Steropodon

Steropodon
Temporal range: Albian
Scientific classification
Kingdom:Animalia
Phylum:Chordata
Class:Mammalia
Order:Monotremata
Family:Steropodontidae
Genus:†Steropodon
Species: †S. galmani
Binomial name
Steropodon galmani
Archer, Flannery, Ritchie, & Molnar, 1985

Steropodon galmani was a prehistoric species of monotreme, or egg-laying mammal, that lived about 105 million years ago (mya) in the Lower Cretaceous period. It is one of the oldest monotremes discovered, and is one of the oldest Australian mammal discoveries.

Taxonomy

Model of Steropodon galmani at the Australian Museum, Sydney

The dentition of Steropodon is somewhat similar to that of therians–the placentals and the marsupials–specifically the presence of the tribosphenic molar tooth which was thought to be exclusive to therians since the Cretaceous. This, along with the tribosphenic molar discoveries of monotreme-relatives Ausktribosphenos and Ambondro 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 subclasses Australosphenida–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.[1]

Description

The Steropodon jaw on display at the American Museum of Natural History, New York City, USA

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 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.[2]

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."

A fine photo of the opalised lower jaw can be found on page 182 in:

See also

Further reading

  • Mackness, Brian, Prehistoric Australia, Sydney, Golden Press, 1987.

References

  1. ↑ Stokstad, E. (2001). "Tooth Theory Revises History of Mammals". Science. 291 (5501): 26. doi:10.1126/science.10.1126/SCIENCE.291.5501.26.
  2. ↑ Archer, M.; Flannery, T. F.; Ritchie, A.; Molnar, R. E. (1985). "First Mesozoic mammal from Australia — an early Cretaceous monotreme". Nature. 318: 363–366.
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