The oldest discovered fossil of the modern platypus dates back to about 100,000 years ago, during the
Quaternary period. The extinct monotremes
Teinolophos and
Steropodon were once thought to be closely related to the modern platypus,
[67] but are now considered more basal taxa.
[71] The fossilised
Steropodon was discovered in
New South Wales and is composed of an opalised lower jawbone with three molar teeth (whereas the adult contemporary platypus is toothless). The molar teeth were initially thought to be
tribosphenic, which would have supported a variation of Gregory's theory, but later research has suggested, while they have three cusps, they evolved under a separate process.
[72] The fossil is thought to be about 110 million years old, making it the oldest mammal fossil found in Australia. Unlike the modern platypus (and
echidnas),
Teinolophos lacked a beak.
[71]