LAWRENCE — 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, 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 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 dataset 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. With fossil animals, you don’t actually know which specimens are males and which are females. You can guess and then test whether or not the ones you guess are males differ from the ones you guess are females, but that’s super circular, because you could just be picking the most extreme ones to be one sex and the least extreme to be the other. Of course they’re going to differ.”
So, the KU researcher decided to use the same statistical techniques he was using on uintatheres on modern-day bison, as he could be sure of the sex of each specimen.
“When you’re dealing with living animals, you can just take the males, take the females and compare them,” he said. “Bison are another really big herbivore, with a similar sort of ecology that’s been proposed for uintatheres, and they also have big horns. What I noticed is that the signals of dimorphism I was picking up in bison were a lot clearer. 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 mistaken assumptions about sexual dimorphism could have roots in the cultural biases of the late 19 th and early 20 th century, when so many fossil species were first described.
“Back then, this is right when evolutionary theory, sexual selection, natural selection, all of that is kind of coming into play,” he said. “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. 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.”
The KU researcher said a weak signal for dimorphism in uintatheres, long a flagship for dimorphism, suggests other species in the fossil record might also be less dimorphic than paleontologists have held.
“Strong, big male-style sexual dimorphism was probably not ancestral at the root of the mammalian tree, and among the mammals where we do see this pattern today, it’s almost certainly shown up independently several times,” Mulcahy said. “It had long been assumed that sexual dimorphism is the norm, that most species show that males are bigger than females. But just a couple years ago, a paper came out showing that among living mammals, most species either don’t show any dimorphism or actually have 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.”
PLOS One