Two teeth found in southern Thailand reveal Yakamys sirindhornae, a rodent that lived about 34–35 million years ago. Researchers estimate it weighed 21.4–47.3 kg, though the estimate is uncertain.
Researchers have identified a giant ancient rodent from two fossil teeth found in southern Thailand. The newly named Yakamys sirindhornae lived about 34–35 million years ago and belonged to an early group of rodents, not the rat family.
Asia’s known fossils from that early group are scarce and mostly come from small animals, particularly from China and India. The new find suggests that one lineage had grown large by the late Eocene, before giant rodents diversified in South America. Researchers said it points to rodents evolving toward larger sizes 15–20 million years earlier than previously understood.
The fossils are a lower second molar and part of a lower incisor, found close together in clay within the main coal seam at the Bang Mark pit of the Krabi lignite mine. To estimate the animal’s mass, the team used seven equations that relate tooth size to body weight. The results ranged from 21.4 to 47.3 kg, with a mean of 31.6 kg; the authors rounded the estimate to about 30 kg and acknowledged substantial uncertainty.
The researchers assigned Yakamys to the extinct Ischyromyidae, one of the earliest major rodent groups. Hulgana, previously considered the largest Asian member of that group, had molars less than half the length and width of those of Yakamys. The closest tooth-size match among previously described members was Manitsha tanka, which lived later, in the Oligocene, in what is now South Dakota.
The teeth were found in 2004, but the team then lacked enough evidence and technology to identify the species, AFP said. Pannipa Tian, a geologist at Thailand’s Department of Mineral Resources who found the incisor, said she initially mistook it for bone because of its unusually large size. Another expedition member found the molar the same day. Detailed analysis of the teeth later allowed the researchers to identify the species.
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The study in Scientific Reports was led by Yaowalak Chaimanee, a mammal paleontologist at France’s National Center for Scientific Research, with researchers from Thailand’s Department of Mineral Resources and colleagues in France and Canada. Thailand’s natural resources and environment minister, Suchart Chomklin, announced the find on 4 November.
The animal’s appearance, diet and behavior remain unknown from the two teeth. The specimens are kept at the Department of Mineral Resources in Bangkok.
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