Steropodon galmani was a prehistoric species of monotreme, or egg-laying mammal, that lived during the middle Albian stage, in the Lower Cretaceous period. It is the earliest known relative of the Platypus. Steropodon is known only from a single opalised jaw with three molars, discovered at the Griman Creek Formation, Lightning Ridge, New South Wales, Australia, by brothers David and Alan Galman. It was a large mammal for the Mesozoic, being 40 - 50 cm long. The lower molars are 5 - 7 mm in length, with a width of 3 - 4 mm. A length of 1 - 2 cm is more typical for Mesozoic mammals. Also from Lightning Ridge is Kollikodon ritchiei.
The molars "bear striking resemblance to the tribosphenic pattern characteristic of living therians..." (Pascual). However, there are also differences: there is no entoconid, and an absence of wear seems to suggest that the upper molars (as yet unknown) did not have a protocone.
Woodburne (2003, p.212) reports that the holotype is a right mandible named AM F66763. The preserved molars are m1 - m3. Page 237 includes: "In Steropodon, the mandibular canal suggests the presence of a bill, with a bill also known in Obdurodon dicksoni and Ornithorhynchus anatinus."