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| Kryoryctes cadburyi Pridmore, Rich, Vickers-Rich & Gambaryan, 2005 in Hand, Wilson, López-Aguirre, Houssaye, Archer, ... et Beck, 2025. Artwork by Peter Schouten. |
Significance
The egg-laying monotremes have played a central role in our understanding of mammalian evolution, but their fossil record is poor and their evolutionary history is controversial. Living monotremes are ecologically very distinct from each other: The platypus is well adapted for a semiaquatic lifestyle, whereas echidnas are fully terrestrial. Here, we show that an isolated mammal humerus from the Early Cretaceous of Australia, from a species called Kryoryctes cadburyi, belongs to a monotreme, and that microscopic features of this bone indicate that this monotreme was a semiaquatic burrower. This suggests that the amphibious lifestyle of the modern platypus had its origins at least 100 Mya, during the Age of Dinosaurs, and that echidnas evolved from semiaquatic ancestors.
Abstract
The platypus and four echidna species are the only living egg-laying mammals and the sole extant representatives of Order Monotremata. The platypus and echidnas are very disparate both morphologically and ecologically: The platypus is a specialized semiaquatic burrowing form that forages for freshwater invertebrates, whereas echidnas are fully terrestrial and adapted for feeding on social insects and earthworms. It has been proposed that echidnas evolved from a semiaquatic, platypus-like ancestor, but fossil evidence for such a profound evolutionary transformation has been lacking, and this hypothesis remains controversial. Here, we present original data about the Early Cretaceous (108 to 103 Ma) Australian mammal Kryoryctes cadburyi, currently only known from a single humerus, that provides key information relating to this question. Phylogenetic analysis of a 536-character morphological matrix of mammaliaforms places Kryoryctes as a stem-monotreme. Three-dimensional whole bone comparisons show that the overall shape of the humerus is more similar to that of echidnas than the platypus, but analysis of microstructure reveals specializations found in semiaquatic mammals, including a particularly thick cortex and a highly reduced medullary cavity, present in the platypus but absent in echidnas. The evidence suggests Kryoryctes was a semiaquatic burrower, indicating that monotremes first evolved an amphibious lifestyle in the Mesozoic, and providing support for the hypothesis that this is ancestral for living monotremes as a whole. The lineage leading to the modern platypus appears to have been characterized by extremely long term (>100 My) niche conservatism, with echidnas representing a much later reversion to a fully terrestrial lifestyle.
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| An artist’s impression of Kryoryctes cadburyi. Artwork by Peter Schouten. |
Suzanne J. Hand, Laura A. B. Wilson, Camilo López-Aguirre , Alexandra Houssaye, Michael Archer, Joseph J. Bevitt, Alistair R. Evans, Amalia Y. Halim, Tzong Hung, Thomas H. Rich, Patricia Vickers-Rich and Robin M. D. Beck. 2025. Bone Microstructure supports A Mesozoic Origin for a semiaquatic burrowing lifestyle in Monotremes (Mammalia). PNAS. 122 (19) e2413569122. DOI: doi.org/10.1073/pnas.2413569122
