Monotremes are the only mammals that hatch from eggs and belong to an ancient order of mammals represented by only five living species: the platypus and four species of echidna (or spiny anteater). They all have things in common other than laying eggs: they are endemic to Australia and have some primitive skeletal features. It is curious because the proposal that some mammals reproduced by laying eggs was considered far- fetched in zoological circles in the early 19th century.
“In the 19th century, many conservative scientists did not want to believe that there could be an egg-laying mammal, because this would support the theory of evolution, the idea that one group of animals was capable of changing another,” said Jack Ashby, deputy director of the Museum of Zoology at the University of Cambridge.
Putting order in the family tree
However, a new look at monotreme evolution suggests that the ancestry of platypuses and echidnas can be traced back to polar regions that experienced months without sunlight, fossils indicate.
Thus, new research led by paleontologists at the Australian Museum shows that monotremes are the last survivors of a diverse set of fossil species that once roamed the southern continents. To chart the history and evolution of monotremes, Professor Kris Helgen and colleagues at the Australian Museum examined all known important monotreme fossils.
They studied the oldest and smallest monotreme Teinolophos trusleri (a mere 40 grams ), classifying it into a new family of mammals, as well as the largest egg-laying mammal that ever lived: a gigantic extinct echidna from Western Australia that has been named as a new genus, Murrayglossus . “Weighing 30kg , about the size of a wombat, Murrayglossus hacketti would have been many times the size of modern Australian echidnas,” Helgen said. “This massive monotreme roamed the Australian landscape in the Pleistocene epoch with megafauna such as giant kangaroos, the marsupial lion, and Diprotodon.”
The smallest and the largest
The oldest monotreme fossil, Teinolophos trusleri , was found in 120-million-year-old Strzelecki Group deposits; a time in history when Australia was joined to Antarctica and the Strzeleckis were close to the Antarctic Circle.
We believe that echidnas originated on an island in what has now become part of New Guinea, and that they arrived in Australia during a period of faunal exchange around the start of the Ice Age ,” the researchers conclude.
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 threatened and in decline as a result of human -induced habitat degradation .
Referencia: Timothy F. Flannery et al. A review of monotreme (Monotremata) evolution. Alcheringa, Australasian Journal of Palaeontology. published online March 16, 2022; doi: 10.1080/03115518.2022.2025900
