Tritylodontidae

Tritylodontidae ("three-knob teeth", named after the shape of animal's teeth) is an extinct family of small to medium-sized, highly specialized mammal-like cynodonts, bearing several mammalian traits like erect limbs, endothermy and details of the skeleton.[1] They were the last-known family of the non-mammalian synapsids, persisting into the Early Cretaceous.[2]

Tritylodontidae Temporal range: 231–113 Ma PreꞒ Ꞓ O S D C P T J K Pg N Late Triassic – Early Cretaceous
Life restoration of Oligokyphus
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Therapsida
Clade:	Cynodontia
Clade:	Mammaliamorpha
Family:	†Tritylodontidae Cope, 1884
Genera
See below

Tritylodontidae probably descended from a Cynognathus-like cynodont. Most tritylodontids are thought to have been herbivorous, feeding on vegetation, such as stems, leaves, and roots, although at least one may have had a more omnivorous diet.[3] Tritylodontid fossils are found in the Americas, South Africa, and Eurasia—they appear to have had an almost global distribution, including Antarctica.[4]

Description

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]

Discovery

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]

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.[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.

Phylogeny

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]

Cynodontia	Charassognathus         Dvinia     Procynosuchus      Epicynodontia    Cynosaurus         Galesaurus     Progalesaurus           Nanictosaurus     Thrinaxodon         Platycraniellus    Eucynodontia    Cynognathia →     Probainognathia →

→ Cynognathia

Cynognathus    Gomphodontia    Diademodon      Trirachodontidae      Beishanodon     Sinognathus         Trirachodon       Cricodon     Langbergia          Traversodontidae      Andescynodon     Pascualgnathus         Scalenodon       Luangwa       Traversodon         "Scalenodon" attridgei       Mandagomphodon       Nanogomphodon       Arctotraversodon     Boreogomphodon              Massetognathinae    Massetognathus       Dadadon     Santacruzodon        Gomphodontosuchinae    Menadon       Gomphodontosuchus       Protuberum       Exaeretodon     Scalenodontoides

→ Probainognathia

Lumkuia       Ecteninion         Aleodon     Chiniquodon         Probainognathus       Trucidocynodon       Therioherpeton      Tritheledontidae    Riograndia         Chaliminia     Elliotherium         Diarthrognathus     Pachygenelus            Brasilodontidae    Brasilitherium     Brasilodon        Tritylodontidae    Oligokyphus       Kayentatherium       Tritylodon     Bienotherium          Mammaliaformes    Sinoconodon     Morganucodon

Genera

Kayentatherium

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

See also

• Embryonic diapause

References

1. Kemp, Tom. 2004. The Origin and Evolution of Mammals. Oxford University Press.
2. Matsuoka, Hiroshige, Nao Kusuhashi, and Ian J. Corfe. "A new Early Cretaceous tritylodontid (Synapsida, Cynodontia, Mammaliamorpha) from the Kuwajima Formation (Tetori Group) of central Japan." Journal of Vertebrate Paleontology (2016): e1112289.
3. Hu, Yaoming; Meng, Jin; Clark, James M (2009). "A New Tritylodontid from the Upper Jurassic of Xinjiang, China". Acta Palaeontologica Polonica. 54 (3): 385–391. doi:10.4202/app.2008.0053.
4. Hammer, WR; Smith, MD (2008). "A Tritylodont Postcanine from the Hanson Formation of Antarctica". Journal of Vertebrate Paleontology. 28: 269–273. doi:10.1671/0272-4634(2008)28[269:atpfth]2.0.co;2.
5. Luo, Zhe-Xi (2002). "In quest for a phylogeny of Mesozoic mammals". Acta Palaeontol. Pol. 47 (1): 1–78.
6. Reilly, S. M. (2003). "Hypaxial Motor Patterns and the Function of Epipubic Bones in Primitive Mammals". Science. 299: 400–402. doi:10.1126/science.1074905. PMID 12532019.
7. Hoffman, Eva A.; Rowe, Timothy B. (2018). "Jurassic stem-mammal perinates and the origin of mammalian reproduction and growth". Nature. 561 (7721): 104–108. doi:10.1038/s41586-018-0441-3. PMID 30158701.
8. Anusuya Chinsamy-Turan, Forerunners of Mammals: Radiation • Histology • Biology, Indiana University Press, 18/11/2011
9. Charlesworth, E (1854). Rept Brit Assoc. 1854 Liverpool Abstracts 80. Missing or empty |title= (help)
10. Panciroli, E.; Walsh, S.; Fraser, N.C.; Brusatte, S.L.; Corfe, I. (2017). "A reassessment of the postcanine dentition and systematics of the tritylodontid Stereognathus (Cynodontia, Tritylodontidae, Mammaliamorpha), from the Middle Jurassic of the United Kingdom". Journal of Vertebrate Paleontology. 37 (5): e1351448. doi:10.1080/02724634.2017.1351448. hdl:10138/230155.
11. Cope, Edward Drinker (1884). "The Tertiary Marsupialia". American Naturalist. 18: 686–697. doi:10.1086/273711.
12. Owen, R (1884). "On the skull and dentition of a Triassic mammal (Tritylodon longaevus) from South Africa". Quarterly Journal of the Geological Society of London. 40: 146–152. doi:10.1144/gsl.jgs.1884.40.01-04.07. hdl:2027/hvd.32044107348633.
13. Liu, J.; Olsen, P. (2010). "The Phylogenetic Relationships of Eucynodontia (Amniota: Synapsida)". Journal of Mammalian Evolution. 17 (3): 151. doi:10.1007/s10914-010-9136-8.
14. Ruta, M.; Botha-Brink, J.; Mitchell, S. A.; Benton, M. J. (2013). "The radiation of cynodonts and the ground plan of mammalian morphological diversity". Proceedings of the Royal Society B: Biological Sciences. 280 (1769): 20131865. doi:10.1098/rspb.2013.1865. PMC 3768321. PMID 23986112.