''Steropodon'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial n ...

of prehistoric

monotreme Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals ( Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their bra ...

, or egg-laying mammal. It contains a single species, ''Steropodon galmani'', that lived about 105 to 93.3 million years ago (mya) in the Early to

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

period. It is one of the oldest monotremes discovered, and is one of the oldest Australian mammal discoveries.

Taxonomy

The dentition of ''Steropodon'' is somewhat similar to that of

theria Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes. C ...

ns—the placentals and the

marsupial Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a ...

s—specifically the presence of the

tribosphenic molar The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...

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 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

subclasses Subclass may refer to: * Subclass (taxonomy), a taxonomic rank below "class" * Subclass (computer science) * Subclass (set theory) See also * Superclass {{disambiguation ...

Australosphenida The Australosphenida are a clade of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of Gondwana. They are thought to have acquired their tribosphenic molars independently from those of Tribospheni ...

—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. It may be only described member of the family Steropodontidae. Another proposed member is '' Teinolophos'' that was moved into its own family, Teinolophidae, by Flannery ''et al.'' (2022). Edentulous partial

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bon ...

from the Finch Clay facies of the Griman Creek Formation was attributed to undescribed steropodontid by Musser (2013). On other side, Flannery ''et al.'' assumed that this mandible may belong to a new genus and species of

stem Stem or STEM may refer to: Plant structures * Plant stem, a plant's aboveground axis, made of vascular tissue, off which leaves and flowers hang * Stipe (botany), a stalk to support some other structure * Stipe (mycology), the stem of a mushr ...

ornithorhynchid. 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".

Description

''Steropodon'' is known only from a single

opal Opal is a hydrated amorphous form of silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms ...

ised jaw with three molars, discovered at the Griman Creek Formation, Lightning Ridge,

New South Wales ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , es ...

, Australia, by brothers David and Alan Galman. It was a large mammal for the

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Creta ...

, being long. The lower molars are in length, with a width of . A length of is more typical for Mesozoic mammals. Woodburne (2003, p. 212) reports that the

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of seve ...

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''.

References

External links

Steropodon
at Fossilworks * * {{Taxonbar, from=Q131338 Prehistoric monotremes Prehistoric mammals of Australia Cretaceous mammals Prehistoric mammal genera Fossil taxa described in 1985 Taxa named by Mike Archer (paleontologist) Taxa named by Tim Flannery Taxa named by Ralph Molnar Cretaceous animals of Australia

Taxonomy

Description

See also

References

Further reading

External links