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Temporal range: Early Jurassic
Kayentatherium wellesi.jpg
Skull of K. wellesi
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Order: Therapsida
Suborder: Cynodontia
Family: Tritylodontidae
Genus: Kayentatherium
Kermack, 1982
Type species
Kayentatherium wellesi
Kermack, 1982
  • Nearctylodon broomi Lewis, 1986

Kayentatherium is an extinct genus of tritylodontid cynodonts that lived during the Early Jurassic. It is one of two tritylodonts from the Kayenta Formation of northern Arizona, United States.

Kayentatherium means "Kayenta Beast", and is named for the geological formation in which it was found, the Kayenta Formation. Kayentatherium is known from several specimens.[1][2] The species name honors paleontologist Samuel Welles, who worked with the University of California Museum of Paleontology in much of the pioneering work on the paleontology of the Kayenta Formation.[1]


The first tritylodontid material found in the Kayenta Formation were collected in the 1950s, and further material was collected in 1977 and 1982 by a team led by Farish Jenkins. Also found in the same rocks were Dinnebitodon amarali and Nearctylodon broomi, but the latter was later considered to be a juvenile specimen of Kayentatherium, and so was synonymised.[2]


Skulls of Kayentatherium sp. in the American Museum of Natural History showing a growth series from young to adult.

It was about a meter long, the skull was over 20 centimetres (7.9 in) in length.[1] It was a robust and stocky animal with a large head and stout backbone.[2] Some researchers think it might have been semi-aquatic, with adaptations formerly thought to indicate digging habits now interpreted as speciation towards limb-powered swimming.[3] Slight flattening and flaring of the tail vertebrae also suggest specialisms for a semi-aquatic ecology.[4] If this was the case, it would be one of the earliest examples of semi-aquatic specialism in mammaliamorphs in the mammal fossil record.


A recent maternal skeleton revealed a litter composed of 38 perinates, a number considerably higher than those of any living mammal litter. These specimens are poorly developed, as in living non-placental mammals due to the presence of epipubic bones. Nonetheless, they do show case that tritylodontids had proportionally smaller brains at this stage than mammal perinates.[5]


  1. ^ a b c Kermack, D. 1982. A new tritylodont from the Kayenta Formation of Arizona. Zoological Journal of the Linnean Society, 76. 1-17.
  2. ^ a b c Sues, Hans-Dieter & F. A. Jenkins. 2006. The Postcranial Skeleton of Kayentatherium Wellesi from the Lower Jurassic Kayenta Formation of Arizona and the Phylogenetic Significance of Postcranial Features in Tritylodontid Cynodonts in: Carrano, Matthew T., Gaudin, T. J., Blob, R. W. and Wible, J. R., Amniote Paleobiology: Perspectives on the Evolution of Mammals, Birds, and Reptiles: The University of Chicago Press. pp. 114-152.
  3. ^ Anusuya Chinsamy-Turan, Forerunners of Mammals: Radiation • Histology • Biology, Indiana University Press, 18/11/2011
  4. ^ Hoffman E, and Rowe TB. 2017 Postcranial anatomy of Kayentatherium wellesi: swimming adaptations in a mammaliamorph from the Early Jurassic. Abstract from the Society of Vertebrate Paleontology Annual Conference.
  5. ^ Eva A. Hoffman; Timothy B. Rowe (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.

See also[edit]