Yakamys sirindhornae, named from the Thai word for giant, weighed about as much as a beaver and lived millions of years before South America's famous rodent giants

A rodent roughly the weight of a beaver lived in what is now southern Thailand about 34 million years ago, scientists report, making it the largest known rodent from the Paleogene period. The species, Yakamys sirindhornae, was identified from just two teeth found at a coal mine in Krabi Province, according to a study in Scientific Reports.

The find suggests rodents evolved large body sizes earlier than scientists thought, and that Southeast Asia played a bigger role in that story than expected. Most famous giant rodents, including the capybara and extinct relatives that grew far larger, are linked to South America and much later periods.

Thailand's natural resources ministry said the discovery shows rodents reached large sizes at least 15 to 20 million years earlier than previously believed, Khaosod English reported.

Two Teeth Found in a Krabi Coal Mine

The fossils, a lower molar and a lower incisor fragment about 2 inches (5 centimeters) long, were collected in 2004 at the Bang Mark Pit of the Krabi lignite mine. Geologist Pannipa Tian of Thailand's Department of Mineral Resources spotted the cream-colored incisor standing out against dark coal.

She first mistook it for bone because of its "unusually large size," she told AFP, as reported by The Standard. A colleague found the dark molar the same day. It took more than two decades, and collaboration with researchers in France and Canada, to identify the animal.

The genus name combines the Thai word "yak," meaning giant, with the Greek "mys," meaning mouse or rat. The species name honors Princess Maha Chakri Sirindhorn for her long support of science and paleontology in Thailand.

Estimating a 66-Pound Animal From Its Teeth

Teeth are often all that survive of ancient mammals, and their size tracks body size reasonably well across living species. The team applied seven established equations that relate tooth measurements to body mass.

The results ranged from about 21 to 47 kilograms (46 to 104 pounds), with a mean near 31.6 kilograms, according to the paper as quoted by IFLScience. The researchers settled on about 30 kilograms, or 66 pounds, roughly twice the mass of other rodents from the same era, including Hulgana.

McGill University biologist Virginie Millien, a co-author, cautioned that the figure is "only a ballpark," given the substantial uncertainty in estimating body mass from fossils, according to the ScienceDaily summary.

Rewriting the Early Timeline of Rodent Gigantism

For comparison, the largest living rodent, South America's capybara, can weigh about twice as much as the estimate for Yakamys. Some extinct South American rodents grew far larger still, but they lived tens of millions of years later. That timing gap is what makes a beaver-sized rodent in the late Eocene of Asia stand out.

Yakamys belonged to the Ischyromyidae, an extinct family that was one of the earliest major rodent groups and is better known from North America and Europe. Its size makes it the largest known member of that family.

Rodents are the most diverse group of mammals, but most are small — think mice, voles and squirrels. Truly large rodents evolved repeatedly later in the group's history, mostly in South America, Millien said. Finding a large rodent this early in Asia suggests body-size experiments began sooner and in more places.

The researchers suggest Yakamys may have lived along lake margins, a habitat consistent with the Krabi basin's coal deposits, which formed in ancient swamps. That basin has already yielded fossils of early primates, carnivores and hoofed mammals, making it one of Southeast Asia's most important windows into the late Eocene. The researchers compared Yakamys's likely habitat with that proposed for Manitsha tanka, a large Oligocene rodent described in 1941 from South Dakota, Gizmodo reported. The comparison hints that wet, food-rich lake edges may repeatedly have favored bigger rodents on different continents, though that idea remains a hypothesis.

The name also carries national significance. Yakamys sirindhornae is the fifth fossil species in Thailand to receive a royally granted name, and Thai officials have highlighted the find as evidence of the country's growing role in international paleontology.

Only Two Teeth, and Many Open Questions

The limits are clear and significant — with only two teeth, scientists cannot reconstruct the animal's appearance, diet, behavior, or exact relationships. A capybara-like artist's image released with the study is a reasonable guess, not evidence.

Body-mass estimates from teeth can mislead if an animal had unusually large or small teeth for its size. More fossils from Krabi or nearby basins would test whether Yakamys was an outlier or part of a wider group of large Asian rodents. The team notes the regional record may hold even older large rodents.

The fossils are stored at the Department of Mineral Resources in Bangkok. For readers, the find is a reminder that some of the most important clues to mammal evolution sit in places like coal mines, where industrial work and science overlap.

What Readers Want to Know

What is Yakamys sirindhornae? An extinct giant rodent that lived in southern Thailand about 34 million years ago.

How big was it? About 30 kilograms, or 66 pounds, roughly beaver-sized, though estimates range from about 21 to 47 kilograms.

What does the name mean? "Yak" is Thai for giant and "mys" is Greek for mouse or rat. The species honors Princess Sirindhorn.

How do scientists know its size? They applied seven equations linking tooth size to body mass in known rodents.

Why is the find important? It suggests rodents grew large millions of years earlier than thought, and in Asia, not just South America.

How certain are the findings? Not very. With only two teeth, size, diet and appearance remain uncertain.