Fossil of 30 kg rodent from 35 million years ago discovered in Thailand

Researchers in Thailand have discovered a fossilized rodent weighing approximately 30 kilograms, dating back 35 million years to the Paleogene period. This colossal rodent, named Yakamys sirindhornae, was described in a paper published in the journal Scientific Reports by a team of scientists from Thailand, France, and Canada. The lead author, mammal paleontologist Yaowalak Chaimanee, revealed that Yakamys is the largest rodent yet found from this era, indicating that rodents had evolved to larger sizes around 15 to 20 million years earlier than previously thought.

The fossil, consisting of just two teeth and a fragment of a lower incisor, was unearthed at the Krabi lignite mine in Thailand's Krabi province. Researchers used seven equations to estimate the rodent's weight based on tooth size, arriving at a mean estimate of 31.6 kilograms, with substantial uncertainty. Despite its Thai nickname, Sirindhorn's Giant Rat, Yakamys is not a true rat but an ischyromyid, an early group of rodents.

Ischyromyids originated in North America but spread to Europe, Asia, and Africa during the Eocene epoch. While Asia's ischyromyid fossil record is sparse, particularly from China and India, Yakamys stands out as exceptionally large for its time. The genus name, Yakamys, is derived from the Thai word "yak," meaning giant, and the Greek "mys," meaning mouse or rat.

The species name honors Princess Maha Chakri Sirindhorn, a major supporter of scientific and paleontological research in Thailand. This find suggests that an early rodent lineage had already grown to large sizes by the late Eocene, well before giant rodents diversified in South America.