Two fossilized teeth pulled from a coal mine in southern Thailand more than two decades ago have been identified as belonging to a 66-pound rodent that lived roughly 34 million years ago — the largest rodent yet found from the Paleogene period, and evidence that giant rodents evolved millions of years earlier than scientists had assumed.
The animal, newly named Yakamys sirindhornae, is known only from a molar and an incisor recovered from the Krabi coal mine in 2004. A geologist with Thailand's Department of Mineral Resources initially mistook the incisor for an ordinary bone fragment; researchers only confirmed the fossils belonged to a previously unknown giant rodent after more than 20 years of analysis, publishing their findings in Scientific Reports on November 4.
Bigger than anything else from its era
At an estimated 30 kilograms — roughly half the weight of a large modern capybara — Yakamys was the biggest rodent known from the Paleogene, the roughly 43-million-year span that followed the extinction of the dinosaurs. It belonged to the extinct Ischyromyidae family, unrelated to modern rats, mice or capybaras, and most likely lived in the freshwater swamps that covered what is now southern Thailand between 33 and 35 million years ago, according to reporting on the study.
The discovery pushes back the timeline for when rodents first evolved large body sizes by an estimated 15 to 20 million years, according to the research team, suggesting the evolutionary pressures that produce giant rodents — likely limited predation and abundant vegetation in wetland habitats — took hold far earlier than the fossil record had previously shown.
Much about the animal remains unknown. With only two teeth to work from, researchers have not been able to reconstruct its appearance, diet or behavior in detail, and coverage of the find notes paleontologists hope further excavation at the Krabi site could turn up more complete remains. The genus name combines the Thai word for "giant" with the Greek root for "mouse"; the species name honors Thailand's Princess Maha Chakri Sirindhorn for her support of paleontological research in the country.