Recent discoveries are reshaping our understanding of the evolutionary journey of monotremes, the unique group of egg-laying mammals that includes echidnas and the platypus. New fossil evidence, particularly the analysis of a 108-million-year-old humerus bone, is challenging long-held beliefs about their origins and the aquatic ancestry of echidnas.
For a long time, the prevailing theory was that monotremes descended from a land-dwelling ancestor. Within this framework, it was believed that the ancestors of platypuses adapted to a semi-aquatic lifestyle, while echidnas remained exclusively terrestrial. However, a groundbreaking re-examination of a humerus (upper arm bone) discovered three decades ago at Dinosaur Cove in southeastern Australia suggests a different narrative.
This fossil, attributed to the extinct monotreme Kryoryctes cadburyi, has been meticulously analyzed by researchers using advanced imaging techniques like CT scans and synchrotron imaging. While the external appearance of the bone initially suggested a closer relationship to modern echidnas, the internal microstructure tells a different story. The bone exhibits thick walls and a reduced medullary cavity (the hollow space inside bones), features characteristic of semi-aquatic mammals like the platypus, which use denser bones as ballast for diving. This contrasts with the lighter, thin-walled bones of modern terrestrial echidnas.
This analysis of Kryoryctes cadburyi's humerus, dating back to the Cenomanian age of the Cretaceous period (around 100-108 million years ago), strongly indicates that this early monotreme was likely a semi-aquatic burrower. This finding supports the hypothesis that the common ancestor of both platypuses and echidnas was adapted to an aquatic or semi-aquatic environment.
If this is the case, it implies a remarkable and rare evolutionary event for echidnas: a transition from an aquatic or semi-aquatic lifestyle back to a fully terrestrial one. While there are numerous examples of land mammals evolving to live in water (like whales and seals), the reverse is exceptionally uncommon.
Several other lines of evidence lend support to the idea of an aquatic ancestry for echidnas:
- Hind limb structure: Echidnas possess backward-facing hind feet, similar to platypuses which use them as rudders for swimming. In echidnas, this feature is utilized for burrowing.
- Sensory receptors: The beaks of echidnas have electroreceptors, though fewer than in platypuses. These are thought to be a remnant of their aquatic heritage, as platypuses use these receptors to detect prey underwater. Embryological studies also show platypus-like bill structures in developing echidnas.
- Diving reflex: Echidnas exhibit a diving reflex when submerged, a physiological adaptation to conserve oxygen.
- Myoglobin studies: Research on the respiratory protein myoglobin in mammals also suggests a semi-aquatic past for echidnas, as certain properties of this protein are associated with enhanced oxygen storage for diving.
The fossil record for monotremes, particularly their early ancestors, remains sparse. Most discoveries have been limited to teeth and jaw fragments, making limb bones like the Kryoryctes humerus exceptionally valuable for understanding their locomotion and ecology.
Recent fossil discoveries in the Lightning Ridge opal fields of New South Wales, Australia, have further expanded our knowledge of monotreme diversity during the Cretaceous period. These opalized fossils, dating back to between 102 million and 96.6 million years ago, have revealed several new genera of monotremes, including:
- Opalios splendens: Nicknamed the "echidnapus," this species exhibits a mix of features found in both modern echidnas and platypuses, suggesting it could be close to the common ancestor.
- Dharragarra aurora: The earliest known platypus-like fossil.
- Parvopalus clytiei: One of the smallest monotremes ever found.
These discoveries indicate that Australia was once home to a much greater diversity of monotremes than previously thought, a period some researchers are calling an "Age of Monotremes" before marsupials became the dominant mammals on the continent. The finds from Lightning Ridge have increased the known diversity of monotremes from that specific time and place to six species, with researchers suggesting that actual diversity was likely even higher.
The evolutionary story of monotremes is also showing a trend from "toothy to toothless." The oldest known monotreme, Teinolophos trusleri (around 126-130 million years old), had five molars. By the time of the Lightning Ridge fossils, some monotremes had reduced this to three molars, and modern platypuses and echidnas are essentially toothless, a change possibly related to dietary shifts.
While the exact timing of when echidna ancestors transitioned to a fully terrestrial lifestyle is still unclear due to the fragmented fossil record, the emerging evidence strongly points towards an aquatic origin for the entire monotreme lineage. Ongoing research, including further analysis of existing fossils using advanced imaging and the hope of new discoveries, continues to shed light on the complex and fascinating evolutionary history of these unique Australian mammals. The first-ever sequencing of the echidna genome, completed around May 2025, is also expected to provide significant insights into their evolutionary biology and unique adaptations.