Rosa Rubicondior: Creationism Refuted - Now It's A Giant Rodent From Thailand

Yakamys sirindhornae

Image by Virginie Millien.

Giant, prehistoric rodent fossils discovered in southern Thailand | Newsroom - McGill University

Two fossil teeth from southern Thailand have added another chapter to the evolutionary history of rodents, set approximately 34 million years before Bible literalists imagine the world was created. Described in Scientific Reports, the newly named Yakamys sirindhornae was an exceptionally large member of an early rodent family. Its discovery also illustrates something creationist propaganda routinely misrepresents: scientists use evolutionary theory to interpret new evidence, revise earlier interpretations and identify questions that remain unanswered.

The fossils come from the Krabi Basin and comprise a lower molar and a fragmentary lower incisor, provisionally assigned to the same species because of their exceptional size and occurrence close together in the same fossil-bearing layer. The deposits date to approximately 35.1–33.7 million years ago, based on their mammalian assemblage and magnetic polarity. That places this animal in the late Eocene, tens of millions of years outside the few thousand years allowed by young-Earth creationism. Uncertainty about its weight does nothing to remove that chronological difficulty.

Comparisons of tooth structure and the microscopic organisation of the incisor enamel support its assignment to the extinct Ischyromyidae. Within that evolutionary framework, its large size becomes informative: substantial increases in body size had occurred in an early rodent lineage long before the better-known giant rodents of South America. The discovery broadens our understanding of when and where rodent gigantism evolved. It does not establish that all giant rodents descended from this species, nor does the limited material resolve its precise position within the family.

The much-publicised estimate of about 30 kilograms deserves equally careful treatment. Seven equations relating the dimensions of living rodents' teeth to body mass produced estimates ranging from 21.4 to 47.3 kilograms. The authors use approximately 30 kilograms as a rounded summary, explicitly acknowledging substantial uncertainty. These are estimates derived from comparative measurements, not a weighing of the extinct animal. Further discoveries could refine them, just as earlier estimates for some South American giant rodents have been revised.

This is science doing what it should: testing interpretations against evidence and distinguishing firm observations from provisional inferences. The fossils expand a previously incomplete picture of rodent evolution, while leaving questions about ancestry, ecology and the causes of large body size open to investigation. Evolutionary theory provides the framework in which those questions can be asked and tested; it does not require palaeontologists to pretend that two teeth reveal an animal's entire history.

There is little comfort here for creationism. Revising the history of gigantism does not undermine common ancestry, and acknowledging uncertainty about body mass supplies no evidence for supernatural creation. A scientific account can become more accurate when new fossils appear. A chronology fixed in advance by a literal reading of Genesis can only become more difficult to reconcile with them.

What can two fossil teeth tell us — and what can’t they? The newly described rodent Yakamys sirindhornae is known from a lower second molar and a fragmentary lower incisor. These were found close together in the same fossil-bearing layer and are provisionally assigned to the same species because of their compatible, exceptionally large dimensions. That does not establish that they came from the same individual.

Identifying evolutionary relationships

A tooth contains much more information than its size. The arrangement of cusps and ridges, the shape of its chewing surface and the microscopic structure of its enamel provide characters that palaeontologists can compare with those of other species. In Yakamys, the combination of molar features and incisor enamel structure supports membership of the extinct rodent family Ischyromyidae, while distinctive differences support recognition of a new genus and species.

Such comparisons are interpreted in an evolutionary framework: similarities may reflect inherited features from common ancestors, although some can evolve independently through convergence. The authors explicitly acknowledge that the available material cannot resolve the animal’s precise relationships within its family. More complete fossils and a broader analysis of anatomical characters are needed to distinguish competing possibilities.

Estimating body mass: the role of allometry

Allometry describes how measurements of an organism change in relation to its size. Researchers can use relationships measured in living rodents to construct equations linking tooth dimensions with body mass, then apply those equations to fossil teeth. These relationships contain variation, however: rodents with similar tooth dimensions do not necessarily have identical body masses.

For Yakamys, seven published equations produced the following results:

Individual model estimates: 21.4–47.3 kg.

Mean of the estimates: 31.6 kg.

Median of the estimates: 28.6 kg.

Rounded central summary: approximately 30 kg.

The familiar “30 kg” figure is therefore a convenient summary, not a precise measurement or the output of one preferred equation. Once the individual models’ prediction errors are included, their uncertainty ranges collectively extend from approximately 10.8 to 69.0 kg. The authors caution that this combined span is not a single statistical confidence interval. The different estimates nevertheless support the interpretation of an exceptionally large early rodent.

What remains unknown?

These teeth cannot establish the animal’s complete appearance, exact body proportions or detailed way of life. Their occurrence in lake deposits suggests a landscape containing lakes, but does not demonstrate that Yakamys was semi-aquatic. A lifestyle resembling that of a capybara is a possibility to investigate, not an established reconstruction. Nor do the fossils reveal exactly which ecological pressures favoured its large size.

Uncertainty about weight is not uncertainty about everything

The age of the deposits was assessed separately, using the associated mammal fauna and the record of Earth’s magnetic polarity preserved in the rocks. The paper assigns them to approximately 35.1–33.7 million years ago, in the late Eocene. Revising a body-mass estimate would not, by itself, alter that geological evidence.

This distinction is fundamental to scientific reasoning. Different conclusions rest on different evidence and carry different uncertainties. Acknowledging what two teeth cannot reveal makes the reconstruction more reliable; it does not erase what those teeth and their geological context can tell us.

The paper in Scientific Reports was also discussed in a McGill University news report:

Giant, prehistoric rodent fossils discovered in southern Thailand
The finding suggests massive rodents evolved in more places worldwide, and much earlier, than previously thought
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. 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.

Professor Virginie Millien, co-author
Department of Biology
McGill University
Montreal, QC, Canada.

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.

Professor Virginie Millien.

https://www.nature.com/articles/s41598-026-76711-6.pdf 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.

Professor Virginie Millien.

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.

Publication:
Chaimanee, Y., Marivaux, L., Millien, V. et al.
A giant eocene ischyromyid from Southeast Asia reveals the early evolution of rodent gigantism.
Sci Rep 16, 34523 (2026). https://doi.org/10.1038/s41598-026-76711-6

Download PDF
Abstract
Gigantism has evolved repeatedly in rodents, but the best-documented examples are predominantly known from Neogene caviomorphs of South America. Evidence for gigantism among early rodents is scarce, limiting our understanding of its evolutionary history. Here we describe Yakamys sirindhornae gen. et sp. nov., a giant ischyromyid from the late Eocene of the Krabi Basin, southern Thailand. The new taxon is represented by a lower second molar and an associated lower incisor that combine exceptionally large dimensions with a distinctive suite of dental characters, including a moderately high crown, a short, isolated anteroconid, an incomplete metalophulid II, a bifid entolophid, and the absence of a mesoconid and hypoconulid. Incisor enamel exhibits a pauciserial Hunter–Schreger band pattern consistent with Ischyromyidae enamel microstructure. Body mass estimates derived from seven allometric equations range from 21.4 to 47.3 kg, with a mean of 31.6 kg and a median of 28.6 kg. We use approximately 30 kg as a rounded summary of the central tendency of these estimates, while acknowledging the substantial uncertainty associated with dental allometry. This estimate indicates that Yakamys had a substantially greater body mass than previously described Paleogene ischyromyids. Its discovery provides evidence for exceptionally large body size in Eocene ischyromyids of Southeast Asia. It demonstrates that substantial body-size increases had occurred in an early rodent lineage by the late Eocene, well before the major diversification of giant Neogene caviomorphs in South America. This finding further highlights the importance of the Southeast Asian fossil record for understanding the early diversification and morphological evolution of rodents.

Geographic and stratigraphic setting of the Eocene Krabi Group, Peninsular Thailand. (A1) Regional location map showing the Krabi coal mine in Krabi Province, southern Thailand. (A2) Detailed map of the Krabi coal mine showing the location of Bang Mark Pit (red star), the fossil locality investigated in this study. Maps were originally prepared by the authors using Adobe Photoshop v.27.2.0.15 (https://www.adobe.com/products/photoshop.html). (B) Stratigraphic section of the Krabi Group, with the red star indicating the fossil-bearing horizon. (C) Photograph of Bang Mark Pit, Krabi coal mine; the red arrow indicates the fossil discovery site.

Lower second molar (m2) of Yakamys sirindhornae gen. et sp. nov. (holotype, TF 2942). (A) Stereopair in occlusal view; (B) mesial view; (C) half-buccal view; (D) buccal view; (E) half-distal view; (F) distal view; (G) half-lingual view; and (H) lingual view. Scale bar = 5 mm.

Pauciserial enamel microstructure of the lower incisor of Yakamys sirindhornae gen. et sp. nov. (TF 2943). The enamel is shown in longitudinal section at different magnifications. (A) Optical photograph of the incisor showing the area sampled for enamel microstructure analysis (red square). (B) Scanning electron micrographs showing the overall enamel thickness. (C,D) Scanning electron micrographs showing the thick Hunter–Schreger bands (HSBs), each composed of up to five prisms, and the inter-prismatic matrix (IPM), which forms a continuous anastomosing network surrounding the prisms and producing a sheath-like IPM. Abbreviations: EDJ, enamel–dentine junction; PE, portio externa; PI, portio interna.

Morphological and size comparisons of the lower molars of Yakamys sirindhornae gen. et sp. nov. from the late Eocene of the Krabi Basin, Thailand, with those of Paleogene ischyromyids of different sizes. (A) Yakamys sirindhornae gen. et sp. nov., right m2 (TF 2942); (B) Asiomys dawsoni, left m1 (IVPP V 17800.8; reversed), left m2 (V 17800.11; reversed), and right m3 (IVPP V 17800.16)18; (C) Hulgana ertnia, right m1 − m3 (AMNH 26085)29; (D) Eosischyromys youngi, right p4 − m2 (V 11376)19; (E) Hartenbergeromys hautefeuillei, left m2 (UM2-138; reversed) and left m3 (SLP-43-PR-119; reversed)16; (F) Manitsha tanka, right p4 − m3 (AMNH 39081)13. Scale bar = 2 mm.

Chaimanee, Y., Marivaux, L., Millien, V. et al.
A giant eocene ischyromyid from Southeast Asia reveals the early evolution of rodent gigantism.
Sci Rep 16, 34523 (2026). https://doi.org/10.1038/s41598-026-76711-6

Copyright: © 2026 The authors.
Published by Springer Nature Ltd. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

The significance of Yakamys sirindhornae extends beyond the striking image of a rodent weighing perhaps 30 kilograms. It adds evidence that substantial increases in body size had already occurred in an early rodent lineage by the late Eocene, long before the better-known South American giants. Evolutionary theory gives these observations their explanatory context: anatomical similarities help identify affinities, differences reveal diversification, and the geographical and geological distribution of fossils helps reconstruct the history of that diversification. The remaining questions concern the details of this history, including precisely where Yakamys belongs within its family and what favoured its large size.

Those questions also demonstrate how scientific understanding develops. An incomplete fossil record inevitably produces an incomplete account, and new discoveries can require that account to change. Here, scientists have expanded the known geographical and morphological diversity of early rodents while acknowledging the limits of their evidence. There is no requirement to defend an earlier interpretation merely because it was once accepted. The obligation is to explain the evidence as accurately as possible and make clear what further discoveries would help resolve.

The uncertainty surrounding body mass illustrates the same principle. Approximately 30 kilograms is a useful summary of several estimates, not a claim to have weighed an animal known from two teeth. Better material may change that figure or clarify the animal’s appearance and ecology. None of this supplies evidence for creationism. Uncertainty about one measurement does not invalidate unrelated evidence, and revising a weight estimate would do nothing to compress the late Eocene into the few thousand years demanded by young-Earth biblical literalism.

That chronological conflict remains stark. These fossils come from deposits dated to approximately 35.1–33.7 million years ago, placing this episode of rodent evolution tens of millions of years before the supposed creation of the world. The contrast is therefore between an account that can improve as evidence accumulates and a chronology whose conclusion has been fixed in advance. Science can acknowledge that two teeth leave much to discover. Biblical literalism still has to explain why those teeth occur in a world with a history it insists never happened.

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