Tritylodontidae

Tritylodonts Temporal range: 221–113 Ma PreЄ Є O S D C P T J K Pg N Late Triassic - Early Cretaceous[1]
Life restoration of Oligokyphus
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Order:	Therapsida
Suborder:	Cynodontia
Clade:	Mammaliamorpha
Family:	†Tritylodontidae Cope, 1884
Genera
See below

Tritylodontids ("three knob teeth", named after the shape of animal's teeth) were small to medium-sized, highly specialized and extremely mammal-like cynodonts, bearing several mammalian hallmarks like erect limbs, endothermy and lactation. They were the last known family of the non-mammalian synapsids. One of the last cynodont lines to appear, the Tritylodontidae descended from a Cynognathus-like cynodont. The Tritylodontids were herbivorous, chewing through vegetation, such as stems, leaves, and roots, though recent studies indicate some may have had more omnivorous diets.^[2] Tritylodontid fossils are found in the Americas, South Africa, and Eurasia - they appear to have had an almost global distribution, including Antarctica.

Description

Skull of Kayentatherium wellesi

The tritylodont's skull had a high flat crest. They retained the reptilian joint between the quadrate bone of the skull and the articular bone of the lower jaw, but they were reduced. It is only through the retention of the vestigial reptilian jawbones that they are technically regarded to not be mammals. 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. The front pair of incisors were enlarged that were very similar to rodents of today. Tritylodontids had a large gap, the diastema, that separated the incisors from the square-shaped cheek. Each of the cheek teeth in the upper jaw had three rows of cusps running along its length that had 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 most earlier cynodonts. It would grind its food between the teeth in somewhat the same way that a modern rodent would with their food, though unlike rodents tritylodontids had a palinal jaw stroke (front-to-back), instead of a propalinal one (back-to-front). The teeth were well suited for shredding plants matter; however, there is evidence that some tritilodontids had more omnivorous diets, much in the same vein as modern mammals with "herbivore dentitions" like modern rats.^[2]

Like Mammaliformes, tritylodontids have epipubic bones, a possible synampomorphy between both clades,^[3] and as such were forced to produce undeveloped fetus-like young like modern monotremes and marsupials. Their tooth replacement patterns suggest that, like modern mammals, tritylodontids produced milk.^[2]

Tritylodonts were active animals that were probably warm blooded and probably burrowed (though in Kayentatherium these supposed burrowing adaptations may be indicative of semi-aquatic habits^[4]). For example, Oligokyphus could be compared to a weasel or mink, with a long, slim body and tail.

Discovery

Tritylodontids were first discovered in the Upper Triassic rocks of South Africa in the late 1800s and were initially thought to be amongst the very earliest mammals. The first genus was named in 1854, Stereognathus ooliticus,^[5] and the family name erected by Cope in 1884.^[6]

Evolutionary history

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.^[7] 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 elusive and 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.

Phylogeny

Because of their extremely mammal-like appearance, tritylodontids were originally placed within Mammalia. Starting with the work of British palaeontologist D. M. S. Watson in 1942, a close relationship was favored between tritylodontids and cynodonts. Watson and other palaeontologists 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.^[8] 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.^[9]

It should be noted that the exact position of Tritylodontidae in relation to Mammalia is still debated, but most researchers agree they are closely related, although Tritylodontidae is considered a non-mammaliamorph cynodont.^[10]

Genera

Kayentatherium

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

See also

• Mammalian embryonic diapause

References