Steropodon is Greek for "lightning tooth", as its opalised lower jaw with molars was found at the mining town of Lightning Ridge in north central New South Wales. Steropodon galmani lived along the forested shores of the inland Eromanga Sea in Australia in the Early Cretaceous (110 million years ago). This ancient monotreme, or egg-laying mammal, was the first Mesozoic mammal to be discovered in Australia. The remains consist of a fragmentary jaw and some teeth, which resemble those of later fossil platypuses, but the molars are more archaic in form. The compound lower jaw had reduced 'accessory jaw bones' (small bones on the inside of the jaw that became the middle ear bones of mammals). These bones are also found in jaws of the ancestors of mammals, other basal mammals and the Cretaceous monotreme Teinolophos trusleri. Steropodon had three lower molars with well-developed, V-shaped transverse ridges, that were similar to those of toothed platypuses, but simpler. The lower molars of Steropodon had two deep roots, as in most other mammals, but platypuses have shallow, multiple roots on all molars. Steropodon was quite similar to a modern platypus and was about the same size, being 30-35 cm long and weighing about 2.2 kg. It probably had short , stout limbs, held out from the body (a primitive mammalian posture), and moved slowly. It probably had a platypus-like bill, which it used to pluck fish and crustaceans and other aquatic invertebrates out of shallow waters. As its molars were more well developed and deeply rooted than in platypuses, Steropodon may have been able to handle larger, more active prey. Steropodon laid eggs. Its closest relatives were Teinolophos, which roamed Australia about 15 million years earlier, and the much later Obdurodon of the Miocene epoch.
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