Monotremata: Fossil Record
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
During the 1970s, the first Tertiary monotreme fossils were uncovered in southern Australia. Over several years, a jaw, a pelvis, and teeth of the Miocene platypus Obdurodon insignis were discovered. Cast of Obdurodon teeth are pictured at left. More recently, a skull with a nearly full complement of teeth has been found in New South Wales (Archer et al. 1992). This new species, O. dicksoni, demonstrates that unlike the modern platypus, Obdurodon kept its teeth at maturity. Like the platypus, however, Obdurodon had an elongated snout, though straighter than in its modern relative. Opinions differ on whether Obdurodon may have been aquatic, based on its association with numerous terrestrial marsupials.
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.