Only 30 million years ago Australian and New Guinean echidnas had an amphibious, platypus-like ancestor, according to a new study.
The results of the study are published in this week’s Proceedings of the National Academy of Sciences. Lead author on the study Dr Matt Phillips, an ARC Postdoctoral Fellow at the ANU Research School of Biology, said its findings shed light on the evolution of monotremes.
“A previous study had suggested that two monotreme fossils from the Early Cretaceous (during the time of the dinosaurs) were close relatives of the platypus. We re-examined the anatomical data and showed that platypuses are, in fact, more closely related to their living cousins, the echidnas, than they are to those Cretaceous fossils,” he said.
“We confirmed this with an examination of DNA sequences. These revealed that platypus and echidnas shared a common ancestor as recently as 30 million years ago. We know from the fossil record that 61 million year old fossil monotremes already looked and would have acted much like platypuses. This tells us that the approximately 30 million year old common ancestor of platypuses and echidnas must also have been an amphibious platypus-like creature, whose echidna descendants sometime later made a transition from feeding in water to feeding on land. The finding that echidnas evolved relatively recently also helps to explain the mystery of their absence from well sampled fossil sites prior to 13 million years ago.”
The study also found evidence to contradict a widely-held view that monotremes have long been evolutionary dead ends.
“The expansion onto land of the ecological territory that monotremes hold also occurred long after marsupials had arrived in Australia,” said Dr Phillips. “This contradicts the common assumption that monotremes are ‘living fossils’ just treading water in an evolutionary sense, waiting to go extinct in the face of competition with ‘superior’ mammals like marsupials.
“Ironically, it is some of the supposedly primitive characteristics of monotremes – such as egg-laying – that probably helped them compete against marsupials. In live-bearing marsupials, the newborn attaches to the nipple with its mouth. This makes feeding in water like a platypus a problem for marsupials and probably prevents marsupials from being able to evolve an echidna-like beak,” he said.
