A new study finds that ancient rodents, previously thought to be the size of a bison, were in fact more like the size of a modern pony.
Modern rodents range in size from the pygmy rat weighing under an ounce to the stocky capybara weighing 80 kg. But even the largest capybara appear "dwarf" when compared to some prehistoric rodents that resemble a cross between a large capybara and a hairy hippopotamus.
Paleontologists estimate that one species, Phoberomys pattersoni, may have weighed about 500 kg, while Josephoartigasia monesi is believed to have weighed 900 kg and was as large as a bison. But predictions of this size have long sparked debate.
"People say this rodent is the size of a bison, but no one has a method that can confidently determine this size," said Russell Engelman, a paleontologist pursuing a Ph.D. from the University of Case Western Reserve, quoted from the New York Times.
Because of this, Engelman proposed a new method to accurately describe the dimensions of this unusual size rodent.
In a study published this week in the journal Royal Society Open Science, he downsized the animals by comparing the joints at the back of the skulls of Phoberomys, Josephoartigasia and other prehistoric rodents with large modern mammals, not their smaller relatives.
Between two million and 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 skull of Josephoartigasia in 2008, these giant rodents had a powerful bite that could produce three times the force of modern tiger bites, potentially protecting them from predators such as terror birds and marsupials. saber-toothed. Much of our understanding of these rodents has to do with their size.
"Body size is a major feature of mammals because everything you can't physically measure in fossils like ecology and physiology is correlated with body size," said Virginie Millien, a zoologist at McGill University who studies rodent body size and was not involved in the research. .
In 2010, Dr. Millien used femur fossils to estimate that Phoberomys was the size of a large antelope. Accurately measuring these giant rodents proved difficult. One reason is the lack of fossils.
While paleontologists have found leg bones and other parts of the Phoberomys skeleton, Josephoartigasia is only known from one skull. Without fossil evidence, researchers often rely on the anatomy of the closest extinct animal relatives.
However, features such as the elongated skull of Josephoartigasia and the large femur of Phoberomys were not found in living rodents. Thus, simply increasing the size of the capybara fails to make accurate anatomical estimates, and can result in similar 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 varies slightly in all mammals to ensure the skull and spine remain securely attached, making it a point of departure for comparing different species.
"Usually paleontologists look for traits that differ between animals. But when you look at body size, you want to know which part has changed the least," he said.
Recently, Engelman measured joint widths in more than 400 mammal species, including African rats and elephants. He found that the width of the occipital condyles was an accurate predictor of their dimensions. Because these joint widths were similar in mammals of a given size, he was able to compare the joint sizes of prehistoric mice with other large mammals without having to estimate.
This led to Engelman's estimate of a drastic reduction in size: Phoberomys maxed out at under 204 kg, and Josephoartigasia weighed about 453 kg, more like a pony than a bison.
"If I make every reasonable assumption that makes the masses taller, I still can't make them as big as people say they are. Even unreasonable assumptions can't make them that big," he said.
Engelman also believes that this decrease in muscle tone could increase the rodents' brains, which are very small for their perceived size. Meanwhile, Dr. Blanco believes these figures are more realistic than previously thought. But he believes more fossil evidence is needed before confirming how big the largest rodent grew.
