The oldest fossil monotremes come from the Lightning Ridge opal fields of New South Wales, Australia. An opalized lower jaw fragment of Steropodon galmani more than 100 million years old (middle Albian, Cretaceous) was found containing three distinctive teeth remarkably similar to those of the juvenile platypus. From the size of the jaw, it is estimated that the living animal was about the size of a cat, making it one of the largest Mesozoic mammals known. This find (Archer et al. 1985) marked the first known Australian mammal from the Mesozoic. A second jaw, described as Kollikodon ritchiei, was found in 1995 from deposits of similar age, suggesting that monotremes had already diversified by the Early Cretaceous.
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Living xenarthrans : On the left, a sloth. At right, a hairy armadillo from Santa Cruz, Argentina.
The Australian fossil record of monotremes also includes some quite good Miocene and Pleistocene fossils of giant echidnas. Three species are known, two assigned to the genus Megalibgwilia (Griffiths et al. 1991). Several nearly complete skulls of M. ramsayi have been recovered from caves in South Australia. The largest of the giant echidnas, Zaglossus hacketti, is known only from a few bones found in Western Australia; it ranks as the largest monotreme ever to have lived.
The time and place of monotreme origin is still largely unkown. Most fossil monotremes have been found in Australia, though a Paleocene platypus tooth (Monotrematum) has recently been recovered from Argentina (Pascual et al. 1992), suggesting they were once distributed across southern Gondwana.
