Study Finds Fossil Sex Differences Less Pronounced Than Previously Thought

Fossil skulls of uintatheres, long cited as a primary example of sexual dimorphism in prehistoric mammals, show far less physical variation between males and females than paleontologists have assumed for decades, according to a study published in PLOS One, Mirage News reported.

Uintathere Skull Analysis Challenges Long-Held Paleontological Assumptions

Uintatheres emerged roughly 58 million years ago, following the extinction of the dinosaurs, and persisted in the fossil record for approximately 20 million years. Known for massive bodies, prominent horns, and large canine teeth, these extinct mammals have traditionally served as a textbook illustration of sexual dimorphism, with males interpreted as significantly larger and possessing more dramatic cranial features than females.

However, Kevin Mulcahy, a doctoral student at the University of Kansas Biodiversity Institute & Natural History Museum, noticed anomalies while examining skulls at the American Museum of Natural History in New York. While comparing a broken upper molar found during fieldwork in Montana, Mulcahy observed that nearly meter-long skulls housed on museum shelves displayed a wide range of variation that did not fit a binary distribution.

Males have been interpreted as being much larger and possessing far more dramatic cranial features than females, Mulcahy said.

Statistical Testing of Extinct Species and Modern Bison

Using a ruler, Mulcahy gathered initial skull measurements at the museum and applied statistical tests to determine if the dimensions fit a unimodal or bimodal distribution. A bimodal distribution with two separate means would indicate strong sexual dimorphism, but the measurements failed to reveal two distinct groups of larger and smaller specimens.

To ensure his methods could accurately detect sexual dimorphism, Mulcahy tested the same statistical techniques on modern-day bison, a large herbivore with a similar proposed ecology and prominent horns. The bison data yielded clear results, successfully separating male and female specimens into distinct categories, whereas uintathere skulls showed only weak signals.

When I was looking at the bison, it was almost perfect that all of the females were most similar to the females, and all the males were most similar to the males, Mulcahy said.

Victorian Biases Shaped Assumptions About Prehistoric Sexual Dimorphism

The study suggests that mistaken assumptions about prehistoric sexual dimorphism stem from the cultural biases of the late 19th and early 20th centuries, when many fossil species were initially described by Victorian-era paleontologists working under the assumption that males would naturally be larger and more threatening.

I think it’s really important to remember that these things were first discovered in the late 1800s by a bunch of male paleontologists working in a very Victorian-era mindset, Mulcahy said. The idea is basically assumed at that point that males are going to be bigger and scarier than females. It’s really interesting: When you step back and actually test that hypothesis, it doesn’t really hold up.

Broader Implications for Mammalian Evolution

Co-authored by K. Christopher Beard, senior curator at the KU institute, the findings imply that other species in the fossil record may also exhibit less sexual dimorphism than previously thought. Mulcahy noted that strong male-biased dimorphism was likely not ancestral at the root of the mammalian tree and that living mammals frequently show no dimorphism or even larger females.

The idea that males are always bigger than females is really just a choice of what animals were people looking at, what are the things that biologists in the past were interested in, Mulcahy said.

Common Questions About Uintathere Dimorphism Research

What is Uintatherium anceps?

Uintatherium anceps is an extinct species of large, horned herbivorous mammal. The study analyzed a dataset of approximately 30 skulls of this species gathered from multiple museum collections.

How did researchers test for sexual dimorphism without knowing the sex of fossils?

Researchers measured skull dimensions and applied statistical models to check for bimodal distributions. To validate the reliability of these statistical methods, the team tested the exact same procedures on modern bison specimens of known sex.

Where was the study published?

The research was published in the scientific journal PLOS One by researchers from the University of Kansas.