Paleontologists working in southern Wyoming have identified a previously unknown species of omomyid primate, adding a new chapter to the story of one of the best-studied fossil lineages in North America.
Tetonius homunculus (colorized) by R. Bruce Horsfall from W.B. Scott’s History of Land Mammals in the Western Hemisphere.
The new species belongs to a family of small, big-eyed, tarsier-like primates called omomyids that lived in the forests of North America roughly 50 million years ago (Eocene epoch).
Named Tetonius varleyorum, the animal was comparable in size to a modern mouse lemur.
“Our paper is about a new species of an omomyid primate, a primate family close to the top of the family tree of modern primates — with lemurs on one side and apes, humans and monkeys on the other,” said Parker Rhinehart, a doctoral student with the Biodiversity Institute and Natural History Museum at University of Kansas.
“The branch point where they meet up at the top is where the omomyids fall.”
“They’re true primates, but they’re extinct. They appear about 55 or 56 million years ago across the northern hemisphere.”
Dozens of jaw and tooth fragments from Tetonius varleyorum was found at a single sandstone locality nicknamed Tim’s Confession, part of the Smiley Draw local fauna in the Great Divide Basin.
The site has already produced nearly 1,000 tooth-bearing jaw fragments representing a wide range of ancient mammals.
“Identifying ancient primate fossils in the field typically requires hunting tiny fossil jaws and teeth — the only body parts tough enough to withstand erosion over geologic time,” Rhinehart said.
“Our paper analyzes fossils from Smiley Draw, a different Wyoming locale outside of the better-sampled Bighorn Basin.”
“This is really important to the Tetonius story,” he added.
“Because so many scientific ideas only really consider the fossils from the Bighorn Basin, they just don’t take in the whole picture.”
“One of the things our paper points out is when you start incorporating fossils from outside of the Bighorn Basin, this picture becomes a little bit more complicated.”
What makes the new discovery of Tetonius varleyorum especially notable is not just the new species itself, but the company it kept.
Alongside Tetonius varleyorum, the same site also yielded two other rare omomyid primates, Arapahovius gazini and Anemorhysis savage, only the second and third times either species has ever been documented.
According to the paleontologists, three separate species of these small primates have never before been found living together at a single early Eocene site, and a fourth omomyid, not yet formally described, was also recovered there.
“Our description of Tetonius varleyorum as a new fossil species gives fresh detail to the biogeographic and evolutionary history of primates,” Rhinehart said.
“A landmark 1987 paper by paleontologists Thomas Bown and Kenneth Rose includes a famous figure depicting the evolution of fossil Tetonius jaws found in the Bighorn Basin.”
“It’s upheld as a great example of an evolutionary transition — reflecting a true transition in the fossil record.”
“While the earlier paper and figure have been vital to the Tetonius story, fossils from Smiley Draw would challenge some of its conclusions.”
“The 1987 paper claims that there is no branching of the tree, going from one intermediate species of Tetonius to the next with no branching speciation events. Evolutionary biologists call this anagenesis.”
“Our analysis reveals that the new fossils of Tetonius varleyorum interrupt the purported anagenetic lineage from the Bighorn Basin.”
The team’s paper appears in the Journal of Human Evolution.
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Parker D. Rhinehart et al. 2026. A diverse new omomyid primate fauna from the early Eocene of southern Wyoming with implications for interpreting macroevolutionary patterns among anaptomorphines. Journal of Human Evolution 218, 103867; doi: 10.1016/j.jhevol.2026.103867
