTS; Tritylodontidae and Tritheledontidae, an internet directory; Un Pterosauro Tamaulipeco; Tritylodontidae; Los dinosaurios en México/Localidades del Período Jurásico en México (Spanish). ATW020601
Yunnanodon: = Yunnania) Y. brevirostre Cui1976.
Range: Early Jurassic of China, from the Dark Red Beds, Lower Lufeng Series of Yunnan, probably of Sinemurian or early Pliensbachian age.
Phylogeny: Tritylodontidae:::::: (Stereognathus + Xenocretosuchus) + *.
Characters: skull length 3.5 - 4-5 cm; periotic bones combined in definitive petrosal with somewhat lengthened cochlea; buccal and labial cusps on uppers tend to form longitudinal crest; 2 cusps on lingual row (of uppers or lowers?); transverse dentine sheet connecting upper postcanine roots in the same row much less developed in Yunnanodon than Lufengia; anterolingual root of the upper postcanines small and rounded in transverse section.
Links: JURASSIC CYNODONTS; Tritylodontidae and Tritheledontidae, an internet directory; bienoyokou.html Japanese); paleng4_99p422abs; Untitled Chinese). ATW030222
Stereognathus: S. ooliticus Charlesworth, 1855.
Range: Middle Jurassic of Europe, Stonesfield slate of Oxfordshire, (& a S. sp., Forest Marble of Dorset), of Bajocian age.
Phylogeny: Tritylodontidae::::::: Xenocretosuchus + *.
Links: JURASSIC CYNODONTS; Tritylodontidae and Tritheledontidae, an internet directory.
Notes: Stereognathus sp. has been recorded from the Forest Marble of Dorset. For a long time Stereognathus was the latest known therapsid, 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, although it is certainly possible. ATW020601. MAK010421.
Xenocretosuchus: X. sibiricus Tatarinov & Matchenko 1999.
Range: Early Cretaceous II of Russia. Shestakovo, Kemerovo Region of Siberia, of late Aptian age.
Phylogeny: Tritylodontidae::::::: Stereognathus + *.
Characters: buccal and labial cusps on the upper teeth form longitudinal crests.
Notes: This form is assigned to the tritylodonts on the basis of the two transverse 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 Early Jurassic of southern China. However, I am somewhat skeptical of a specimen based only some isolated teeth. It may very well be a tritylodont (and if so it would be the youngest stratigraphic 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. MAK010421.
Links: paleng4_99p422abs; JURASSIC CYNODONTS; Tritylodontidae and Tritheledontidae, an internet directory; RE: Details on SVP Thursday posters (Part 2). ATW031002.