The platypus and echidna-two of the most unusual mammals on Earth-have long puzzled scientists due to their mix of primitive and unique traits. Both are monotremes, the only mammals that lay eggs, and have no teeth. While the platypus is semiaquatic with duck-like features, the echidna lives on land and is covered in spines. A new study based on a fossil humerus bone discovered in 1993 in southeastern Australia reveals new insights into their evolution. Although the fossil looked like it belonged to a land-dwelling echidna, internal 3D imaging showed it shared features with the semiaquatic platypus.

The fossil belongs to an extinct animal called Kryoryctes cadburyi, believed to have lived over 100 million years ago. The study, published in Proceedings of the National Academy of Sciences, showed that the fossil&rsquos dense bone structure helped reduce buoyancy, a trait useful for aquatic animals. This supports the theory that Kryoryctes may be a common ancestor to both the platypus and echidna, with an amphibious lifestyle emerging far earlier than previously believed. Scientists suggest that echidnas later transitioned back to a land-based life, possibly inheriting features like their backward-facing hind feet from aquatic ancestors.A cross-section comparison shows (from left) a Kryoryctes (A), platypus (B) and echidna (C) humerus bone. Hand et al.

This discovery gives evolutionary biologists rare evidence of a water-to-land transition, which is much less documented than the reverse. Understanding how monotremes evolved sheds light on the broader history of mammals. As relics of an ancient lineage, platypuses and echidnas may hold critical clues about the biology of the earliest mammals. Researchers believe that studying these &ldquoliving fossils&rdquo could help us better understand our own evolutionary past.