Modern rodents range in size from pygmy mice weighing less than 30 grams to strong capybaras that carry 80 kilograms. But even the biggest capybara is a flea compared to some prehistoric rodents that looked like a cross between a giant capybara and a hairy hippopotamus. Paleontologists estimate that one of them, the Phoberomys pattersoni, could weigh up to 600 kilograms. It is believed that another Josephoartigasia monesiweighed about 900 kilograms, as big as a bison.
But these size predictions have long provoked debate. “Some people said they were the size of a bison, but no one had methods that could reliably determine those sizes,” said Russell Engelman, a paleontologist who is a doctoral student at Case Western Reserve University in Cleveland.
Engelman then proposed a new method to accurately describe the dimensions of these unusually sized rodents. In a study published in the journal Royal Society Open Science, he scaled down the animals by comparing a joint in the back of the skulls of Phoberomys, Josephoartigasia and other prehistoric rodents with those of large modern mammals rather than their tiny relatives.
Between 2 million and 8 million years ago, giant rodents like Phoberomys and Josephoartigasia inhabited the swamps of South America. According to Ernesto Blanco, a paleontologist at the University of the Republic in Uruguay, who discovered the skull of the Josephoartigasia in 2008, these giant rodents had a powerful bite capable of generating three times the force of a modern tiger bite, potentially protecting them from predators like terror birds (Phorusrhacidae) and saber-toothed marsupials.
Much of our understanding of these rodents is tied to their size.
“Body size is a key feature in mammals, because everything you can’t physically measure in the fossil, like ecology and physiology, is correlated with body size,” said Virginie Millien, a zoologist at McGill University in Montreal ( Canada), which studies the body size of rodents and was not involved in the new study. In 2010, Millien used fossilized femurs to calculate that the Phoberomys it was the size of a large antelope.
Accurately sizing these gigantic rodents proved difficult. One of the reasons is the lack of fossils. While paleontologists have unearthed leg bones and other pieces of the skeleton of PhoberomysO Josephoartigasia is known for a single skull. Without fossil evidence, researchers often rely on the anatomy of an extinct animal’s closest living relatives.
However, features such as the elongated skull of the Josephoartigasia and the bulky femurs of Phoberomys are not found in live rodents. Thus, simply increasing the size of a capybara does not provide accurate anatomical estimates and can produce distorted sizes.
So Engelman turned to the occipital condyle, the joint that helps connect an animal’s skull to its spine. The size of this joint, which ensures that the skull and spine remain firmly attached, varies little among all mammals, making it a guide for comparing species. “Generally, paleontologists look for traits that are different between animals,” Engelman said, “but when you look at body size you want to identify the parts that have changed the least.”
Recently, Engelman measured joint width in more than 400 species of mammals, including mice and African elephants. He found that the width of the occipital condyle gave an accurate estimate of the animal’s dimensions. Because the width of these joints was similar in mammals of a specific size, he was able to compare the size of the joints of prehistoric rodents with that of other large mammals without needing to extrapolate.
This left Engelman with drastically reduced sizes: the Phoberomys reached less than 204 kilos, and the Josephoartigasia it weighed about 450 kg—much closer to the size of a pony than a bison. “If I made all reasonable assumptions to make the masses bigger, I still couldn’t make them as big as people said,” Engelman said. “Even irrational assumptions couldn’t make them that big.”
Engelman also says he believes that this decrease in physical size can promote the brains of these rodents, which are tiny for their perceived size. “They have small brains, but maybe they’re not as ridiculously small as people believed,” he said.
Blanco believes these numbers are more realistic than previous estimates of these rodents that weigh a ton. But he believes more fossil evidence is needed to be sure of the size of the largest rodents. “Even with this excellent method, we will have significant uncertainties until we have more than one skull,” he said.
While the new findings are less surprising than previous estimates, Millien said 450 kilograms “is still a pretty big rat.”
