Two Krabi teeth point to a 30 kg Thai rodent at 34 million years ago — large then, smaller than a capybara

An international team including Virginie Millien of McGill University reports Yakamys sirindhornae from the Bang Mark Pit at the Krabi lignite mine in southern Thailand. The material is a lower second molar and a lower incisor, dated to about 34 million years ago, latest Eocene rather than a vague middle Paleogene. Seven tooth-to-mass equations produced a ballpark weight near 30 kilograms, or 66 pounds. The name pairs Thai yak, meaning giant, with Greek mys, and honors Princess Maha Chakri Sirindhorn.

That mass is not a weighed animal. It is an allometric extrapolation from two isolated teeth, and Millien correctly calls it a ballpark with substantial uncertainty. Scaling equations fitted mostly on living rodents can fail when tooth shape, diet, or clade differ from the calibration set, and a single molar cannot show whether the animal was robust or merely large-toothed. No limb bones, skull, or jaw are described in the release, so locomotion, posture, and even a secure family placement remain open.

Coverage that calls this a challenge to mammal evolution oversells a mass smaller than a typical capybara, often 35 to 66 kilograms, and far below Josephoartigasia monesi, the South American giant described by Rinderknecht and Blanco in 2008. Thirty kilograms is about twice some same-age rodents the team cites, including Hulgana. Gigantism evolved repeatedly, mostly later in South America. The sharper point is place and time: a large rodent in Southeast Asia at the Eocene-Oligocene boundary, a global cooling that reshuffled mammal faunas. Krabi’s lignite records swamp forest, but teeth alone do not show a capybara-like life.

What the release underplays is sampling. Paleogene mammals in Southeast Asia are known from few productive basins; Krabi has long been worked by Thai, French, and Canadian teams, so a new large rodent there may reflect where people dig, not where giants first evolved. A single species cannot show that the region was an early center of rodent gigantism. Stronger evidence would be a jaw with several teeth, any postcrania, an explicit confidence interval on mass, and a phylogeny tested against other Asian hystricognaths. Until then, Yakamys is a provocative data point, not a rewritten tree.