The three newly-identified species show combinations of features not previously seen before in other living or fossil monotremes, according to Professor Kris Helgen from the Australian Museum Research Institute and colleagues.
Six monotremes living in the same place at the same time, 100 million years ago at Lightning Ridge, New South Wales, Australia; clockwise from lower left: Opalios splendens; Stirtodon elizabethae, the largest monotreme of the time; Kollikodon ritchiei, with hot-cross-bun shaped molars; Steropodon galmani, now known from additional opalized fossils; Parvopalus clytiei, the smallest monotreme of the time; and Dharragarra aurora, the earliest known species of platypus. Image credit: Peter Schouten.
Professor Helgen and co-authors unearthed and examined a large assemblage of monotreme fossils in the Lightning Ridge opal fields, New South Wales, Australia.
The fossils date back to the Cenomanian age of the Cretaceous period, between 102 million to 96.6 million years ago.
They were found by Elizabeth Smith and her daughter Clytie of the Australian Opal Centre, who have spent decades working and searching over the opal fields.
“Opal fossils are rare, but opalized monotreme fossils are infinitely more rare, as there’s one monotreme fragment to a million other pieces,” Elizabeth Smith said.
“We don’t know when, or exactly where, they’ll turn up.”
“Our research reveals that 100 million years ago, Australia was home to a diversity of monotremes, of which the platypus and the echidna are the only surviving descendants,” said Australian Museum’s Professor Tim Flannery.
“Today, Australia is known as a land of marsupials, but discovering these new fossils is the first indication that Australia was previously home to a diversity of monotremes. It’s like discovering a whole new civilization.”
One of the newly-described species, Opalios splendens, retains characteristics of the earliest known monotremes, but also some that foreshadow adaptations in the living monotremes, the echidnas and platypus.
“Opalios splendens sits on a place in the evolutionary tree prior to the evolution of the common ancestor of the monotremes we have today,” Professor Helgen said.
“Its overall anatomy is probably quite like the platypus, but with features of the jaw and snout a bit more like an echidna — you might call it an ‘echidnapus’.”
“The story of how our egg-laying mammals evolved is ‘toothy to toothless’ on the oldest monotreme, Teinolophos trusleri, which dates back to Victoria 130 million years ago.”
“What we see at Lightning Ridge is that by 100 million years ago, some of the monotremes still have five molars but some of them are down to three.”
“Today, echidnas have no teeth, and platypuses too are essentially toothless,” Professor Flannery said.
“Adult platypuses have no teeth, though juveniles have rudimentary molars.”
“Just when and why adult platypus lost their teeth after nearly 100 million years is a mystery we think we have solved.”
“It may have been competition with the Australian water rat, which arrived in Australia within the last 2 million years, which caused platypus to seek out softer, slipperier food best processed with the leathery pads that adults use today.”
“What is so unusual about this uniquely Australian story is that in one snapshot we see six different egg-laying mammals living together in Lightning Ridge over 100 million years ago.”
“All of them are holding potential evolutionary destinies that can go off in different directions, and all of them are deep distant ancestors and relatives of the current living monotremes.”
“The discovery of three new genera of monotremes helps to piece together their remarkable evolutionary story,” said Dr. Matthew McCurry, curator of paleontology at Australian Museum.
“There are six species of monotremes, including the three newly described here, within the Cenomanian Lightning Ridge fauna of New South Wales making it the most diverse monotreme assemblage on record.”
“Four species are known from a single specimen, suggesting that diversity remains underrepresented.”
“This discovery adds more than 20% to the previously known diversity of monotremes.”
“We have very few monotreme fossils, and so finding new fossils can tell us more about where they lived, what they looked like and how changes in the environment influenced their evolution.”
“Every significant monotreme fossil currently known fits into this evolutionary story, from Teinolophos, the tiny shrew-like creature in Antarctica 130 million years ago to the present day.”
“The platypus and echidna are iconic Australian species,” said Museums Victoria Research Institute paleontologists Thomas Rich and Patricia Vickers-Rich AO.
“The discovery of these several new species in one small area suggests that the family tree of the egg laying monotremes is far more complicated than the living platypus and echidna alone suggest.”
“As the fieldwork continues in the Mesozoic of Australia, we continue to increase our understanding of how life changed over time. This, to me, is what makes science so exciting.”
The team’s paper is published in the Alcheringa, an Australasian Journal of Palaeontology.
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