Plate Nº 96 · recorded November 11, 2026
Biology & EvolutionReported finding
Two Teeth From a Thai Coal Mine Rewrite Rodent Giant History
A single 11.6 mm molar from a Thai coal mine reveals a 30 kg Eocene rodent, pushing rodent gigantism back to 35 million years ago in Southeast Asia.
By James Calloway4 min read751 words
In brief
- New rodent species Yakamys sirindhornae dated to 35–34 million years ago (Late Eocene)
- Estimated body mass of ~30 kg, based on a single 11.6 mm lower molar and incisor fragment
- Fossils found in a clay layer within a coal seam at the Krabi lignite mine, Thailand
- Incisor enamel measured only about 235 micrometers thick despite the tooth's size
- South America's later giant rodent Josephoartigasia monesi was estimated at about 350 kg
A single molar roughly 11.6 mm long has pushed the story of giant rodents back in time by tens of millions of years — and moved it halfway around the globe. Paleontologists working in southern Thailand have described a new Eocene species, Yakamys sirindhornae, estimated at about 30 kg. That's the weight of a large dog, from an era when rodents were supposedly still small-timers.
The animal is known from exactly two fossils: one lower molar and a fragment of a lower incisor. Both came from a clay layer inside a coal seam at the Krabi lignite mine, in Thailand's Krabi province. That's the entire evidence base. Keep that in mind as the claims get bigger.
How big are we talking?
The molar measures about 11.6 mm long and 11.3 mm wide. The largest previously known Asian relatives of this animal had molars less than half that size, so this tooth genuinely stands out in the fossil record of the region.
From that lone tooth, the team estimated a body mass of roughly 30 kg. Professor Virginie Millien of McGill University, co-author of the paper published in Scientific Reports, was quick to flag the limits of that number. "This is only a ballpark figure, as there is substantial uncertainty associated with fossil body-mass estimation," she said. Tooth-to-body-mass equations carry wide error bars. Thirty kilograms is a starting point, not a verdict.
Where does it fit on the family tree?
The researchers placed the new species within the Ischyromyidae, an early rodent family. They based that assignment on two lines of evidence: the shape of the teeth and the microscopic structure of the incisor's enamel.
Here's the odd detail. Despite the incisor's impressive size, its enamel is remarkably thin — about 235 micrometers. Thin enamel on a big tooth is not what you'd casually expect, and the paper doesn't pretend the anatomy is fully explained by two fragments.
Before this find, one of the few documented examples of a very large early rodent was Manitsha tanka, an ischyromyid from the Oligocene of South Dakota. The researchers estimate the two animals had similarly large body masses. Yet their molars differ markedly in shape and size. The practical implication: large ischyromyids did not share a single tooth design. Gigantism, in other words, kept reinventing the hardware.
What does this change for rodent history?
Plenty, potentially. When people picture a giant rodent, they think of the capybara, or of the enormous extinct rodents that roamed South America within the last 10 million years. The real monster is Josephoartigasia monesi, a Plio-Pleistocene South American species estimated in later studies at about 350 kg.
The Thai find suggests big body size evolved in rodents well before those South American giants appeared. "Today, the largest living rodent is the capybara, but very large rodents are not restricted to recent ecosystems," Professor Millien said. "Giant rodents evolved repeatedly during the later history of the group, mostly in South America. Our discovery suggests that Southeast Asia also played an important role in the early evolutionary history of large-bodied rodents."
So the map of early rodent gigantism now has a Southeast Asian pin in it, dated between about 35 and 34 million years ago, in the Late Eocene. That's tens of millions of years before Josephoartigasia.
What don't we know?
A lot. Two teeth cannot tell you how an animal lived, moved, or related to its relatives. "Because the fossil material is limited, it is difficult to determine exactly how Yakamys sirindhornae evolved," Professor Millien said. "Further discoveries will be important for understanding the animal's biology, ecology and evolutionary history."
That's an honest assessment, and it's the right one. The species name and the family placement rest on dental anatomy alone, which is standard practice for fossil rodents but still a narrow foundation. The body-mass figure inherits the uncertainty of its estimation method. And a single site in Krabi province is a thin sample of an entire continent's Eocene fauna.
What would settle things? More teeth, ideally jaws, ideally from more than one locality. Until then, Yakamys sirindhornae stands as a suggestive data point — an 11.6 mm molar suggesting Southeast Asia was experimenting with rodent gigantism long before South America made it famous.
The paper, by Y. Chaimanee and colleagues, appears in Scientific Reports (2026), volume 16, article 34523, doi: 10.1038/s41598-026-76711-6.
via sci.news (Source)
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Staff writer covering regulation, policy and their impact on general science news at SciBeat.
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