Study: Giant 66-Pound Rodent Once Roamed Southeast Asia

Fossil Teeth Show a Dog-Sized Rodent Lived in Ancient Thailand

A rodent weighing around 66 pounds, about as much as a large dog, lived near ancient lakes in what is now Thailand about 34 million years ago. That is millions of years before South America produced its giant rodents, some of which weighed hundreds of pounds. Paleontologists identified it from just two fossilized teeth, one of them broken, pulled from a coal mine in the country’s south.

Researchers named the new species Yakamys sirindhornae. It lived during a period called the late Eocene, which makes it one of the earliest known oversized rodents. The find pushes the history of giant rodents further back and into Asia. The work appeared in the journal Scientific Reports.

Two Teeth in a Thai Coal Mine Were Enough to Identify a Giant Rodent

Both fossils came from the Krabi Basin in southern Thailand, from a layer of clay inside a coal seam. A right lower molar (a back chewing tooth) and a lower incisor (a front gnawing tooth) lay close together. Because the pieces matched in size and sat in the same spot, the team tentatively linked them to one animal, though no skeleton was attached to confirm it.

Other mammal fossils from the site and magnetic patterns in the rock dated the layer to between 35.1 million and 33.7 million years ago.

Researchers measured the molar at nearly half an inch across. That sounds small, but a rodent called Hulgana, previously regarded as the largest Asian member of the same family, had molars less than half as long and wide. The family, known as ischyromyids, is one of the earliest rodent groups. The tooth also had a combination of ridges and bumps that set it apart from related rodents the team compared it with. That earned it a new species name instead of a spot under an existing one.

Giant Rodent Likely Weighed Around 66 Pounds, Based on Tooth Size

Estimating weight from teeth alone relies on formulas built from living rodents, which link tooth size to body weight. Different formulas give different answers. The team ran seven, using measurements from both teeth. Estimates ranged from about 47 pounds to 104 pounds, with an average near 70 pounds and a middle value near 63 pounds. Splitting the difference between those two figures and rounding produced the 66-pound number. The authors say that figure “should not be interpreted as a precise point estimate.”

Closest in size was Manitsha tanka, a South Dakota rodent that lived later, in the epoch after the Eocene. Its second molar measured roughly a third of an inch across, compared with nearly half an inch for Yakamys. The two animals’ molars also look quite different despite similar estimated weights, so big bodies in this family seem not to have come with one standard tooth design. How closely the two are related remains unsettled, since two fossils are too few for a full family-tree analysis.

Tooth Enamel Links the Giant Rodent to an Early Rodent Family

To pin down what kind of rodent this was, researchers sliced the incisor and studied its enamel under a microscope. Rodent incisor enamel forms layered bands, and the arrangement of those bands helps identify rodent groups. Yakamys showed a pattern consistent with ischyromyids, which first appeared in North America before spreading to Europe, Asia, and Africa during the Eocene.

Fossils from this family are common in North America and Europe, but Asia’s record is thin and mostly made up of small animals from China and India. A giant member in Thailand fills part of that gap.

Giant Rodent Fossil Shows Rodents Were Already Growing Large 34 Million Years Ago

South America’s largest documented rodents came millions of years later. One of them, Josephoartigasia monesi, was first estimated at more than 2,200 pounds, then revised to about 770 pounds. That revision shows how hard it is to weigh an animal from its remains, and the Yakamys team treated its own number with similar caution.

Giant bodies evolved only rarely in rodents, since most stayed small to medium-sized. Each early example helps scientists work out when and where big sizes first appeared.

An unpublished, unidentified rodent incisor from Myanmar, from slightly earlier in the Eocene, is bigger than other rodent incisors known from that region, according to the authors. Its owner remains unknown, so it cannot be tied to Yakamys or used to push the timeline back.

How Yakamys lived is guesswork. The fossils sat in clay deposited in an ancient lake, so the animal may have spent time at the water’s edge, somewhat like the modern capybara, South America’s water-loving giant rodent. The authors note, though, that the setting alone does not demonstrate a semi-aquatic lifestyle.

Whatever Yakamys actually weighed, its teeth show that big rodents lived in Southeast Asia long before South America’s largest ones, and the region’s sparse fossil beds may be hiding more. Pinning down its true size and family ties will take more complete fossils.

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