A giant rodent weighing around 66 pounds roamed what is now southern Thailand 33 to 35 million years ago, scientists said, after identifying a previously unknown species from two fossilized teeth.
The ancient animal, which one geologist on the team called a “giant rat,” was among the largest known from the Paleogene period, offering new clues about how early rodents evolved to enormous sizes.
The findings, published Wednesday in the journal Scientific Reports, were based on a molar and an incisor discovered in 2004 at a coal mine in southern Krabi province.
Pannipa Tian, a geologist with Thailand’s Department of Mineral Resources, who found the incisor, said she initially misidentified the fossil because of its “unusually large size.”
“At first, I thought it was a bone. I showed it to my supervisor, who told me it was a giant rat’s tooth,” Pannipa, 51, told AFP on Thursday.
The cream-colored fossil stood out against the dark coal deposits, while another expedition member found the dark-colored molar the same day.
The Krabi basin has previously yielded fossils of ancient primates, carnivores and hoofed mammals.
But at the time of the discovery, the team lacked sufficient evidence and technology to identify the species.
More than two decades later, researchers from Thailand, France and Canada were able to do so through detailed analysis of the teeth.
The animal weighed around 30 kilograms — an estimated half as much as a large capybara, the world’s largest living rodent, they said.
Scientific Reports / Chaimanee et al 
Unlike the South American capybara, however, the prehistoric rodent belonged to the now-extinct Ischyromyidae family, one of the earliest major groups of rodents.
“It tells a lot about the diversity of life in the past,” Pannipa said, suggesting ancient rodents may have evolved in what is now Thailand before spreading to other parts of the world.
But with only two teeth recovered, the animal’s appearance, diet and behavior remain unknown.
The species was named Yakamys sirindhornae in honor of Thailand’s Princess Sirindhorn, a supporter of scientific and paleontological research.
While the new species may seem massive, other discoveries around the world have revealed even bigger rodents. In 2020, scientists announced the discovery of a prehistoric human-sized rat that lived in the Amazon rainforest 10 million years ago.
In 2008, Uruguayan scientists said they had uncovered fossil evidence of the biggest species of rodent ever found, a 1-ton creature that scurried across wooded areas of South America about 4 million years ago.
In 2003, researchers said they had found fossils for a 1,545-pound “Rat-zilla” that thrived millions of years ago in a swampy South American forest.
