Weekly Spotlight: Steropodon

Good tidings and well-wishes!

Throughout its limited history, I’ve attempted to utilize “The Theatrical Tanystropheus” for the purposes of lending coverage towards bizarre prehistoric creatures who, for a variety of inexplicable reasons, have received relatively little coverage from the scientific community and the media at large. Given this favoritism towards long-extinct oddities, I figured that an ancient relative of one of the modern world’s most beloved biological eccentrics would be most fitting. With that, I give you Steropodon galmani, a Mesozoic precursor of the modern duck-billed platypus (Ornithorhynchus anatinus).

As most paleo-enthusiasts are well aware, Mesozoic mammal remains are often be maddeningly difficult to come by, as evidenced by the fact that Steropodon‘s 1985 discovery marked the first occasion in which a member of the class had ever been discovered in an Australian bed of the era’s sediment. This initial excavation was made in the mid-Albanian Girman Creek formation near the town of Lightning Ridge in New South Wales and consisted of a solitary lower jaw fragment containing three lower molars. In a taphonomic sense, the fossil is noteworthy due to the fact that the material had been long-since opalised, as seen in the following image.

Steropodon sported a compound lower jaw: a trait possessed by many ancestral mammals from this point in time including Teinolophos trusleri, another Cretaceous monotreme known from the region. The fact that the two little mammals also share the derived feature of having a notably deep dentary along with double-rooted molars (as opposed to their multi-rooted counterparts in subsequent monotremes) has led many authors to suggest that the two should be placed into their own family, the Steropodontidae which, in addition to the Ornithorhynchidae, Tachyglossidae, and extinct Kollikodontidae, makes up the Monotremata order, though some researchers have asserted that Steropodon could be more accurately viewed as an ornithorhynchid. Steropodon‘s molars are also noteworthy for their tribosphenic arrangement comparable to that found in modern insectivores and, more importantly from a cladistic standpoint, young platypuses and Obdurodon, their toothed Miocene forebear. However, this particular dental arrangement is considerably more advanced in Steropodon than in its extant duck-billed kin; a fact which has given rise to a considerable amount of debate concerning the phylogenetic affiliations of monotremes as a whole. Intriguingly, the advanced nature of these teeth in Steropodon implies to many Mesozoic paleo-mammologists that the Monotremata split from the Therian subclass of mammals (whose members give birth to live young in lieu of a shelled egg) far more recently than had been previously assumed.

With regards to the animal’s lifestyle, the dietary and ecological habits of Steropodon were likely quite analogous to those of modern amphibious platypuses, although it’s been said that despite their theoretically identical overall size, the deeply rooted teeth of this Cretaceous monotreme may have enabled it to tackle larger forms of fish and other aquatic denizens than those pursued by their present-day descendants.