Kayentatherium, “the beast from the Kayenta Formation”, was a cynodont (one of a group of mammal ancestors which are sometimes called “mammal-like reptiles”) which lived in Arizona during the early Jurassic Period approximately 185-180 million years ago. Kanyentatherium was big, measuring nearly 5 feet long, and it also appears to have been semi-aquatic like a modern-day beaver or otter.
In 1953, geologists from the US Geological Survey discovered several individuals of a prehistoric animal within the upper layers of the Kayenta Formation, which dates to the early Jurassic Period. The first specimen was found at Comb Ridge, Arizona, 6 miles (9.6 km) east of the type locality for the Kayenta Formation, and was found 8-10 feet below the Kayenta Formation’s contact with the overlying Navajo Sandstone Formation. Upon examination, these fossils belonged to a group of primitive proto-mammals from the family Tritylodontidae. That in itself was noteworthy, because while tritylodontids were known from Europe, South Africa, and China, none had been found within North America up to that point. Even more excitingly, the specimens which were found in Arizona didn’t seem to match any known tritylodontid. This could be a new species! As an aside, it was also noted that the remains were found near other fossils which were ascribed to the basal crocodylomorph Protosuchus richardsoni (Lewis 1958, page 1,735; Sues 1986, page 218).
At first glance, it looked as though the tritylodontid specimens from Arizona were most similar morphologically to Bienotherium from southwestern China, which dates to the early Jurassic Period (mistakenly written in Lewis 1958 as the upper Triassic) (Lewis 1958, page 1,735). However, when a more thorough examination of the bones was made in the 1960s, it appeared that the shape and arrangement of the skull bones in particular were most similar to those of the eponymous genus Tritylodon rather to any of the other tritylodontid genera. Tritylodon comes from South Africa and dates to the early Jurassic Period; the South African connection is important because comparisons have often been made between the faunae of the Kayenta Formation and South Africa’s Elliot Formation. It was noted that the skull remains which were recovered from Arizona lacked the first and third upper incisors, that the premaxillae did not meet the nasal bones but instead extended upwards to the suture that joined the maxilla to the septomaxilla, the nasals were narrow anteriorly and broad posteriorly (2.5 times the anterior width), and both nasals were narrow across the squamosal region compared to the snout. George Edward Lewis stated in 1966 that this new North American tritylodontid was about the same size as Tritylodon or Bienotherium (Lewis 1966, pages 96-97).
In 1982, the British paleontologist Doris Mary Kermack named the creature from Arizona Kayentatherium wellesi, “Welles’ Kayenta beast”, named in honor of the paleontologist Samuel P. Welles, presumably in recognition of all of the work that he had done within the Kayenta Formation. The holotype specimen UCMP V6897 is housed within the University of California Museum of Paleontology (Kermack 1982, page 10).
Hypothetical reconstruction of Kayentatherium wellesi from Sues and Jenkins (2006), page 141. Scale bar = 10 cm. The skull, pre-sacral postcranial skeleton, and individual caudal vertebrae are based on MCZ 8812. Other postcranial bones are reconstructed on the basis of additional tritylodontid specimens from the Kayenta Formation: ilium (anterior portion from MCZ 8812 and remainder from indeterminate tritylodontid, MCZ 8835), ischium (Dinnebitodon amarali, MNA V3232), femur (indeterminate tritylodontid, MCZ 8832 and 8838), and tibia (Kayentatherium wellesi, MNA V3141). Length of the tail is conjectural. Note that the tail illustrated here is much shorter than it actually was.
Skull of Kayentatherium wellesi (MCZ 8812), right lateral view and dorsal view. Scale bar = 3 cm. Images from Sues 1986, pages 221-222.
An examination of all known members of the family Tritylodontidae including Kayentatherium confirmed that the tritylodontids were indeed cynodonts – that is proto-mammals, not true mammals. Specifically, the tritylodontids were the sister group to the cynodont family Traversodontidae, which includes creatures like Traversodon, Exaeretodon, and Massetognathus. The families Tritylodontidae and Traversodontidae are grouped together under the super-family Tritylodontoidea, which in turn is part of the sub-order Cynodontia. Members of the family Tritylodontidae are distinguished from other cynodonts by the shape of their molars (Sues 1985, pages 205-206, 215).
Much has been written about Kayentatherium’s skull and teeth (Lewis 1966, pages 96-97; Kermack 1982, pages 1-17; Sues 1985, pages 207, 210, 213; Sues 1986, pages 221-222), which I will not put here as it would be long-winded and tedious. Suffice to say the skull was strongly built with large hefty teeth ideal for crushing up plant material. Kayentatherium’s skull measured 7.41 inches (18.82 cm) long (Sues 1986, pages 221-222). Based upon the size of the skull, the whole animal probably measured almost 5 feet long!
Kayentatherium’s coracoid and procoracoid (two of the bones which form the shoulder girdle) are much reduced in size compared to less-advanced cynodonts (Sues 1985, page 211). Kayentatherium also possessed “amphicoelous” dorsal vertebrae, meaning that the front and rear surfaces of the vertebra’s centrum are concave like a bowl (Sues 1985, page 210).
A report from 2006 stated that Kayentatherium’s arm bones had features which suggested digging or burrowing abilities (Sues and Jenkins 2006, page 146). Then again, such movements could also be employed in swimming. In 2009, Sebastian Egberts and his colleagues stated that a recently-discovered specimen of Kayentatherium showed evidence that it was highly-adapted for a semi-aquatic lifestyle. The specimen included parts of the skeleton that hadn’t been seen before, and consisted of three dorsal vertebrae, the pelvis, both hind limbs, and a complete tail consisting of twenty-nine caudal vertebrae, far longer than earlier estimates of the tail’s length as seen in Sues and Jenkins (2006). The authors stated that the structure of Kayentatherium’s tail vertebrae were very similar to the tail bones of modern-day beavers and the prehistoric semi-aquatic mammal Castrocauda. Examination of the caudal vertebrae’s range of motion showed that the tail would have been good at moving up-and-down like a beaver or otter (Egberts et al. 2009, page 91A). An abstract written by Eva Hoffman and Timothy B. Rowe which was presented at a 2017 paleontology conference reiterated much of what was said by Egberts and his colleagues, but also added a few more details. Hoffman and Rowe hypothesized that Kayentatherium sporteda flattened paddle like a beaver or otter. They also looked closer at Kayentatherium’s tail vertebrae, and concluded that caudal vertebrae’s range of motion showed that the tail would have been good at moving up-and-down as Egberts et al. stated, and also added that it wouldn’t have been good at moving side-to-side. Additionally, Kayentatherium’s hind feet were broad and flat, and might have had webbing between the toes, and the structure of the foot bones themselves were very similar to the foot bones seen in semi-aquatic animals (Hoffman and Rowe 2017, page 130).
Kayentatherium wellesi. © Jason R. Abdale (July 1, 2025).
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Bibliography
Egberts, Sebastian; Rowe, Timothy B.; Sues, Hans-Dieter; Luo, Zhe-Xi; Jenkins, Farish A. (2009). “The first semi-aquatic synapsid from the Early Jurassic of Arizona”. Journal of Vertebrate Paleontology, volume 29, issue 3 (supplement) (January 2009). Page 91A.
https://www.researchgate.net/publication/296142211_THE_FIRST_SEMI-AQUATIC_SYNAPSID_FROM_THE_EARLY_JURASSIC_OF_ARIZONA.
Hoffman, Eva; Rowe Timothy B. (2017). “Postcranial anatomy of Kayentatherium wellesi: Swimming adaptations in a mammaliamorph from the Early Jurassic”. 77th Annual Meeting of the Society of Vertebrate Paleontology – Calgary, Canada (August 23-26, 2017). Page 130.
https://vertpaleo.org/wp-content/uploads/2021/03/SVP-2017-program-book-v6-Print-Ready-with-front-cover.pdf.
Kermack, D. M. (1982). “A new tritylodont from the Kayenta Formation of Arizona”. Zoological Journal of the Linnean Society, volume 76, issue 1 (September 1982). Pages 1-17.
Lewis, George Edward (1958). “American Triassic mammal-like vertebrates”. Bulletin of the Geological Society of America, volume 69, issue 12, part 2 – Abstracts of papers submitted for the meeting in Golden, Colorado, May 8-10, 1958 (Rocky Mountain Section GSA) (December 1958). Page 1,735.
https://archive.org/details/sim_geological-society-of-america-bulletin_1958-12_69_12/page/1735/mode/1up.
Lewis, George Edward (1966). “American Tritylodontidae from the Kayenta Formation of Arizona”. Geological Society of America Special Paper 87 – Abstracts for 1965 (January 1, 1966). Pages 96-97.
https://pubs.geoscienceworld.org/gsa/books/book/730/chapter-abstract/3809000/Abstracts-of-Papers-Submitted-for-the-Meeting-in?redirectedFrom=fulltext.
Sues, Hans-Dieter (1985). “The relationships of the Tritylodontidae (Synapsida)”. Zoological Journal of the Linnean Society, volume 85, issue 3 (November 1985). Pages 205-217.
Sues, Hans-Dieter (1986). “The skull and dentition of two tritylodontid synapsids from the Lower Jurassic of western North America”. Bulletin of the Museum of Comparative Zoology at Harvard College, volume 151 (1986). Pages 217-268.
https://www.biodiversitylibrary.org/page/4779034#page/237/mode/1up.
Sues, Hans-Dieter; Jenkins, F. A. (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.; Wible, J. R., eds. Amniote Paleobiology: Perspectives on the Evolution of Mammals, Birds, and Reptiles. Chicago: The University of Chicago Press, 2006. Pages 114-152.
https://repository.si.edu/bitstream/handle/10088/7533/paleo_SUES_JENKINS.2006.pdf.
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