Steropodon galmani, a platypus-like monotreme from the Early Cretaceous of Australia, was the first Mesozoic mammal discovered from Australia. It is known from an opalised lower jaw with molar teeth found at the mining town of Lightning Ridge in north central New South Wales. The teeth of Steropodon are similar to those of later fossil platypuses although its molars are more archaic in form. Steropodon lived alongside dinosaurs, crocodiles, early birds and other early mammals on the forested shores of the inland Cretaceous Eromanga Sea.
Judging from the size of its jaw, Steropodon would have been a small mammal about the size of the living platypus. Like other monotremes, it probably had short, stout limbs held out from the body (a primitive mammalian posture) and would have been an egg-layer. Aside from the lower jaw, the rest of its anatomy is unknown.
Steropodon had a compound lower jaw, with reduced 'accessory jaw bones' (small bones on the inside of the jaw that later in mammalian evolution become the middle ear bones). The jaw of Steropodon was similar in this respect to the jaw of the ancestors of mammals, to other basal mammals and the Cretaceous monotreme Teinolophos trusleri. Steropodon had three lower molar teeth with well-developed, V-shaped transverse ridges, simpler in form than the molars of toothed platypuses but otherwise similar. The lower molars of Steropodon have two deep roots, as in most other mammals but not as in platypuses, which have shallow, multiple roots on all molar teeth.
Lightning Ridge in the Early Cretaceous was close to the Antarctic Circle and along the shores of the Eromanga Sea, an inland sea that covered vast areas of inland Australia 110 million years ago. This high latitude position meant that the area experienced extremes of daylight during winter and summer months, although the climate was much milder then than it is today. Preserved plant material (tree branches, conifer cones) are evidence that the Lightning Ridge area was forested, with araucarian conifers the dominant canopy trees. The understory would have included ferns and cycads.
Steropodon was found at Lightning Ridge in north-central New South Wales. To date, Steropodon has not been found at any other locality.
Feeding and diet
Steropodon probably had a diet similar to that of the living platypus, including aquatic invertebrates, yabbies (freshwater crayfish) and other small aquatic animals such as fish. The molar teeth of Steropodon were more well developed and deeply rooted than in platypuses, however, and Steropodon may have been able to handle larger, more active prey.
The opalised, fragmentary jaw of Steropodon has three molar teeth and a partial tooth socket for the last premolar. This specimen is held by the Australian Museum.
Relationships of monotremes to other mammals are hotly debated. They may be related to other Southern Hemisphere mammals with triangulated teeth and primitive jaws (the Australosphenida of Luo et al. 2001, 2002). They may alternatively have evolved from some other Mesozoic group, as yet unknown. They are undoubtedly very ancient - perhaps Jurassic or even Triassic in origin - and there is much that is still unknown about mammalian evolution in the Southern Hemisphere during this time.
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