Steropodon is a genus of prehistoric platypus-like monotreme, or egg-layingmammal. It contains a single species, Steropodon galmani, that lived about 105 to 93.3 million years ago (mya) during the Cretaceous period, from early to middle Cenomanian.[1] It is one of the oldest monotremes discovered, and is one of the oldest Australian mammal discoveries.
The dentition of Steropodon is somewhat similar to that of therians—the placentals and the marsupials—specifically the presence of the tribosphenic molartooth which was thought to be exclusive to therians since the Cretaceous. This, along with the tribosphenic molar discoveries of monotreme-relatives Ausktribosphenos and Ambondro mahabo of which the latter evolved in the Jurassic, led to the conclusion that the molar evolved independently in the two lineages. This inspired the creation of the subclassesAustralosphenida—the monotremes and extinct relatives—and Tribosphenida—placentals and marsupials. However, given this classification is based only on jaw and lower-tooth remains, insufficient evidence may exist to definitively make this conclusion.[2]
It may be only described member of the family Steropodontidae. Another proposed member is Teinolophos that was moved into its own family, Teinolophidae, by Flanneryet al. (2022).[3] Edentulous partial mandible from the Finch Clay facies of the Griman Creek Formation was attributed to undescribed steropodontid by Musser (2013).[4] On other side, Flannery et al. assumed that this mandible may belong to a new genus and species of stemornithorhynchid.[3]
The specific epithet is derived from the surname of the jaw's collectors, when combined with the genus name Steropodon is intended to translate as "Galman's lightning tooth".[5]
Steropodon is known only from a single opalisedjaw 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 centimetres (16–20 in) long. The lower molars are 5–7 millimetres (0.20–0.28 in) in length, with a width of 3–4 millimetres (0.12–0.16 in). A length of 1–2 centimetres (0.39–0.79 in) is more typical for Mesozoic mammals.[6]
Woodburne (2003, p. 212) reports that the holotype is a right mandible named AM F66763. The preserved molars are m1–m3. Examination of the jaw fragment revealed a mandibular canal, which has been proposed to indicate the presence of a bill, similar to those of the extinct species Obdurodon dicksoni and the modern platypus Ornithorhynchus anatinus.
^Musser, A. M. (2013). "Classification and evolution of the monotremes". In Ashwell, K. (ed.). Neurobiology of Monotremes: Brain Evolution in Our Distant Mammalian Cousins. Melbourne: CSIRO Publishing. pp. 1–17. ISBN9780643103153.
^M. Archer, F. A. Jenkins, S. J. Hand, P. Murray, and H. Godthelp. 1992. Description of the skull and non-vestigial dentition of a Miocene platypus (Obdurodon dicksoni n. sp.) from Riversleigh, Australia, and the problem of monotreme origins. Platypus and Echidnas 15-27.
