Scientists studying fossils from northwestern Queensland say they have identified an entire branch of the marsupial family tree that fits none of the groups alive today. The evidence is small: a handful of teeth and jaw fragments. The implications are large.
Writing in the Journal of Paleontology, a team from the University of New South Wales describes three new species of small, insect-eating marsupials from the Riversleigh World Heritage Area, one of the world's richest fossil sites. The researchers argue the animals are distinct enough to warrant a new order, which they have named Keeunamorphia.
Small animals, big claim
The three species lived in lush rainforest about 18 million years ago and weighed between 25 and 200 grams, ranging in size from a shrew to a mouse. Fully preserved skeletons are rare at Riversleigh, so the team worked from teeth and jaw pieces, combining them with genetic data from living species to reconstruct how the animals were related.
What caught the researchers' attention was the shape of the teeth. They closely resembled those of Djarthia murgonensis, an extinct marsupial that lived roughly 35 million years earlier and is often treated as a prototype for Australia's marsupials. That similarity, the team says, points to a separate lineage that branched off very early and then quietly survived for a long time.
“"Whatever these things were, they seemed to be primitive compared to other marsupials at the time, and they seem to have been doing their own thing and surviving well enough alongside them," said Dr Tim Churchill, the study's lead author.”
A more tangled family tree
Marsupials are pouched mammals, and Australia is their stronghold, with about 160 living species filling almost every ecological niche. The textbook account holds that they descended from a single ancestral lineage that reached Australia from South America, via Antarctica, more than 50 million years ago, when the southern continents were still joined as part of Gondwana.
The new order complicates that neat picture. UNSW reports that Keeunamorphia may have Gondwanan origins and could be one of the oldest known lineages of Australian marsupials. If so, it suggests several primitive groups coexisted early on, rather than one line giving rise to everything seen today.
“"Not only is it a new order, but it could also be one of the most ancient lineages of Australian marsupials," Dr Churchill said.”
Why it matters
Australia's living marsupials fall into orders grouped within a superorder called Australidelphia. Churchill is proposing a sixth order and estimates it endured for around 35 million years, from deposits at Murgon about 55 million years ago to Riversleigh around 18 million years ago, before disappearing about 15 million years ago as the climate cooled and rainforest gave way to open woodland.
No fossils from the group have turned up in younger rocks, so the lineage appears to have died out leaving no descendants. The researchers stress that much of this history remains hidden in gaps in the fossil record, which means the earliest chapters of marsupial evolution are still being written.
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