A tiny chunk of Australia seeded Southeast Asia's tropical forests 50 million years ago, plant fossils preserved in amber reveal

Southeast Asia's tropical forests owe their rich biodiversity to a tiny fragment of Earth's crust that split from what is now northwestern Australia roughly 150 million years ago, a new study reports.

The fragment transported hundreds of flowering species and a flourishing collection of other ancient plants north across the prehistoric Tethys Ocean, the research reveals. The findings contradict previous theories that the fragment originated closer to what is now Southeast Asia and that Southeast Asia's tropical forests evolved sometime after the extinction of the dinosaurs around 66 million years ago.

At the time the fragment formed, Australia was connected to Antarctica and India within the ancient supercontinent Gondwana, which the Tethys Ocean separated from another ancient supercontinent, called Laurasia.

After drifting northward for 100 million years, the microplate eventually collided with Southeast Asia, seeding tropical forests that evolved into the lush ecosystems found in this part of the world today, the researchers said.

"The rich tropical rainforest flora of [Southeast Asia] is not endemic but owes much of its biota to the great southern land, Gondwana," study co-author Byron Lamont, a distinguished professor emeritus in Curtin University's School of Molecular and Life Sciences in Australia, told Live Science in an email.

But the collision with Southeast Asia was far from smooth, Lamont said. The fragment, which is known as the West Burma Block (WBB) or the Burma terrane due to its location in western Myanmar, wasn't the only piece of crust crossing the Tethys Ocean around 100 million years ago. The Indian subcontinent was also moving north, and it was doing so at a relatively rapid speed of 4 inches (10 centimeters) per year.

Around 80 million years ago, the Indian Plate bumped into the WBB, altering the fragment's orientation. The Indian Plate continued to shove the WBB until they both collided with Asia. The Indian Plate then slid beneath the Eurasian Plate to form the Himalayas, and the WBB ground to a halt to the west of present-day Myanmar, where it created a deep valley between mountain ranges about 35 million years ago, Lamont explained.

Both the Indian Plate and the WBB "took some time to stop," he said.

From the time the WBB broke off from Gondwana until its collision with Southeast Asia starting 50 million years ago, the microplate spent tens of millions of years hovering near the equator, where it was humid and hot like it is today. This climate and the fragment's isolation provided a refuge where ancient plant lineages could survive and diversify, Lamont said.

For the study, Lamont and his colleagues examined more than 3,000 plant fossils from Myanmar that were preserved in amber around 100 million years ago. The fossils included flowers, leaves, fruits and seeds. They belonged to at least 392 types of flowering plants and a vast array of conifers, ferns, mosses, cycads and liverworts that reached Southeast Asia by floating on the WBB.

Amber has been mined from the north end of the WBB valley for at least a century, Lamont said. "Its beautiful fossils, preserved in 3D, have long attracted interest for their novelty value alone and there has been much commercial interest in them, especially from Chinese traders," he noted.

Some of the study's co-authors acquired chunks of amber from traders at least a decade ago, and they have been studying these pieces ever since. Advice was often required from experts at other institutions around the world, which is how Lamont got involved in the study, he said.

The findings, published Oct. 27 in the journal Nature Ecology & Evolution, reveal the deep evolutionary roots of the tropical forests that grow in Southeast Asia today. The WBB didn't strip Australia of its ancient flora, because the descendants of many plants found in the 100 million-year-old amber still exist in the country's Northeast, which is hot and humid, Lamont said.

It's hard to say what kinds of animals arrived in Southeast Asia via the WBB, because only creatures small enough to be preserved in amber are represented in this fossil assemblage.

"The animals present in the amber are small insects, such as ants and mosquitoes," Lamont said. "Typical Australian vertebrates, such as marsupials, emus and nectar-feeding birds, arrived from South America in Australia over 100 million years later [after the WBB split off], and of course they would not have got caught in the resin even if they had been around."

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