Researchers have pinpointed exactly how and when echidnas likely arrived in Australia as part of a fossil analysis that sheds new light on the origins of egg-laying mammals.
The platypus and four species of echidnas are the only remaining living monotremes – mammals that lay eggs instead of giving birth to live young. A new analysis of every monotreme fossil discovered to date has recast the earliest animal history.
The research, published in the journal Alcheringa, was led by the Australian Museum’s chief scientist, Professor Kristofer Helgen, and his honorary associate, Professor Tim Flannery.
Mysteriously, their analysis revealed that the oldest echidna in the Australian fossil record was only 2 million years old, relatively recent in the species’ evolutionary history. “It’s been tens of millions of years since echidnas and platypus shared a common ancestor,” Helgen said.
The study authors believe the echidnas may have migrated to Australia from the Bird’s Head Peninsula on the island of New Guinea at the start of the last ice age, around 2.6 million years ago. .
“This [region] is a good candidate because there are multiple species of echidnas out there today,” Helgen said. All four species of living echidnas are found in New Guinea, while only the short-beaked echidna occurs in Australia.
Periodically throughout history, New Guinea has been physically connected to the Australian mainland. “Ten thousand years ago you could walk in between,” Helgen said. “It almost certainly had to be some sort of Lost Ark Island of New Guinea that was home to echidnas…because they’re invisible thanks to a pretty good Australian fossil record. [for millions of years]for a group of animals that fossilized quite well.
The researchers also classified the first monotreme, a tiny insectivorous creature that lived around 126 million years ago, in its own family of mammals. animal, Teinolophos trusleriweighed about 40g and lived at a time when southeastern Australia was near the South Pole, as part of the supercontinent Gondwana.
Helgen said Teinolophos trusleri probably possessed an “unusual sixth sense that monotremes use” – a large set of nerves in their faces that are able to detect electric fields, which aid in finding prey. “We think that in a place where there would have been polar night for much of the year…that might have been the right condition for this kind of monotreme mark to evolve.”
“Even though it was so long ago… [Teinolophos] was already well on its way to evolving the characteristic beak that we will see later in platypus and echidnas,” he added. Monotreme fossils were first discovered at Flat Rocks in eastern Victoria.
The separate categorization as a new taxonomic family provides “a very good indication that this was a completely different way of being a mammal,” Helgen said. “Dogs are family, cats are family.”
Examining the fossil record, the researchers found that monotremes later diversified in Australia, with body masses varying between 4 and 8 kg. “These are some of the largest mammals that existed anywhere on the planet during the age of the dinosaurs,” Helgen said.
The team also named a new genus, Murrayglossus, for the largest egg-laying mammal that ever lived: an extinct giant echidna from Western Australia. “We think they probably hit something like 30kg,” Helgen said. In comparison, short-beaked echidnas averaged 3.5 kg, while New Guinea long-beaked echidnas averaged 7-8 kg, he said.
“We think it has adaptations that show it was probably the best of the echidnas…at climbing,” Helgen said, adding, “Modern echidnas are actually very good climbers – they can climb trees and at the fences.”
The comprehensive analysis of the fossils, he said, gave the team the opportunity to “tell this unique story of how monotremes came about.”