Modern rodents range in size from harvest mice weighing less than an ounce to stocky capybaras weighing 175 pounds. But even the largest capybara is tiny compared to some prehistoric rodents that resemble a cross between an oversized capybara and a hairy hippopotamus. Paleontologists estimate that one, Phoberomys pattersoni, may have weighed up to 1,300 pounds. Another, Josephoartigasia monesi, was thought to weigh around 2,000 pounds, the size of a bison.
But these size predictions have sparked long debates. “People have said they were the size of bison, but nobody had methods to reliably determine those sizes,” said Russell Engelman, a paleontologist doing his Ph.D. from Case Western Reserve University.
So Mr. Engelman proposed a new method to accurately describe the dimensions of these unusually sized rodents. In a study published Wednesday in the Royal Society Open Science journal, he downsized the animals by comparing a joint on the back of the skulls of Phoberomys, Josephoartigasia and other prehistoric rodents to those of large modern mammals rather than their small relatives.
Two to eight million years ago, giant rodents such as Phoberomys and Josephoartigasia inhabited the wetlands of South America. According to Ernesto Blanco, a paleontologist at the Universidad de la República in Uruguay who discovered the Josephoartigasia skull in 2008, these giant rodents had a powerful bite that could generate three times the force of a modern tiger bite, potentially protecting them from predators like Terror birds and saber-toothed marsupials.
Much of our understanding of these rodents is related to their size. “Body size is a key feature in mammals because everything that you can’t physically measure in the fossil, like ecology and physiology, correlates with body size,” said Virginie Millien, a zoologist at McGill University who studies rodent body sizes and not involved in the new study. In 2010, Dr. Millien fossilized femurs to estimate that Phoberomys was the size of a large antelope.
Determining the exact size of these gigantic rodents has proven difficult. One reason is a lack of fossils. While paleontologists have unearthed leg bones and other parts of the skeleton of Phoberomys, Josephoartigasia is only known from 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 Josephoartigasia and the bulky femurs of Phoberomys are not found in living rodents. Therefore, simply increasing a capybara’s size does not provide accurate anatomical estimates and can result in skewed sizes similar to those seen in a carnival mirror.
So Mr. Engelman turned his attention to the occipital condyle, the joint that helps connect an animal’s skull to its spine. The size of this joint varies little across mammals to ensure the skull and spine remain securely connected, making it a reference point for comparing different species. “Typically, paleontologists look for traits that differ between animals,” Mr. Engelman said, “but when you look at body size, you want to pin down the parts that have changed the least.”
Recently, Mr. Engelman measured joint width in more than 400 species of mammals, including mice and African elephants. He discovered that the width of the occipital condyle was an accurate indicator of their dimensions. Because the width of these joints was similar in mammals of a given size, he was able to compare the size of the prehistoric rodents’ joints to those of other large mammals without having to extrapolate.
This left Mr. Engelman with drastically reduced sizes: Phoberomys peaked at under 450 pounds and Josephoartigasia weighed about 1,000 pounds — much closer to the size of a pony than a bison. “If I made every reasonable guess to make the masses taller, I still couldn’t make them as big as people say,” Mr. Engelman said. “Even unreasonable assumptions couldn’t make them that big.”
Engelman also believes this reduction in muscle strength may strengthen the brains of these rodents, which are puny for their perceived size. “They have small brains, but they might not have those ridiculously tiny brains that people think they have,” he said.
dr Blanco believes these numbers are more realistic than previous estimates of these rodents, which weigh a ton. But he believes more fossil evidence is needed before it is certain how large the largest rodents have become. “Even with this excellent method, we will have significant uncertainties until we have more than one skull,” he said.
While the new results are less striking than previous estimates, Dr. Millien that £1,000 is “still a really big rat”.