For decades, paleontologists viewed Uintatherium anceps—a massive, horned prehistoric mammal—as a classic textbook case of sexual dimorphism. It was widely assumed that males were far larger than females and possessed much more prominent horns and canine teeth. However, a study published in PLOS One by researchers at the University of Kansas challenges this longstanding belief.
The Discovery
The investigation began when Kevin Mulcahy, a doctoral student at the KU Biodiversity Institute & Natural History Museum, and his adviser, K. Christopher Beard, sought to identify a broken upper molar recovered during fieldwork in Montana. They traveled to the American Museum of Natural History in New York to compare the fossil tooth with other specimens. While examining entire shelves of skulls—some measuring nearly one meter long—Mulcahy noticed that physical variations in the skulls did not divide neatly into two separate male and female categories.
What Scientists Found
Mulcahy gathered measurements from roughly 30 Uintatherium anceps skulls across multiple museum collections to perform statistical analyses. Rather than finding a distinct bimodal distribution—where measurements split into two clear statistical groups representing males and females—the data showed no conclusive separation. To test whether his statistical tools were capable of detecting real sexual differences in animals, Mulcahy applied the same methods to modern bison. Because bison are large horned herbivores with known sexes, the analysis successfully picked up clear signals of dimorphism, confirming that the statistical approach was reliable.
Why It Matters
Uintatherium anceps first appeared around 58 million years ago, less than 10 million years after the asteroid impact that ended the era of nonavian dinosaurs, and remained in the fossil record for approximately 20 million years. Because the testing methods easily identified sex differences in modern animals but found inconclusive evidence in Uintatherium anceps, the research indicates that physical differences between males and females in this species were far less pronounced than previously taught.
Source: prehistoric animals new study - BingNews · Published October 21, 2026 · Photo by Wretch Fossil via Openverse