Monotremes, which include platypus and echidnas, may be traced back to arctic locations with months of darkness, according to fossil evidence. This is only one of the discoveries made as part of an effort to organize the monotreme family tree. Monotremes are unlike other mammals in that they have hair and produce milk. Platypuses were often thought to be hoaxes by British naturalists: how else could traits resembling so many different species be blended into one animal? Monotremes are only represented now by the platypus and four echidna species, but they were more diversified during the Cretaceous, despite dinosaur competition.
However, because the locations monotremes inhabited were either not good for fossil preservation or buried beneath so much ice now, it’s unclear how diversified they were. In a research published in Alcheringa: An Australasian Journal of Palaeontology, some of Australia’s top paleontologists collaborated to make sense of the fossils we do have. Teinolophos trusleri, the oldest known monotreme fossil, is also the tiniest. It was discovered in 120 million-year-old Strzelecki Group deposits in south-eastern Australia, weighing about 40 grams (1.4 ounces) — about the size of a mouse. The Strzeleckis lived near the Antarctic Circle while Australia was still tied to Antarctica.
Teinolophos is “pretty much what you would anticipate of an archetypal monotreme,” according to first author Professor Tim Flannery of the University of Melbourne. It lacked a functional beak and instead had a patch of electrosensitive skin.” This patch developed into the platypus bill, which is used to hunt for insect food in typically murky streams, and echidna beaks, which are used to detect ants and termites. The ability to sense electrical impulses has remained as important as egg-laying in differentiating monotremes from other mammals. “There was probably a fossil record before [Teinolophos], but probably not much,” Flannery noted.
Despite the discovery by two of the paper’s co-authors of polar “dinosaurs of darkness,” the poles in the early Cretaceous may have been one of the most inviting areas for mammals at the time. At least four monotreme taxa originated in the same region during the late Cretaceous, one of which comprised some of the biggest animals of the time. However, it appears that just one person survived the asteroid impact’s deeper darkness.
The loss of non-avian dinosaurs presented an opening for mammals, but monotreme paws appear to have been ill-equipped to seize it. Although one monotreme fossil, Asfaltomylos patagonicus, was discovered in South America 63 million years ago, monotremes have been relegated to Australia and occasionally-connected islands like New Guinea, where they have always been overwhelmed by marsupials. Although the echidna’s habitat covers more of the continent than any other native animal, monotremes may have been slower to adapt to non-polar environments when Australia moved north. All known monotremes, extant and extinct, are divided into five families, one of which, the Teinolophidae, has never been officially characterized.
Echidnas are thought to have developed from platypus in New Guinea at a period when it was isolated from Australia, then moving south when water levels were low enough for the two to meet. Despite keeping mostly to echidna and platypus forms, there was considerable variety in the post-asteroid world, as seen by Murrayglossus hacketti, the world’s biggest monotreme at a meter long and an estimated 20-30 kilograms (44-66 pounds). It most likely climbed trees to feed on arboreal termites in southwest Australia, according to Flannery.
