New Research Sheds Light on Origins of Monotremes

New research led by Australian Museum paleontologists shows that monotremes are the last survivors of a diverse set of fossil species that once roamed the southern continents. In particular, the authors investigated the oldest and smallest known monotreme Teinolophos trusleri — classifying it in a new mammalian family — as well as the largest egg laying mammal that ever lived: a gigantic extinct echidna from Western Australia for which a new genus, Murrayglossuswas named.

Murrayglossus hacketti, a giant echidna that lived in Western Australia during the Pleistocene. The largest egg-laying mammal ever discovered, this 30-kg animal may have specialized on termites. Image credit: Peter Schouten.

Living monotremes (order Monotremata) are the only surviving egg-laying mammals.

With the exception of the Patagonian monotreme species Monotrematum sudamericanumall living and fossil monotremes are restricted to Australia and New Guinea.

The five living species are placed in two families: Ornithorhynchidae, with a single living species, the platypus (Ornithorhynchus anatinus); and Tachyglossidae, which includes the short-beaked echidna (Tachyglossus aculeatus) and the long-beaked echidnas (genus Zaglossus).

The long-beaked echidnas include three living species: Zaglossus bruijnii, Zaglossus bartoniand Zaglossus attenboroughi.

In order to chart the history and evolution of monotremes, Australian Museum’s Professor Kris Helgen and colleagues examined every significant monotreme fossil known.

They investigated the oldest and smallest known monotreme Teinolophos trusleri — classifying it in a new mammalian family — as well as a gigantic extinct echidna from Western Australia for which a new genus, Murrayglossuswas named.

“Weighing 30 kg in size, roughly the size of a wombat, Murrayglossus hacketti would have been many times the size of modern Australian echidnas,” Professor Helgen said.

“This massive monotreme roamed the Australian landscape in the Pleistocene epoch with megafauna like gigantic kangaroos, the marsupial lion, and Diprotodon.”

The research also pinpoints the origins of monotremes, dating back to the South Pole 130 million years ago, when the Pole was positioned over southern Australia.

“We have also firmly classified the oldest known monotreme, Teinolophos trusleriwhich likely possessed an electro-sensitive mechanism for feeding on insects in the seasonally dark Early Cretaceous polar forests,” said Australian Museum’s Professor Tim Flannery.

At the time Teinolophos trusleri lived, 130 million years ago, southeastern Australia lay close to the South Pole. Yet forests grew there, surviving at least 3 months of freezing darkness each year.

“This is an incredible survival adaptation for this mini monotreme, which we estimate had a body mass of approximately 40 g — roughly the weight of a slice of bread — imagine it using its snout to plunge into moss and snow to find food it can’ t see and hasn’t yet touched,” Professor Flannery said.

The restriction of the oldest monotremes to polar forest may explain why they did not spread to other continents, which lay further from the pole where conditions were not right for them.

The research also illuminates the likely migration of echidnas from New Guinea to Australia.

“Echidnas are found in the Australian fossil record going back only two million years, yet genetic studies indicate that echidnas evolved from platypus-like ancestors tens of millions of years before that,” Professor Helgen said.

“So, their absence from the Australian fossil record is mysterious.”

“We think that echidnas originated on an island in what has now become part of New Guinea, and that they reached Australia during a period of faunal exchange around the onset of the Ice Ages.”

“Monotremes are such iconic Australian species, and this research not only reveals their unique origin story, but also helps us understand how they came to live in Australia in all shapes and sizes throughout the fossil record,” Professor Flannery added.

“Although this research focuses on the past of these remarkable mammals, we also want to highlight the urgent need for protection of our modern platypuses and echidnas, which are under threat and in decline as a result of human-induced habitat degradation.”

The study was published in the Alcheringa, an Australasian Journal of Palaeontology.

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Timothy F Flannery et al. A review of monotreme (Monotremata) evolution. Alcheringa, published online March 16, 2022; doi: 10.1080/03115518.2022.2025900

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