“This is only a ballpark figure, as there is substantial uncertainty associated with fossil body-mass estimation,” said Virginie Millien, a biology professor at McGill University and a co-author of the study.
An international team of researchers has discovered the fossil remains of a new genus and species of prehistoric giant rodent in southern Thailand, dating back approximately 33 to 35 million years. Named Yakamys sirindhornae, or the “Sirindhorn giant rat” (Yakamys sirindhornae) in Thai reports, the animal is estimated to have weighed about 30 kilograms, making it the largest rodent ever discovered in the Paleogene period.
The discovery was announced on Nov. 4, 2026, by Suchart Chomklin, Thailand’s Minister of Natural Resources and Environment, with the research findings published the same day in the international academic journal Scientific Reports.
According to the Ministry of Natural Resources and Environment, the species name “sirindhornae” was granted royal permission to honor Princess Maha Chakri Sirindhorn for her ongoing support of science, geology, and paleontology in Thailand. This marks the fifth fossil species to receive her royal name.
The fossils were recovered from the Bang Mark Pit at the Krabi lignite mine in Krabi Province by an ongoing collaborative project investigating ancient mammals in the Krabi Basin. The international research team includes scientists from McGill University, Thailand’s Department of Mineral Resources, and French research organizations CNRS and Université de Poitiers, along with contributions from researchers in Canada and Université de Montpellier.
The physical evidence examined consists of only two teeth: a lower second molar and a lower incisor found in the mine’s sediment layers. Led by Dr. Yaowalak Chaimanee, a mammalian paleontology expert from the French National Centre for Scientific Research (CNRS), the researchers analyzed the specimens using advanced scientific techniques.
Because only two teeth have been recovered, researchers said that available details regarding the animal’s appearance, lifestyle, and evolutionary relationships are limited, and additional fossils will be required to develop a comprehensive understanding. To calculate the animal’s mass, the team applied seven established methods based on tooth measurements, placing the rodent at approximately twice the weight of other known contemporary rodents from the same era, such as Hulgana.
Despite the calculated 30-kilogram mass, Millien cautioned that the weight remains a rough approximation due to the inherent limitations of calculating body mass from fossil teeth.
The researchers said that the discovery provides evidence that unusually large rodents evolved in Southeast Asia earlier than previously recognized, pushing back the timeline for rodent gigantism. According to the study, the massive body size indicates that rodents evolved larger bodies 15 to 20 million years earlier than previous scientific assumptions suggested, broadening the known geography of early gigantism outside of South America, where giant rodents predominantly evolved later in history.
The fossils are currently stored safely at Thailand’s Department of Mineral Resources. Government and academic representatives indicated that further fossil discoveries in the region will help clarify how Yakamys sirindhornae lived and how it relates to large rodents from other parts of the world.