A newly described rodent from ancient Thailand may have weighed about 66 pounds, yet nearly everything scientists know about it comes from two teeth. Researchers named the animal Yakamys sirindhornae after analyzing a lower molar and incisor recovered from a coal mine in Krabi Province. The fossils date to the late Eocene, roughly 33 to 35 million years ago.
The discovery story is almost as strange as the animal. The teeth were found in 2004, but their size made them difficult to identify with confidence. Geologist Pannipa Tian told AFP that she initially thought the enormous incisor was a piece of bone. Another member of the expedition found the molar on the same day. Twenty-two years later, those two specimens became the evidence for a new genus and species.
The “giant rat” was not a modern rat
“Giant rat” is the wonderfully direct nickname used in reports about the discovery, but it should not be mistaken for a supersized version of the rats living in cities today. The new animal belonged to Ischyromyidae, an extinct family from one of the earliest major rodent radiations. Its family tree is much older and stranger than the familiar brown rat’s.
The peer-reviewed study in Scientific Reports, published November 4, describes a distinctive combination of features in the molar, including its moderately high crown and the arrangement of its ridges and cusps. Microscopic examination of the incisor enamel also revealed a pattern consistent with ischyromyids. Together, those details gave researchers more than size alone: they provided anatomical evidence that the fossils belonged to a previously unknown animal.
The name Yakamys sirindhornae recognizes Thailand’s Princess Maha Chakri Sirindhorn and her support for science, geology and paleontology. The fossils themselves remain with Thailand’s Department of Mineral Resources, according to Khaosod English’s report on the announcement.
How two teeth produced a 66-pound estimate
There is no complete skeleton, so 30 kilograms is not a measurement taken from the animal’s bones. The researchers applied seven equations that compare rodent tooth dimensions with body mass. Those models produced estimates from 21.4 to 47.3 kilograms. The mean was 31.6 kilograms, the median was 28.6, and the team used about 30 kilograms—roughly 66 pounds—as a convenient summary.
That number comes with a large warning label. When the prediction errors for the individual models are included, the possible ranges become much broader. The authors explicitly say that 30 kilograms should not be treated as a precise point estimate. Dental allometry is closer to reconstructing the size of a vehicle from its tires than placing the whole animal on a scale: informative, but dependent on comparisons and assumptions.
Even with that uncertainty, Yakamys was substantially larger than other known Paleogene ischyromyids. Its central estimate is around half the mass of a large capybara, the heaviest living rodent. More importantly, it pushes a major increase in rodent body size back into the Eocene, long before the better-known giant rodents that evolved in South America during the Neogene.
The teeth reveal a giant—and preserve the mystery
The study’s real payoff is not a monster-rat portrait. It is the realization that early rodents experimented with enormous size much earlier than the fossil record had securely shown. The discovery also expands the known range of ischyromyids into Southeast Asia, a region whose early rodent record is far less complete than those of North America and Europe.
The molar pictured here is the holotype, the specimen used to define the new species. Its multiple views show the ridges, worn surfaces and broken edges that researchers compared with other early rodents. The incisor supplied another line of evidence through its enamel microstructure. Two pieces that could fit in a hand ended up carrying information about anatomy, mass, geography and tens of millions of years of evolution.
They do not reveal everything. The paper says the fossils are insufficient to resolve Yakamys’ exact relationships within Ischyromyidae. Its appearance, diet and behavior also remain unknown. A moderately high-crowned molar may have resisted wear, but the researchers do not claim that it proves what the animal ate or explains why it became so large.
A 22-year scientific delayed reveal
That restraint makes the story better, not smaller. The fossils were not waiting for someone to invent a dramatic creature around them; they were waiting for comparisons, imaging and the right analytical framework. A tooth first mistaken for bone now helps document one of the largest rodents known from its era.
Phys.org’s November 5 report includes the researchers’ account of the 2004 discovery, while the Scientific Reports paper provides the measurements, anatomical diagnosis and uncertainty behind the headline number. The honest picture is both vivid and incomplete: a roughly 66-pound ancient rodent known from two remarkable teeth, with the rest of the animal still hiding in deep time.
Featured image: Multiple views of the lower second molar of Yakamys sirindhornae, holotype TF 2942. Credit: Chaimanee et al., Scientific Reports (2026), CC BY 4.0. Image used unchanged.
