Were there rainforests in Australia’s deserts?

Australia’s dry, red centre used to be green, according to fossil evidence of plants that thrive in rainforests. An international research team says it’s the first time the plants have been noted in the Gondwanan southern hemisphere.

“These fossils show that the evolutionary history of the family is much more complex than previously thought,” says Andrew Rozefelds from the Queensland Museum, lead author of a paper in the journal Historical Biology.

The Eocene fossils, dated from 56 to 34 million years ago, were from distinctive Icacinaceae fruits discovered near Lake Eyre in northern South Australia and Tasmania’s Tamar Valley.

Icacinaceae is a flowering plant family that includes tropical vines, low shrubs and trees, with 23 known genera worldwide. Modern plants are found in tropics across southeast Asia, the central Americas, central Africa and Madagascar.

Two species of the fruits were discovered in Australia, and combined with other leaf and fruit fossils and the pollen record suggest the continent was prehistorically dominated by rainforest communities.

Although the plant family isn’t well known, palaeobotanists recognise its importance for understanding the history of the world’s tropical forests.

It was a serendipitous discovery. Rozefelds spotted two specimens from Tasmania in London’s Natural History Museum, which had been collected by natural historian Joseph Milligan around the 1840s.

Collaborating with Icacinaceae expert Greg Stull from Washington’s Smithsonian Institution, Rozefelds and colleagues matched the fossils to specimens that he had been sent many years previously from South Australia’s geological survey, which they attributed to the Phytocreneae tribe.

It’s an example of the rich trove of biological diversity languishing in museums that can go undetected without close inspection by experts.

Stull remarks that the new discoveries “are the first unequivocal evidence of the family from Australia and they show that at least two species occur in the continent”.

The South Australian fossils had unique features, earning them a new genus named Machesteria australis in honour of palaeobotanist Steven Manchester from the University of Florida.

Most Icacinaceae fossils have been dated to the Eocene, an era with warmer and wetter climates than today, with higher global temperatures. In Australia, the most closely related modern plants are limited to the tropics of northeast Queensland.

The fossil record suggests that the family – at least the Phytocreneae tribe – might have originated in North America and Europe before diversifying and moving south.

The latest insight, made possible by modern molecular analysis, challenges that view, the authors say, suggesting the plants were distributed more broadly by the middle of the Eocene – possibly migrating through Antarctica.

“This suggests that the modern distribution of Phytocreneae has largely been shaped by regional extinction/extirpation since the Eucoene,” they write, “likely driven by post-Eocene climatic cooling.”

In Australia, they say, this probably happened during the later Cenozoic as the continent became drier and cooler.