Southeast Asia's tropical forests may owe much of their diversity to a fragment of crust that split from what is now northwestern Australia about 150 million years ago, Live Science reported. A new study argues that the fragment, known as the West Burma Block or Burma terrane, carried hundreds of flowering species and other ancient plants north across the prehistoric Tethys Ocean and later collided with Southeast Asia, starting about 50 million years ago. The findings contradict earlier ideas that the fragment originated closer to Southeast Asia and that the region's tropical forests evolved only after the dinosaur extinction around 66 million years ago.
"The rich tropical rainforest flora of [Southeast Asia] is not endemic but owes much of its biota to the great southern land, Gondwana," co-author Byron Lamont, a distinguished professor emeritus at Curtin University in Australia, told Live Science. At the time the fragment formed, Australia was still connected to Antarctica and India within Gondwana. The journey was not a straight drift. The Indian subcontinent was also moving north, at about 4 inches (10 centimeters) a year. Around 80 million years ago, the Indian Plate bumped the West Burma Block, altering its orientation, then shoved it until both collided with Asia. The Indian Plate later slid beneath Eurasia to form the Himalayas, and the block ground to a halt west of present-day Myanmar, where it created a deep valley between mountain ranges about 35 million years ago, Lamont said.
From the time it broke off until that collision, the microplate spent tens of millions of years near the equator, humid and hot, a refuge where ancient plant lineages could survive and diversify. For the study, published Oct. 27 in Nature Ecology & Evolution, Lamont and colleagues examined more than 3,000 plant fossils from Myanmar preserved in amber around 100 million years ago: flowers, leaves, fruits, and seeds from at least 392 types of flowering plants, plus conifers, ferns, mosses, cycads, and liverworts. Amber has been mined from the north end of the West Burma Block valley for at least a century. Some co-authors acquired chunks from traders at least a decade ago and have been studying them since.
The block did not strip Australia of those lineages, Lamont said, because descendants of many plants in the amber still grow in the country's hot, humid northeast. It is harder to say what animals made the same trip. Only creatures small enough to be trapped in resin are in this assemblage: "small insects, such as ants and mosquitoes," he said. Typical Australian vertebrates such as marsupials, emus, and nectar-feeding birds arrived from South America more than 100 million years later, after the block had already split off, and would not have been caught in the resin anyway. The verified argument is about plants, timing, and a raft of crust, not a complete passenger list of the Australian fauna.