The ancestry of platypus and echidnas, collectively known as monotremes, can be traced to polar regions experiencing months without sunlight, fossils indicate. This is just one of the findings emerging from an effort to put some order into the monotreme family tree.
Although they have fur and produce milk, monotremes are so unlike other mammals. British naturalists famously believed platypus specimens were hoaxes: how else could features resembling such differing creatures be combined in one animal? Today, monotremes are represented purely by the platypus and four echidna species, but during the Cretaceous, they were more diverse, despite the competition from dinosaurs.
Just how diverse remains a mystery, however, because the regions monotremes inhabited were either not great for fossil preservation, or lie beneath so much ice today. However, some of Australia's leading paleontologists have combined to make sense of the fossils we do have in a study published in Alcheringa: An Australasian Journal of Palaeontology.
The oldest known monotreme fossil, Teinolophos trusleri, is also the smallest. Having been found in 120 million-year-old Strzelecki Group deposits in south-eastern Australia, it is estimated to have weighed just 40 grams (1.4 ounces) – around the size of a mouse. Back then, Australia was attached to Antarctica and the Strzeleckis were near the Antarctic circle.
First author Professor Tim Flannery of the University of Melbourne told IFLScience that Teinolophos is “very much what you would expect of a prototypical monotreme. It had no proper beak, but a patch of electrosensitive skin.” This patch evolved into the bill the platypus uses to hunt for insect prey in often murky streams and the beaks echidnas use to sense ants and termites. The capacity to detect electrical signals has remained as much a feature distinguishing monotremes from other mammals as their egg laying.
“Doubtless there was a fossil record before [Teinolophos] but possibly not much before,” Flannery added.
Despite the polar “dinosaurs of darkness” discovered by two of this paper's co-authors, the poles in the early Cretaceous may have been one of the more welcoming places for mammals at the time. During the late Cretaceous, at least four genera of monotremes evolved in the same region, one of which included some of the largest mammals of their day. However, it seems only one made it through the deeper darkness after the asteroid impact.
The extinction of the non-avian dinosaurs created an opening for mammals, but it seems monotreme paws were ill-suited to grasping it. Although one monotreme fossil Asfaltomylos patagonicus has been found in South America from 63 million years ago, they have otherwise been restricted to Australia and sometime-connected islands like New Guinea, and have always been far outnumbered, even there, by marsupials.
As Australia drifted north the monotremes may have been slower to adapt to non-polar conditions, although today the echidna's habitat covers more of the continent than any other native mammal.
Flannery and co-authors place all known monotremes, living and extinct, into five families, one of which, the Teinolophidae, has not been scientifically described before. They reason echidnas evolved from platypus in New Guinea during a time when it was isolated from Australia, before spreading south when water levels were low enough to join the two.
Despite sticking fairly tightly to the echidna and platypus forms, some diversification did take place in the post-asteroid world, highlighted by Murrayglossus hacketti, at a meter long and an estimated 20-30 kilograms (44-66 pounds) the largest monotreme that ever lived. Flannery told IFLScience it probably climbed trees to feed off southwest Australia's arboreal termites.