Two teeth found in a Thai coal mine reveal a giant ancient rodent that weighed almost seventy pounds

Two teeth from a Thai coal mine point to a new giant rodent of about 66 pounds that lived some 34 million years ago, long before South America's giants.

Two teeth lay in a clay band inside the main coal seam of the Krabi lignite mine in southern Thailand. A Thai-French expedition between Thailand's Department of Mineral Resources and French research institutions collected them, and they belong to a giant rodent weighing about 66 pounds (30 kilograms).

Researchers from Thailand, France and Canada have now named the animal Yakamys sirindhornae. A molar and an incisor are all they have, so that weight is a loose summary, not a measurement.

Clay inside the Krabi coal

The molar and the incisor turned up close together in the same fossil-bearing layer. The team links them provisionally to one animal because both are exceptionally large and come from the same horizon.

The clay band sits at Bang Mark Pit, part of the Krabi lignite mine, which the Electricity Generating Authority of Thailand manages and where it gave permission to work.

The Department of Mineral Resources holds them in Bangkok as TF 2942 and TF 2943. Mammal fossils from the pit and a magnetic reversal in the rock, Chron 13r, date the layer to the late Eocene. That means 35.1 to 33.7 million years ago.

A molar half an inch long

Yaowalak Chaimanee of CNRS and the University of Poitiers is first author of the paper. Five co-authors from Poitiers, Montpellier, McGill and Thailand's Department of Mineral Resources worked with her. They chose the molar as the holotype, the specimen that anchors the species name.

The tooth is 0.46 inch (11.56 millimeters) long and 0.44 inch (11.27 millimeters) wide. Its crown stands 0.31 inch (7.85 millimeters) high. Contact marks on two sides show that it had erupted and touched its neighbors.

R. Lebrun of the Montpellier evolution institute ISEM scanned it on an EasyTom 150 micro-CT machine, at 9.03 micrometers.

The model is headed to MorphoMuseuM, an open 3D archive. A lone tooth from the Hudson Bay region did similar work for woolly mammoths in Canada, moving their range farther east.

Hulgana had been considered the largest Asian member of the family. Its molars are less than half as long and wide. The nearest match is Manitsha tanka, an Oligocene rodent from South Dakota, with a molar of about 0.32 by 0.35 inch (8.2 by 8.9 millimeters).

Thin enamel, Thai name

The second tooth is a fragmentary left lower incisor, 0.44 inch (11.23 millimeters) across. The lab set it in epoxy, cut it lengthwise, polished it, etched the surface with phosphoric acid for 30 seconds and then coated it in palladium.

A Hitachi S-4000 scanning electron microscope then showed enamel about 235 micrometers thick, under 0.01 inch. Hunter-Schreger bands, sheets of enamel rods that cross each other, were up to five rods wide. Ischyromyidae, an extinct family of very early rodents, have that pattern.

Yak means giant in Thai, and mys means mouse or rat in Greek. Princess Maha Chakri Sirindhorn receives the species name for backing science and paleontology in Thailand.

Seven equations, one weight

Seven published equations based on living rodents gave the mass. Virginie Millien of McGill University, a co-author here, wrote the incisor equations with a colleague in 2010. Molar equations came from a 2024 paper by Boivin and colleagues.

Results ran from 47 to 104 pounds (21.4 to 47.3 kilograms). The mean was 70 pounds (31.6 kilograms), the median 63 pounds (28.6 kilograms).

The authors took the midpoint and rounded it to 66 pounds, a value they wrote "should not be interpreted as a precise point estimate."

Later giants in South America weighed far more, though their numbers have also been revised, with Josephoartigasia monesi first put above 2,200 pounds (1,000 kilograms) and then cut to about 770 pounds (350 kilograms).

Phoberomys pattersoni fell from as much as 1,634 pounds (741 kilograms) to about 617 pounds (280 kilograms) at most.

Older than Phoberomys

Age is the other half of the argument. The authors say an early rodent line had grown exceptionally large by the late Eocene. South America's giant caviomorphs, the group of the capybara, the largest rodent alive, diversified well after that.

Teeth from South China suggest that mammals grew larger before their teeth specialized. A tooth from Chile put a capybara relative in the Atacama 8.8 million years ago.

Most ischyromyids known from Asia were small and came from China and India, so Krabi adds Southeast Asia to the map.

The team also holds a very large unpublished rodent incisor from the Pondaung Formation of Myanmar, from the late middle Eocene. Nobody has identified it yet.

No skull, no skeleton yet

One molar and one incisor were too little for a formal family-tree analysis. The authors could not test whether similarities with the genus Hartenbergeromys reflect kinship or convergence.

Lake clay surrounded the teeth, but the sediment alone "does not demonstrate a semi-aquatic lifestyle," the paper says. A lakeside life like a capybara's stays only a possibility.

Size on islands, a common route to rodent gigantism, does not fit either, since nothing suggests Krabi was an island then.

More complete skull and limb material would be needed to test those ideas. Until it turns up, the species consists of one molar and a fragmentary incisor.

The full study was published in the journal Scientific Reports.

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