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| Yakamys sirindhornae Chaimanee, Marivaux, Millien, Yamee, Tian & Jaeger, 2026 |
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.
Keywords: Rodentia, Ischyromyidae, Gigantism, Body mass, Incisor enamel microstructure, Krabi, Thailand
Systematic palaeontology
Class Mammalia Linnaeus, 175821.
Order Rodentia Bowdich, 182122.
Clade Ischyromyiformes Marivaux et al., 200423.
Family Ischyromyidae Alston, 187624.
Subfamily indet.
Yakamys gen. nov.
Etymology: The generic name is derived from the Thai yak, meaning “giant”, and the Greek mys, meaning “mouse” or “rat”. The species epithet honors HRH Princess Maha Chakri Sirindhorn for her longstanding support of scientific and paleontological research in Thailand.
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| 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. |
Yakamys sirindhornae gen. et sp. nov.
“หนูยักษ์สิรินธร”
Holotype: Right lower second molar (m2; TF 2942) (Fig. 2).
Diagnosis: Large-bodied ischyromyid characterized by a moderately high-crowned lower m2 measuring 11.56 mm in mesiodistal length and 11.27 mm in buccolingual width, with a subquadrate occlusal outline, a large incompletely enclosed trigonid, a short isolated anteroconid lacking an anterolophid, a short transversely oriented metalophulid II terminating before the metaconid, a strongly curved ectolophid lacking a mesoconid, a shallow posteriorly directed hypoflexid, a bifid entolophid, and a posterolophid lacking a hypoconulid.
Yaowalak Chaimanee, Laurent Marivaux, Virginie Millien, Chotima Yamee, Pannipa Tian and Jean-Jacques Jaeger. 2026. A giant Eocene ischyromyid from Southeast Asia reveals the early Evolution of Rodent Gigantism. Scientific Reports. 16: 34523. DOI: doi.org/10.1038/s41598-026-76711-6 [04 November 2026]
