‘Echidnapus’ hints at a lost age of egg-laying mammals

The Australian platypus is one of Earth’s most unusual creatures — but there was a time when it might not have stood out in a crowd. In roughly 100-million-year-old rocks in Australia, scientists have unearthed three new species of monotremes, a group of egg-laying mammals that today include only the platypus and another Australian oddball, the echidna (SN: 11/18/16).

The fossil discoveries double the number of known monotreme species during this brief span of the Cretaceous Period, hinting at a bygone Age of Monotremes, mammal biologist Timothy Flannery and colleagues report May 26 in Alcheringa: An Australian Journal of Palaeontology.

Australia today is thought of as a land of marsupials, mammals that nurture developing young in pouches, including kangaroos, koalas and wombats. But a whole “civilization” of diverse monotremes, ranging from pig-sized to rat-sized, may have radiated across the continent first, says Flannery, of the Australian Museum in Sydney.

The modern platypus, native to eastern Australia and Tasmania, is one of the weirdest creatures on Earth (SN: 12/3/14). The creatures have a toothless, ducklike bill, a beaverlike tail and otterlike feet; their bills are also electro-sensory organs that allow them to detect prey in murky waters. Male platypuses even produce venom, delivered via spurs on the rear feet. The platypus’ Frankensteinian combination of parts was so surprising to 18th century European biologists that many initially thought it to be a hoax.

The three new Cretaceous species, along with new specimens of three previously identified extinct monotremes, were identified from fossil teeth and jaws found in the Lightning Ridge opal fields in New South Wales. These modest fossils, particularly when compared with previously found fossil monotremes, provide a wealth of information, the team notes: the animals’ relative sizes, number and orientation of teeth, even likely position on the family tree.

One of the newfound species, officially named Opalios splendens, earned the nickname “echidnapus” for its mix of features, which are found in modern echidnas and platypuses. These traits suggest it was an ancestor of both. Another of the new species, Parvopalus clytei, is among the smallest monotremes ever found, roughly the size of a rat. The third, Dharragarra aurora, is the earliest known species of platypus.

The discoveries also reveal a slow progression in monotremes from toothy to toothless. The earliest known monotreme, Teinolophos trusleri, was a beakless, shrew-sized creature that lived about 130 million years ago. It had five molar teeth — but by 100 million years ago, the newly discovered species suggest, some monotremes had only three molars, while modern platypuses and echidnas are essentially toothless. That shift may be related to a change in diet to softer, more slippery food — leaving crunchier fare like crustaceans and insects to a newly arrived competitor from New Guinea, the water rat.