Two Teeth Reveal a Rodent the Size of a Large Dog

A rodent that probably weighed about 30 kilograms, roughly the build of a large dog, lived in what is now southern Thailand about 34 million years ago, and two teeth are all that is left of it.

A study published on November 4, 2026 in Scientific Reports names the animal Yakamys sirindhornae. ^[2] The authors say it pushes evidence of very large rodents back into the late Eocene, well before the giant South American rodents that evolved millions of years later. ^[2]

Found in 2004, named in 2026

The teeth are not a fresh find. Geologist Pannipa Tian pulled the incisor from a coal mine in 2004, and she told AFP she first took it for a bone because it was so large. ^[1] Her supervisor identified it as the tooth of a giant rat, she said. ^[1] Another member of the expedition found the molar the same day. ^[1]

Then the fossils waited. According to AFP, the team lacked the evidence and the technology to identify the species at the time. ^[1] Researchers from Thailand, France and Canada made the identification more than two decades later. ^[1]

Both teeth came from a clay layer inside the main coal seam at Bang Mark Pit in the Krabi lignite mine. ^[2] The study dates the layer to between about 35.10 and 33.7 million years ago, using the other mammals found there and the reverse magnetic polarity locked in the rock. ^[2] The fossils now sit in the collections of Thailand's Department of Mineral Resources in Bangkok. ^[2]

The name carries the place. The genus joins the Thai word yak, meaning giant, to the Greek mys, meaning mouse or rat. ^[2] The species honours Princess Maha Chakri Sirindhorn for her support of science and paleontology in Thailand. ^[2]

How two teeth make a species

Teeth are the hardest thing an animal grows, and they survive when everything else rots. They also carry a great deal of information. Each cusp and ridge on a molar has a shape, and the pattern of those shapes works like a signature that differs from one lineage to the next.

The molar here is a right lower second molar, 11.56 millimetres long and 11.27 wide, just over a centimetre each way. ^[2] The researchers compared it with other members of a family of early rodents called ischyromyids. It stands apart through a combination of features: an isolated front cusp, an incomplete ridge and a divided crest, plus the absence of certain cusps found in its relatives. ^[2]

The molar is the holotype, the single specimen that formally anchors the species name. ^[2] The incisor, the long gnawing tooth at the front of the jaw, supplied a second line of evidence. The team examined the microscopic structure of its enamel. The enamel is built in bands of prisms, and the banding pattern here, called pauciserial, is consistent with Ischyromyidae, the family where the researchers place the animal. ^[2]

There is a wrinkle. The incisor is fragmentary, and nobody can show that the two teeth came from the same animal.

The researchers found them close together in the same layer and assign them to one species provisionally, on the strength of their shared large size and where they lay in the rock. ^[2] The incisor is the paratype, a supporting specimen. ^[2]

Weighing an animal that is mostly missing

The 30 kilograms is a calculation, not a measurement. The researchers ran seven published equations that relate tooth dimensions to body mass. ^[2] The answers ranged from 21.4 to 47.3 kilograms, with a mean of 31.6 and a median of 28.6. ^[2] The top of that range is more than double the bottom.

The study rounds the result to about 30 kilograms and says plainly that estimating mass from teeth carries substantial uncertainty. ^[2] Even the low end would make Yakamys far larger than the rodents most people know.

That is the point of the paper. The authors say the Asian fossil record for rodents is comparatively sparse and dominated by small species. ^[2] Against that background, a lineage with exceptionally large body size by the late Eocene is a surprise. ^[2] The animal also predates the major diversification of the giant caviomorph rodents of the Neogene in South America. ^[2]

What the teeth do not say

The teeth do not say what the animal looked like, what it ate or how it lived. ^[1] No skull or limb bones have turned up, so any picture of the living animal is imagination, not evidence.

They also do not show why it grew so large. The study gives the fossil's age and size but does not tie that size to a warmer climate, and it does not test any rule about how body size responds to heat. ^[1][2] The authors say further fossils and phylogenetic analyses, which map how species are related, are needed to clarify where Yakamys sits in the rodent family tree. ^[2]

Still, the find makes a plain argument about how little we know. One rodent of this size has now surfaced in an Asian record that had looked like a gallery of small animals. If it came out of a single mine that sat unidentified for 22 years, other large animals may be waiting in other drawers and other seams.

The Krabi basin has already yielded ancient primates, carnivores and hoofed mammals, according to AFP. ^[1]

The next test is simple to state. A skull, a jaw with both teeth in place, or a limb bone from the same layer would settle whether the molar and incisor belong together, and would begin to show what this giant actually was.