The Ornithorhynchidae are one of the two extant families in the order Monotremata, and contain the

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

and its extinct relatives. The other family is the Tachyglossidae, or

echidna Echidnas (), sometimes known as spiny anteaters, are quill-covered monotremes (egg-laying mammals) belonging to the family Tachyglossidae . The four extant species of echidnas and the platypus are the only living mammals that lay eggs and the ...

s. Within the Ornithorhynchidae are the

genera 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 nomenclat ...

Monotrematum ''Monotrematum sudamericanum'' is an extinct monotreme species from the Paleocene (Peligran) Salamanca Formation in Patagonia, Argentina. It is the only monotreme found outside Oceania. Taxonomy A species described in 1992 and assigned to a new ...

'', ''

Obdurodon ''Obdurodon'' is a genus of extinct platypus-like Australian monotreme which lived from the Late Oligocene to the Late Miocene. Three species have been described in the genus, the type species ''Obdurodon insignis'', plus ''Obdurodon dicksoni'' ...

'', and ''

Ornithorhynchus 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 monotyp ...

'': *Family Ornithorhynchidae **Genus † ''Monotrematum'' ***†'' Monotrematum sudamericanum'' **Genus †''Obdurodon'' — an ancient branch of the platypus family ***†''

Obdurodon dicksoni ''Obdurodon'' is a genus of extinct platypus-like Australian monotreme which lived from the Late Oligocene to the Late Miocene. Three species have been described in the genus, the type species ''Obdurodon insignis'', plus ''Obdurodon dicksoni'' ...

'' ***†''

Obdurodon insignis ''Obdurodon'' is a genus of extinct platypus-like Australian monotreme which lived from the Late Oligocene to the Late Miocene. Three species have been described in the genus, the type species ''Obdurodon insignis'', plus ''Obdurodon dicksoni'' ...

'' ***†''

Obdurodon tharalkooschild ''Obdurodon'' is a genus of extinct platypus-like Australian monotreme which lived from the Late Oligocene to the Late Miocene. Three species have been described in the genus, the type species ''Obdurodon insignis'', plus ''Obdurodon dicksoni'' a ...

'' **Genus ''Ornithorhynchus'' ***''

Ornithorhynchus anatinus 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 monotyp ...

'' (the modern

) Another two genera, ''

Steropodon ''Steropodon'' is a genus of prehistoric monotreme, 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 period. It is one of the oldest ...

'' and ''

Teinolophos ''Teinolophos'' is a prehistoric species of monotreme, or egg-laying mammal, from the Teinolophidae. It is known from four specimens, each consisting of a partial lower jawbone collected from the Wonthaggi Formation at Flat Rocks, Victoria (Aus ...

'', were originally thought to belong to the Ornithorhynchidae. However, they were both placed into a new family, the Steropodontidae. This decision was made based on differences in the

dentary 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 tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...

recovered from the Griman Creek Formation, Lightning Ridge, New South Wales, Australia. This dentary is 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 several ...

for the genus ''Steropodon'', thus the lack of information led to the original misclassification. Further research on ''Teinolophos'' has indeed shown it to be an animal much different from ornithochrynchids, lacking a beak, possessing a more complete mammalian dentition, and retaining primitive ears connected to the jaw as in more basal mammals. In 2022 it was proposed to move ''Steropodon'' into its own family,

Teinolophidae Teinolophidae is an extinct family of small, carnivorous mammals that were the earliest known monotremes and were endemic to what would become Australia. Two genera are known: ''Teinolophos'', and possibly also '' Stirtodon''. ''Teinolophos'' is ...

. The extinct ''Ornithorhynchus maximus'' has been included in ''Ornithorhynchus'', but later placed with the echidna family Tachyglossidae as ''

Zaglossus robustus ''Zaglossus robustus'' is an extinct species of long-beaked echidna known from the middle Miocene (about 13 or 14 million years ago) of Gulgong, New South Wales, Australia. It may belong in the genus '' Megalibgwilia''. The supposed fossil plat ...

''.

References

* {{Taxonbar, from=Q21811 Taxa named by John Edward Gray Extant Selandian first appearances