A small fossil bone discovered three decades ago in southeastern Australia has now emerged as a groundbreaking piece of evidence that could revolutionize our understanding of the evolutionary history of monotremes—the extraordinary group of egg-laying mammals that includes the platypus and echidnas. Until recently, the scientific consensus held that these enigmatic creatures descended from a terrestrial ancestor, with the platypus lineage transitioning to a semi-aquatic lifestyle while echidnas remained strictly land-bound. However, a meticulous re-examination led by researchers from the University of New South Wales (UNSW) challenges this long-standing theory, proposing instead that echidnas and platypuses share a common ancestor that was primarily aquatic.
This paradigm-shifting claim stems from an in-depth analysis of a single humerus bone—the upper arm bone situated between the shoulder and elbow—unearthed at Dinosaur Cove in Victoria during the early 1990s. The fossil has been attributed to Kryoryctes cadburyi, an extinct monotreme species named in 2005, and represents the only known limb bone of Mesozoic Australian monotremes. Traditionally, comparisons of the bone’s external morphology aligned Kryoryctes more closely with modern echidnas. Yet, there has been ongoing debate whether Kryoryctes was an early stem-monotreme common to both the platypus and echidnas, or a terrestrial echidna ancestor.
In an ambitious collaborative study, palaeontologists led by Emeritus Professor Suzanne Hand applied an array of advanced imaging techniques, including computed tomography (CT) scans and high-resolution synchrotron imaging, to investigate not just the external contours but also the intricate internal microstructure of the fossilized humerus. Unlike surface morphology, which primarily elucidates taxonomic relationships, bone microanatomy provides compelling insights into the lifestyle and ecological niche of extinct species. This approach reveals features associated with locomotion, habitat preferences, and physiological adaptations that are otherwise hidden.
The internal structure of the Kryoryctes humerus uncovered an unexpected story. While living echidnas exhibit thin-walled bones with large medullary cavities—adaptations consistent with their terrestrial, burrowing life—platypuses are renowned for their dense, thick-walled bones with constricted marrow cavities. This heavy bone structure in platypuses functions as ballast, facilitating their ability to sink and maneuver underwater during foraging. Strikingly, the fossil bone from Kryoryctes shares more in common with this dense microanatomy than with the lighter skeletal framework of echidnas, strongly supporting the view that stem-monotremes were semi-aquatic.
Monotremes today are exceptional among mammals for their oviparous reproduction and relictual traits, often deemed evolutionary curiosities. The discovery that their early ancestors may have been adapted to submerged or amphibious environments reshapes not only the phylogenetic narrative but also provides important context for the evolution of unique monotreme life history strategies. The transition of echidnas back to land, implied by this study, would constitute a remarkably rare evolutionary reversal from aquatic to terrestrial life—an event scarcely documented in mammalian evolution.
This reverse ecological transition also correlates with curious physiological and anatomical traits in modern echidnas that echo their proposed aquatic heritage. For instance, the electroreceptive capabilities embedded in the platypus bill, which enable detection of prey via minute electrical fields in water, have vestigial counterparts in echidnas, whose beaks retain fewer, yet detectable, electroreceptors. Embryological studies have also revealed residual platypus-like structures in developing echidna bills, suggesting a shared ancestral morphology timed to a semi-aquatic past.
Moreover, echidnas display hind feet oriented backward, a trait unique among mammals except for platypuses, where this adaptation serves as a rudder during swimming. In echidnas, this inversion aids burrowing, yet its origin likely reflects an ancient adaptation for aquatic locomotion. Additional physiological clues come from studies of myoglobin, a respiratory protein crucial for oxygen storage during dives. Both platypuses and echidnas uncover elevated myoglobin concentrations with positively charged residues that enhance oxygen affinity, allowing extended underwater foraging. Such convergent molecular evidence aligns perfectly with the new fossil-based theory.
Paleontological records from the Mesozoic era in Australia remain sparse, especially regarding mammalian fauna. Monotremes and their relatives seemingly dominated primitive mammalian communities over 100 million years ago, but fossils predominantly consist of teeth and jaw fragments, making the Kryoryctes humerus an exceptional find. Its analysis provides a rare window into the locomotor and ecological adaptations of early monotremes at a time when dinosaurs still roamed the continent.
Given these revelations, the research team plans to delve deeper into the histology of the bone through non-destructive yet cutting-edge imaging techniques such as synchrotron radiation microtomography. This will allow unprecedented resolution of the bone’s growth patterns, vascularization, and microstructural intricacies without compromising the fragile and unique fossil. By extending these analyses, the researchers hope to refine the timeline and ecological transitions of primitive monotremes, filling gaps in the fossil record and evolutionary history.
Parallel investigations are also underway at the opal-rich fossil beds of Lightning Ridge in New South Wales, whose unique preservation conditions offer the potential to unveil additional Mesozoic monotreme fossils. These efforts aim to piece together the morphological and ecological evolution of basal monotremes, testing the hypothesis that semi-aquatic life strategies were the ancestral norm before echidnas diverged onto terrestrial niches.
This groundbreaking work not only challenges established views but underscores the complex evolutionary pathways mammals have traversed in adapting to diverse environments. The Mesozoic origin of a semi-aquatic, burrowing lifestyle in monotremes articulates a narrative of ecological flexibility and remarkable evolutionary reversals, expanding our understanding of mammalian biology through deep time. As precision imaging technologies improve and more fossils come to light, the story of monotreme origin will undoubtedly continue to evolve, captivating both scientists and the public alike.
Subject of Research: Animals
Article Title: Bone microstructure supports a Mesozoic origin for a semiaquatic burrowing lifestyle in monotremes (Mammalia)
News Publication Date: 28-Apr-2025
Web References:
https://doi.org/10.1073/pnas.2413569122
Keywords: Paleontology, Animal research, Humerus, Evolutionary biology
