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Modern rodents come in a variety of sizes, from dwarf mice that weigh less than an ounce to stocky capybaras that push 175 pounds. But even the largest capybara is a pipsqueak compared to some prehistoric rodents, which resemble a cross between an oversized capybara and a feathered hippo. Paleontologists estimate that one named Phoberomys pattersoni may have weighed 1,300 pounds. Another, Josephoartigasia monesi, was believed to weigh about 2,000 pounds, the size of a bison.

But these size estimates have long sparked controversy. “People said they were about the size of a bison, but no one had any method that could safely pin those dimensions,” said palaeontologist Russell Engelman, who goes on to Ph.D. From Case Western Reserve University.

That’s why Mr. Engelman proposed a new method for accurately describing their size. unusually sized rodents. In a study published Wednesday in the journal Royal Society Open ScienceHe downsized the animals by comparing the articulation at the back of the skulls of Phoberomys, Josephoartigasia, and other prehistoric rodents with those in large modern mammals rather than their smaller relatives.

Between two million and eight million years ago, giant rodents such as Phoberomys and Josephoartigasia lived in the wetlands of South America. According to paleontologist Ernesto Blanco, who discovered the Josephoartigasia skull at the Universidad de la República in Uruguay 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 such as the following. terror birds and saber-toothed marsupial.

Much of our understanding of these rodents depends on their size. “Body size is an important trait in mammals because everything you can’t physically measure in a fossil, like ecology and physiology, relates to body size,” said Virginie Millien, a zoologist at McGill University who studies rodent body sizes and was unrelated to this research. new work. In 2010, Dr. Millien, Fossilized femurs to estimate Phoberomys was the size of a large antelope.

It has been difficult to size these huge rodents accurately. One reason for this is the lack of fossils. Paleontologists have uncovered leg bones and other fragments of Phoberomys’ skeleton, while 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 long skull of Josephoartigasia and the voluminous thighbone of Phoberomys are not found in living rodents. Therefore, simply increasing the size of a capybara fails to make accurate anatomical predictions and may yield distorted dimensions similar to those seen in a carnival mirror.

So Mr. Engelman turned to the occipital condyle, the joint that helps connect an animal’s skull to its spine. The size of this joint varies little in all mammals to keep the skull and spine securely attached, making it a precursor for comparing different species. “Often, paleontologists look for features that differ between animals,” said Mr. Engelman, “but when you look at body size, you want to detect the parts that change the least.”

Recently, Mr. Engelman Measured joint width in over 400 mammalian speciesincluding mice and African elephants. He discovered that the width of the occipital condyle was an accurate indicator of their size. Because the width of these joints was similar in mammals of a given size, he could compare the size of the joints of prehistoric rodents with those of other large mammals without guessing.

This left Mr. Engelman in greatly reduced dimensions: Phoberomys dropped to just under 450 pounds, and Josephoartigasia weighed about 1,000 pounds—far closer to the size of a pony than a bison. “If I made every reasonable assumption I could to raise the masses, I still wouldn’t be able to make them as big as people say,” Engelman said. “Even illogical assumptions couldn’t make them this big.”

Mr. Engelman also believes this reduction in muscle strength could strengthen the brains of these rodents. stingy for perceived dimensions. “They have small brains but they may not have these ridiculously small brains that people think they have,” he said.

Dr. Blanco believes these numbers are more realistic than previous estimates of these one-metric ton rodents. But he believes more fossil evidence is needed before it can be definitively determined how big the largest rodents got. “Even with this excellent method, we will have significant uncertainties until we have more than a skull,” he said.

While the new findings are less remarkable than previous estimates, Dr. Millien said the 1,000 pounds was “still a really big mouse”.