This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: For decades, paleontologists have held up uintatheres as a prime illustration of sexual dimorphism, or the tendency for males and females of a species to be physically different. Notable for their large size, prominent horns and canine teeth, uintatheres emerged about 58 million years ago, fewer than 10 million years after a space rock killed the dinosaurs, and show up in the fossil record for the next 20 million years or so.
But a study from the University of Kansas published today in the journal PLOS ONE shows the differences between male and female uintatheres might not be as vast as paleontologists have long believed. "Males have been interpreted as being much larger and possessing far more dramatic cranial features than females," said study author Kevin Mulcahy, a doctoral student at the KU Biodiversity Institute & Natural History Museum. Mulcahy and his doctoral adviser, K.
Christopher Beard, senior curator at the institute, were trying to identify a single broken upper molar from fieldwork in Montana that the researchers suspected could have belonged to the uintathere group. "We had a research visit at the American Museum of Natural History in New York, where our main goal was just to compare that tooth to teeth from other uintatheres and figure out what type of animal we had," Mulcahy said. "They have this big collection—entire shelves occupied with these skulls, and each one of them is almost a meter (3.3 feet) long.
These are famous specimens that are in the literature, and I was originally just looking at them to compare their teeth, and I noticed a ton of variation in a way that didn't appear binary." The KU scholar began recording dimensions of the skulls at the AMNH. "I just, really hastily, with a ruler, gathered a bunch of measurements of the skulls while we were there," Mulcahy said. "I started playing around with statistics, and there are ways you can look at different measurements to see if they're best fit by a classic unimodal distribution, or if you were to think you're dealing with something that exhibits strong sexual dimorphism, you'd see something with a bimodal distribution—that there are two means, one for males and one for females." However, after analysis, the KU researcher wasn't seeing that distribution in the traits he'd measured in New York.
"It wasn't like there's one group of specimens that look bigger than another, and then there's another group that are smaller with smaller horns and canines," he said. "So I was kind of interested—are these really good examples of sexual dimorphism after all? Is there actual evidence in the morphology of these skulls, in the distribution in particular of the ranges of morphology we're seeing, that that's what's going on?" Mulcahy branched out, gathered more measurements from specimens at other institutions and used new statistical procedures.
In the end, his data set included about 30 skulls of one extinct species, Uintatherium anceps. "Maybe there were some little signals of dimorphism here and there, but nothing really concrete," he said. "I had an idea: Maybe it's just that the techniques I'm using aren't robust enough to actually measure sexual dimorphism.
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