The evolutionary history of the monotremes

Monotremes arose in southern polar environments and were restricted to high latitudes for most of their evolutionary history, explaining their limited global distribution. They may be specialised dryolestids, primitive mammals with a near-global distribution in the Jurassic–Cretaceous. The oldest monotreme fossils are from the Early Cretaceous – 126 million-year-old (Ma) teeth and jaws from southern Victoria, representing a mouse-sized species (Teinolophos trusleri) which had an elongated snout that was used to probe for food. It lived at 76°S, enduring freezing temperatures and 3 months of total darkness each year. The next oldest monotremes, also from southern Victoria, date to 113–108 Ma. They were at least an order of magnitude larger than Teinolophos, around the size of modern Platypus Ornithorhynchus anatinus and echidnas. By the Late Cretaceous (ca. 100 Ma), an age of monotremes had opened in the Australian part of the Gondwanan continent, with six monotreme genera (but no other mammals) co-existing at 60°S at what is now Lightning Ridge, northern NSW. They included an ancestral platypus, another species that was structurally annectant to platypus and echidnas, and a variety of rat to small pig-sized types that appear to have occupied aquatic and terrestrial niches. There are no monotreme fossils yet known from Australia between 100 and 24 Ma, but ancestral platypus, dating to around 70–64 Ma, are known from southern South America. Fossils of toothed platypus (Obdurodon) from central Australia date to around 26 Ma. These are the earliest known non-polar monotremes, living at around 40°S. Rare toothed platypus fossils occur 24–13 Ma, but the oldest known toothless platypus (Ornithorhynchus) date to the Pleistocene (2.5 Ma – 10 ka). After having teeth for 100 million years, Platypus may have lost them because of competition with Water Rats Hydromys chrysogaster. The oldest well-dated echidna fossils are Pleistocene, but genetic studies suggest that the lineage dates back to ca. 50 Ma. Echidnas may have evolved from platypus-like relatives that became isolated on the margin of the paleo-continent Sahul such as on the Vogelkop Island 50–30 Ma. They may have then re-invaded mainland Sahul (i.e. Australia and New Guinea) in the Late Pliocene–Early Pleistocene.