Fossil Teeth From a Thai Coal Mine Reveal a 66-Pound Rodent That Lived 34 Million Years Ago
Yakamys sirindhornae, about twice the size of its known rivals, suggests giant rodents evolved earlier, and in Southeast Asia, than scientists believed.
Paleontologists have identified a rodent that weighed roughly 66 pounds and lived in what is now Thailand about 34 million years ago. The animal, named Yakamys sirindhornae, is about twice as heavy as any other known rodent of its time, and its discovery pushes back the story of rodent gigantism.
The evidence is thin but striking: two "remarkably large" fossil teeth, a lower second molar and a lower incisor. They were recovered from the Bang Mark Pit at the Krabi lignite mine in Thailand's Krabi Province by an international team that includes scientists from McGill University in Canada, Thailand's Department of Mineral Resources, and French research groups including CNRS and the Université de Poitiers.
The name blends "yak," the Thai word for giant, with "mys," the Greek word for mouse or rat.
To estimate size, the researchers applied seven established methods that convert tooth measurements into body mass. The results put Yakamys at about 30 kilograms, or 66 pounds, roughly double the mass of contemporaries such as Hulgana. The team stresses that extrapolating weight from a couple of teeth carries considerable uncertainty.
"Today, the largest living rodent is the capybara, but very large rodents are not restricted to recent ecosystems," said Virginie Millien, a biology professor at McGill and co-author of the study. "Giant rodents evolved repeatedly during the later history of the group, mostly in South America. This discovery suggests that Southeast Asia also played an important role in the early evolutionary history of large-bodied rodents."
The find dates to the middle of the Paleogene period. Until now, evidence of very large rodents came mostly from later periods and from South America. A giant in Thailand at 34 million years ago means the trait appeared earlier and in more parts of the world than previously believed.
The limits are real. With only two teeth, the team cannot say how Yakamys looked, how it lived or exactly how it relates to other rodents. The researchers say more fossils will be needed to work out its physical characteristics, its role in its ecosystem and its family tree.
The fossils were identified through an ongoing collaboration investigating ancient mammals in Thailand's Krabi Basin.
Future finds from the basin could show how these giants lived and whether they connect to the enormous rodents that later appeared elsewhere in the world.
Rodent gigantism, the evolutionary process by which rodents develop unusually large bodies, has mostly been studied in South American species. Even two teeth offer valuable clues about it, the researchers say, and they highlight the role Southeast Asia played in the early history of large-bodied rodents. For now, the capybara remains the largest rodent alive,.
