''Steropodon'' is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of prehistoric

platypus The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to eastern Australia, including Tasmania. The platypus is the sole living representative or monotypi ...

-like

monotreme Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

, or egg-laying

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

. It contains a single species, ''Steropodon galmani'', that lived about 100.2–96.6 million years ago during the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

period, from early to middle

Cenomanian The Cenomanian is, in the International Commission on Stratigraphy's (ICS) geological timescale, the oldest or earliest age (geology), age of the Late Cretaceous epoch (geology), Epoch or the lowest stage (stratigraphy), stage of the Upper Cretace ...

. It is one of the oldest monotremes discovered, and is one of the oldest

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

n mammal discoveries. Several other monotremes are known from the Griman Creek Formation, including '' Dharragarra'', '' Kollikodon'', '' Opalios'', '' Parvopalus'', and '' Stirtodon''.

Taxonomy

The

dentition Dentition pertains to the development of teeth and their arrangement in the mouth. In particular, it is the characteristic arrangement, kind, and number of teeth in a given species at a given age. That is, the number, type, and morpho-physiology ...

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

theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...

ns—the placentals and the

marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...

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 toot ...

tooth A tooth (: teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tea ...

which was thought to be exclusive to therians since the Cretaceous. This, along with the tribosphenic molar discoveries of monotreme-relatives ''

Ausktribosphenos ''Ausktribosphenos'' is an extinct genus of mammals from the Early Cretaceous of Australia, sometime between 121 - 113 Ma during the Aptian. The only recorded species, ''Ausktribosphenos nyktos'', was found on Flat Rocks in the Wonthaggi Formati ...

'' and ''

Ambondro mahabo ''Ambondro mahabo'' is a mammal from the Middle Jurassic (Bathonian) Isalo III Formation (about 167 million years ago) of Madagascar. The only described species of the genus ''Ambondro'', it is known from a fragmentary mandible, lower jaw with ...

'' of which the latter

evolved Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

in the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

, led to the conclusion that the molar evolved independently in the two lineages. This inspired the creation of the subclasses

Australosphenida The Australosphenida are a clade of mammals, containing mammals with tribosphenic molars, known from the Jurassic to Mid-Cretaceous of Gondwana. Although they have often been suggested to have acquired tribosphenic molars independently from those ...

—the monotremes and extinct relatives—and

Tribosphenida Tribosphenida is a clade of mammals that includes the ancestor of ''Hypomylos'', Aegialodontia and Theria (the last common ancestor of marsupials and placentals plus all of its descendants). It belongs to the group Zatheria. The current definitio ...

—placentals and marsupials. However, given this

classification Classification is the activity of assigning objects to some pre-existing classes or categories. This is distinct from the task of establishing the classes themselves (for example through cluster analysis). Examples include diagnostic tests, identif ...

is based only on

jaw The jaws are a pair of opposable articulated structures at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth ...

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 ''Teinolophos'', from Ancient Greek τείνω (''teínō''), meaning "extend", and λόφος (''lóphos''), meaning "crest", is a genus of Teinolophidae, teinolophid monotreme that lived during the late Barremian Age (geology), age of the Ear ...

'' that was moved into its own family, Teinolophidae, by

Flannery Flannery is a bridge convention using a 2 opening bid to show a hand of minimal opening bid strength (11-15 high card points) with exactly four spades and five (or sometimes six) hearts. It was invented by American player William (Bill) L. Flann ...

''et al.'' (2022). Edentulous partial

mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...

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

may belong to a new genus and species of

stem Stem or STEM most commonly refers to: * Plant stem, a structural axis of a vascular plant * Stem group * Science, technology, engineering, and mathematics Stem or STEM can also refer to: Language and writing * Word stem, part of a word respon ...

ornithorhynchid The Ornithorhynchidae are one of the two extant families in the order Monotreme, Monotremata, and contain the platypus and its extinct relatives. The other family is the Tachyglossidae, or echidnas. Within the Ornithorhynchidae are the main Cen ...

. 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 silicon dioxide, silica (SiO2·''n''H2O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as a ...

ised

with three molars, discovered at the Griman Creek Formation,

Lightning Ridge, New South Wales Lightning Ridge is a small outback town in north-western New South Wales, Australia. Part of Walgett Shire, Lightning Ridge is situated near the southern border of Queensland, about east of the Castlereagh Highway. The Lightning Ridge area ...

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

for the

Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...

, 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 (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

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 The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal endemic to Eastern states of Australia, eastern Australia, including Tasmania. The platypus is the sole livi ...

''.

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 mammals of Australia

Taxonomy

Description

See also

References

Further reading

External links