Giant, prehistoric rodent fossils discovered in southern Thailand

An international team of paleontologists, including researchers from McGill University, has discovered “remarkably large” rodent fossils in the pit of a mine in southern Thailand. The finding provides a new perspective on the early manifestation of rodent gigantism – the process by which animals evolve to become larger – demonstrating that the evolutionary history of large-bodied rodents is considerably older and more geographically widespread than previously recognized, the researchers said. 

The new species, Yakamys sirindhornae, lived approximately 34 million years ago, during the mid-Paleogene period. The researchers discovered fossils of its teeth – specifically, the lower second molar and lower incisor – at the Bang Mark Pit of the Krabi lignite mine in Krabi Province.  

“Today, the largest living rodent is the capybara, but very large rodents are not restricted to recent ecosystems,” said Virginie Millien, study co-author and Professor of Biology at McGill University. “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 new genus name, Yakamys, combines the Thai word “yak,” meaning giant, with the Greek “mys,” meaning mouse or rat.  

“The species name, sirindhornae, honours Princess Maha Chakri Sirindhorn, in recognition of her longstanding support for science and paleontological research in Thailand,” Millien said.

A rodent far larger than its contemporaries 

The fossils were discovered as part of a collaborative research project on fossil mammals in Thailand’s Krabi Basin. The project involves researchers from McGill University, the Department of Mineral Resources of Thailand and French research institutions, including CNRS and Université de Poitiers. 

Using seven established approaches for calculating body mass from dental dimensions, the researchers estimate that the Yakamys weighed approximately 30 kilograms – about twice the size of other known rodents from this era, such as the Hulgana. 

“This is only a ballpark figure, as there is substantial uncertainty associated with fossil body-mass estimation,” Millien said. 

Because the fossil material is limited, the researchers also noted that it is difficult to determine exactly how the Yakamys evolved. Further discoveries will be important for understanding the animal’s biology, ecology and evolutionary history, they said. 

About the research 

“A giant Eocene ischyromyid from Southeast Asia reveals the early evolution of rodent gigantism,” by Yaowalak Chaimanee, Laurent Marivaux, Virginie Millien, Chotima Yamee, Pannipa Tian and Jean-Jacques Jaeger, was published in Scientific Reports. 

The research was supported by CNRS, Université de Poitiers, Université de Montpellier, McGill University and the Department of Mineral Resources of Thailand.