Platypus and echidna diverged from same ancestor between 19 and 48 mln yrs ago
By ANITuesday, September 22, 2009
SYDNEY - DNA analysis of the land-loving, spiny echidna has shown that the platypus and echidna diverged from the same ancestor between 19 and 48 million years ago.
According to a report by ABC News, the finding contradicts the widely held view that monotremes, or egg-laying mammals, are “living fossils” that have not evolved.
Monotremes are a unique group of animals made up of the echidna and the platypus.
These animals lay eggs from which their young emerge, like a reptile, but are ‘warm-blooded’ and suckle their young, like a mammal.
Lead author Dr Matt Phillips, of the Australian National University (ANU) in Canberra, said that the echidna has evolved from the semi-aquatic platypus-like form to the modern terrestrial spiny anteater in a relatively short time.
The research was prompted by a paper last year that suggested two Australian fossils from the early Cretaceous - the time of the dinosaurs - were close relatives of the platypus.
The linking of this fossil to the platypus lineage would have meant that platypuses and echidnas diverged more than 112.5 million years ago, reinforcing the notion of monotremes as living fossils.
Phillips and his colleagues challenged this view by using molecular dating of DNA sequences from the modern-day platypus and echidna to show the two species shared the same ancestor about 30 million years ago.
Under molecular dating, the researchers look at how different modern DNA sequences are and estimate when those DNA sequences would have been similar.
“We know the common ancestor would have been more platypus than echidna because fossils dating back to 60 million years reveal a monotreme that was slightly larger than the modern-day platypus, but very anatomically similar,” said Phillips.
He said that the result also explains the lack of fossil record for the echidna before 13 million years ago.
“There is no fossil record dating before 13 million years because they simply hadn’t evolved yet,” he said. (ANI)
