New research reveals a surprising twist in the evolution of echidnas. A 100-million-year-old fossil found in a rocky area of Australia suggests that echidnas might have descended from ancestors that once thrived in water. This idea flips the usual narrative, as most land mammals evolve to return to the water, not the other way around.
The fossilized arm bone, belonging to the species Kryoryctes cadburyi, has been a subject of debate among paleontologists since its discovery in the early 1990s. Scientists agree that it belonged to a monotreme, a group of mammals known for laying eggs, but differing views exist on whether it was a land-dweller or an aquatic creature.
Dr. Suzanne Hand, a paleontologist from the University of New South Wales, led a study analyzing this fossil. "The external structure of a bone helps us compare it with similar animals and understand its relationships, while the internal structure reveals clues about its lifestyle," she explains.
Recent advanced imaging techniques like microCT scans uncovered that the tiny humerus bone has a heavy wall and a small medullary cavity. These features are typical in semi-aquatic animals like sea otters and dugongs, as they help control buoyancy. Hand notes that the bone structure closely resembles that of the platypus, which also has heavy bones to help it dive underwater easily.
Conversely, such dense bones would be a hindrance on land, making them more prone to fractures. Today’s echidnas have light, thin-walled bones due to their terrestrial lifestyle.
Further phylogenetic analysis confirmed that K. cadburyi shares a common ancestor with both echidnas and the platypus. This suggests that they evolved from a swimming, burrowing ancestor — an intriguing idea that changes our understanding of echidna evolution.
Hand also highlights that the echidna’s beak bears a resemblance to that of the platypus, albeit with fewer sensitive receptors. She speculates that these features are remnants of their shared history. Additionally, the backward orientation of echidnas’ hind feet aids in digging, similar to how the platypus uses its feet in water.
This research paints a picture of echidnas as unique creatures that transitioned from a semi-aquatic environment to life on land. It’s a rare occurrence in evolutionary history, and this study opens a new chapter in understanding how these fascinating animals adapted over time.
For more details on this research, you can view it in the journal PNAS here.
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