Animal Species:Steropodon galmani

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.

Steropodon galmani

Anne Musser © Anne Musser

Standard Common Name

Steropodon

Identification

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.

Size range

Approx. 35 cm long (head to tail)

Distribution

Steropodon was found at Lightning Ridge in north-central New South Wales. To date, Steropodon has not been found at any other locality.

Habitat

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.

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.

Fossils

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.

Era / Period

Cretaceous Period

What does this mean?

Evolutionary Relationships

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.

Classification

Species:
galmani
Genus:
Steropodon
Family:
Steropodontidae
Division:
Monotremata
Superdivision:
Australosphenida
Infraclass:
Holotheria
Subclass:
Mammaliaformes
Class:
Mammalia
Series:
Amniota
Superclass:
Tetrapoda
Subphylum:
Vertebrata
Phylum:
Chordata
Kingdom:
Animalia

What does this mean?

Further Reading

  • Long, J. A. et al. 2002. Dinosaurs of Australia and New Zealand and Other Animals of the Mesozoic Era. New South Wales University Press, Sydney; 188 pp.
  • Long, J. A. et al. 2002. Prehistoric Mammals of Australia and New Guinea: One Hundred Million Years of Evolution. Johns Hopkins University Press, Baltimore, 240 pp.

References

  • Archer, M., Flannery, T.F., Ritchie, A. & Molnar, R.E. 1985. First Mesozoic mammal from Australia - an early Cretaceous monotreme. Nature 318, 363-366.
  • Luo, Z., Cifelli, R.L. & Kielan-Jaworowska, Z. 2001. Dual origin of tribosphenic mammals. Nature 409, 53-57.
  • Luo, Z., Kielan-Jaworowska, Z. & Cifelli, R. 2002. In quest for a phylogeny of Mesozoic mammals. Acta Palaeontologica Polonica 47, 1-78.
  • Musser, A.M. 2006. Furry egg-layers: monotreme relationships and radiations. pp. 523-550 in Merrick, J., Archer, M., Hickey, G. and Lee, M. S. Y. (eds) Evolution and Biogeography in Australasia. Australian Scientific Publishing, Sydney.
  • Rich, T. H., Vickers-Rich, P., Trusler, P., Flannery, T. F., Cifelli, R. L., Constantine, A., Kool, L. and van Klaveren, N. 2001b. Monotreme nature of the Australian Early Cretaceous mammal Teinolophos trusleri. Acta Palaeontologica Polonica 46, 113-118.


Anne Musser
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Tags platypuses, monotremes, extinct, Lightning Ridge, fossils, mammals, Cretaceous,

2 comments

Elanor McCaffery - 4.08 PM, 13 August 2010

Hi Xion,

Thanks, I also think Anne Musser makes nice info sheets. What sort of other info are you looking for?

Xionyuffie - 5.03 PM, 25 March 2010
Nice info but could have more pictures and elaborate on the subject.

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