Tritylodontidae

Tritylodontids ("three knob teeth", named after the shape of animal's teeth) were small to medium-sized, highly specialized and extremely mammal-like cynodonts. They were the last 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. Some scientists believe that the mammals arose from this group of cynodonts, however, some say that mammals arose from the Tritheledontidae, another group of specialized cynodonts. One of the interesting facts about the Tritylodontids is that they are the longest living of all the non-mammalian therapsids. They appeared in the latest Triassic period, and persisted through the Jurassic until the Middle Cretaceous. This shows that the Tritylodontids were a very successful group of therapsids, even though they lived right beneath the ruling dinosaurs' feet. No one knows why the Tritylodontids went extinct by the Middle Cretaceous. Perhaps the Tritylodontids were outcompeted by their relatives, the mammals. Some mammals have developed herbivory during the Late Jurassic and Cretaceous. Or, the Tritylodontids may have gone extinct because of the new plant group, the angiosperms or flowering plants because they weren't used to eating new type of plants. Chronoperates may be one exception, it may be a Tritylodontid, and it lived in the Paleocene, long after the Middle Cretaceous, and after the Cretaceous–Tertiary extinction event. If true, then the Tritylodontids were elusive and rare during the Late Cretaceous, because no Tritylodonts were found by that time. However, the Chronoperates 's anatomy almost closely resembles to that of symmetrodonts - a mammalian lineage. It is very clear that the Tritylodontids were warm-blooded. The Tritylodontid fossils were found in the Americas, South Africa, and Eurasia. They may have managed to live worldwide, including Antarctica.

Genera

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

The Tritylodonts can very much be seen as Mesozoic rodents.

The Tritylodontidae were mammal-like herbivorous cynodont synapsids. They were among the last of the cynodonts, which evolved from the Traversodontidae in latest Triassic period, and their lineage survived into the Jurassic period. They were the longest-lived group of the therapsids and along with the Tritheledontidae (Ictidosaurs), the only mammal-like reptiles to endure into the Jurassic period and the only non-mammalian Theropsida (Synapsida) to survive into the late Jurassic period.

Tritylodonts were first discovered in the Upper Triassic rocks of South Africa in the late 1800’s and were initially thought to be amongst the very earliest mammals.

The tritylodont's skull had a high flat crest. They retained the reptilian joint between the quadrate bone of the skull and the auricular 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 as reptiles. 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. Traversodonts 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 meet in a precise bite. It would grind its food between the teeth in somewhat the same way that a modern rodent would with their food. The teeth were well suited for shredding plants matter.

Tritylodonts were active animals that were probably warm blooded and burrowed like modern day rodents. For example, Oligokyphus could be compared to a weasel or mink, with a long, slim body and tail. Its legs had evolved directly beneath the body, as they have in mammals.

See also

• Cynodontia
• Cynognathia
• Tritheledontidae
• Mammalian embryonic diapause