Tritylodontidae

Skull of Kayentatherium wellesi

The tritylodont's skull has a high sagittal crest. They retained the reptilian joint between the quadrate bone of the skull and the articular bone of the lower jaw[1]—the retention of the vestigial reptilian jawbones is one of the reasons they are technically regarded to not be mammals, but are instead mammaliamorphs.[5] The back of the skull had huge zygomatic arches for the attachment of its large jaw muscles. They also had a very well-developed secondary palate. The tritylodont dentition was very different from that of other cynodonts: they did not have canines, and the front pair of incisors were enlarged and were very similar to rodents of today.[1] Tritylodontids had a large gap, called a diastema, that separated the incisors from their square-shaped cheek teeth. The cheek teeth in the upper jaw had three rows of cusps running along its length, with grooves in between. The lower teeth had two rows of cusps which fitted into the grooves in the upper teeth. The matching of the cusps allowed the teeth to occlude more precisely than in earlier cynodonts. It would grind its food between the teeth in somewhat the same way as a modern rodent, though unlike rodents tritylodontids had a palinal jaw stroke (front-to-back), instead of a propalinal one (back-to-front).[1] The teeth were well suited for shredding plant matter; however, there is evidence that some tritylodontids had more omnivorous diets, much in the same vein as modern mammals with "herbivore dentitions" like modern rats.[3]

Like mammaliaformes, tritylodontids have epipubic bones, a possible synapomorphy between both clades,[6] and this suggests they may also have laid eggs, or produced undeveloped fetus-like young like modern monotremes and marsupials. A recent Kayentatherium showcases that they were indeed produced undeveloped young, but at litter sizes much larger than any monotreme or marsupial, at around 38 perinates.[7]

Tritylodonts were active animals that were likely warm-blooded and possibly burrowed.[1] The small early tritylodontid, Oligokyphus has been compared to a weasel or mink, with a long, slim body and tail. In Kayentatherium the burrowing adaptations seen in the skeleton have been re-interpreted as possibly suggesting a semi-aquatic ecology.[8]

The first tritylodontid named was Stereognathus, from the Middle Jurassic of England[9][10] and the family name was erected by Cope in 1884.[11] Shortly after, another tritylodontid was discovered in the Upper Triassic rocks of South Africa.[12]

Tritylodontids appeared in the late Triassic period, and persisted through the Jurassic until the early Cretaceous. Montirictus was the latest surviving tritylodontid, from the Cretaceous Barremian-Aptian Kuwajima Formation of Japan.[2] Some authors argue Chronoperates is a tritylodontid member that persisted into the Paleocene after the Cretaceous–Paleogene extinction event, but this is not widely accepted. If true, then tritylodontids would have to be extremely rare during the Late Cretaceous, because no tritylodontids were found by that time. Chronoperates's anatomy closely resembles that of symmetrodonts—a mammalian lineage. It is therefore more likely to belong to this group.

Because of their extremely mammal-like appearance, tritylodontids were originally placed within Mammalia. Starting with the work of British paleontologist D. M. S. Watson in 1942, a close relationship was favored between tritylodontids and cynodonts. Watson and other paleontologists noted that tritylodontids lacked the dentary and squamosal jaw articulation that was characteristic of early mammals. Haughton and Brink (1954) were the first to classify tritylodontids within Cynodontia. Later studies identified close similarities between the teeth of tritylodontids and traversodontids, and tritylodontids were eventually thought to be descendants of traversodontids. Under this classification, which was widely accepted in the following decades, Tritylodontidae was previous considered to be part of Gomphodontia, a larger group within Cynognathia. The name Tritylodontoidea was previously used for the group, which traditionally included the families Diademodontidae, Trirachodontidae, Traversodontidae, and Tritylodontidae.

More recently, tritylodontids have been reinterpreted as close relatives of mammals. Beginning with Kemp (1983), Tritylodontidae has been proposed by numerous studies as a member of Probainognathia, the closest sister group to Mammalia. Gomphodontia is still used for the cynognathian group containing traversodontids and is preferred over Tritylodontoidea now that tritylodontoids are not part of it. A phylogenetic analysis performed by Liu and Olsen (2010) places Tritylodontidae very closely to Mammalia, as the sister taxon of the clade formed by Brasilodontidae and Mammalia.[13] Ruta et al. (2013) phylogenetic analysis which is partially based on Liu and Olsen (2010) places Tritylodontidae in a more derived position than Brasilodontidae. Below is a cladogram from this analysis.[14]

The exact position of Tritylodontidae in relation to Mammalia is still debated, but most researchers agree they are closely related, usually considering Tritylodontidae to be non-mammaliaform, mammaliamorph cynodonts.[5]

Kayentatherium

• Bienotherium
• Bienotheroides
• Bocatherium
• Dianzhongia
• Dinnebitodon
• Kayentatherium
• Lufengia
• Nearctylodon
• Montirictus
• Oligokyphus
• Polistodon
• Shartegodon
• Stereognathus
• Tritylodon
• Xenocretosuchus
• Yuanotherium
• Yunnanodon

• Embryonic diapause