RESOLVING THE LEDGER
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Add as preferred source A new study from the University of Kansas complicates the picture of primates living in North America some 55 million years ago during the Eocene, a warm period that allowed mammals to diversify quickly.
The research, published in the Journal of Human Evolution , introduces a new species of ancient primate and challenges long-held assumptions about the sequence of evolutionary traits in a key fossil primate genus of the epoch.
"The 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 lead author Parker Rhinehart, a doctoral student at the KU Biodiversity Institute and Natural History Museum. "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."
The new paper describes a new species of the genus Tetonius, an omomyid comparable in size to a modern mouse lemur. The findings suggest its evolution might be more complex than the step-by-step progression earlier suggested by fossils from a single basin.
"We named it Tetonius varleyorum," Rhinehart said. "Varley is the name of a family that runs a truck stop near the campground where we stay when we're collecting fossils, and they've been a great help to us."
The KU researcher said 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. He added that the KU paper analyzes fossils from Smiley Draw, a different Wyoming locale outside the better-sampled Bighorn Basin.
Co-author Robert Anemone, a professor of anthropology at the University of North Carolina at Greensboro, discovered the Smiley Draw site and has conducted fieldwork there for several years. Co-author K. Christopher Beard, senior curator at the KU Biodiversity Institute and Foundation Distinguished Professor in KU's Department of Ecology & Evolutionary Biology, is advising Rhinehart's doctoral research.
"This is really important to the Tetonius story," Rhinehart said. "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."
Rhinehart said the paper's description of Tetonius varleyorum as a new fossil species gives fresh detail to the biogeographic and evolutionary history of primates.
For instance, the KU researcher cited a landmark 1987 paper by paleontologists Thomas Bown and Kenneth Rose, which includes a famous figure depicting the evolution of fossil Tetonius jaws found in the Bighorn Basin.
"One of the premier papers from that basin has this figure depicting a proposed evolutionary transition from an early form of Tetonius," he said. "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, Rhinehart said fossils from Smiley Draw described in the new research 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," he said. "Evolutionary biologists call this 'anagenesis.'"
The KU paper's analysis reveals that the new fossils of T. varleyorum interrupt the purported anagenetic lineage from the Bighorn Basin.
Rhinehart plans more fieldwork at the Smiley Draw locale.
Parker D. Rhinehart et al, 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 (2026). DOI: 10.1016/j.jhevol.2026.103867
Journal information: Journal of Human Evolution
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