'Extraordinary' Fossil Discoveries Reveal Plants From 30 Million Years Ago

A study of fossil sites in Australia has revealed "exquisitely preserved" plants that lived millions of years ago.

The material in which the fossils described in the study occur is called silcrete. This hard substance is formed when soil, sand or gravel is cemented together by dissolved silica—a common mineral that is the major constituent of sand and many kinds of rocks.

In the study, published in the journal Gondwana Research, a team of scientists wanted to better understand silcrete plant fossil sites in Australia. In particular, their aim was to shed light on how plants were preserved in these localities, Andrew Rozefelds, an author of the study affiliated with Central Queensland University and the Queensland Museum, told Newsweek.

To do this, the researchers reviewed the published scientific literature and provided new data on additional sites. Previously, little research had been conducted on silcrete plant fossil sites.

The study authors concluded that such sites occur commonly in eastern Australia and are always associated with volcanic materials.

While Australia is now geologically stable, this was not always the case. Between around 40 and 20 million years ago, the eastern parts of Australia experienced significant volcanic activity.

"Erupting volcanoes can have devastating consequences for human settlements, as we know from Pompeii in Italy, which was buried by ash when Mount Vesuvius erupted in A.D. 79," Rozefelds wrote in a piece for The Conversation. "But ash falls and lava flows can also entomb entire forests, or at least many of the plants within them."

Fossil plants are often found in a flattened and compressed state, or they are preserved as impressions. But the plants from the silcrete sites—which mostly date to around 30-20 million years ago—are preserved in three dimensions and display little or no signs of compression.

"What is perhaps most extraordinary is that in some specimens soft tissue preservation results in the arils on seeds, the outer fruit wall, the bark of vines and root nodules to be preserved," Rozefelds said. (Arils are a type of covering found around some seeds, such as the red flesh covering pomegranate seeds. Root nodules, meanwhile, are knob-like structures formed on and from roots of certain plants, primarily legumes.)

This type of "exceptional" preservation is rarely found or evident in fossil plants that have been noticeably compressed, according to Rozefelds.

In order for such fine preservation to occur, the researchers determined that the plants would have been buried quickly by volcanic materials. There would also have been an abundant source of silica, which almost certainly resulted from the weathering of volcanic rocks, according to the study. The process in which silica infiltrates and preserves plant structures is known as "silicification."

The fact that the plants were rapidly entombed by volcanic material indicates that they would have been actively growing in their original location at the time. This scenario provides researchers with a golden opportunity to shed light on the nature of prehistoric plant communities.

"In other areas where plant fossils might accumulate—such as river deltas—we can never be sure how far the bits of plants were carried, and whether they were from different types of vegetation," Rozefelds wrote for The Conversation.

"Silicification not only preserves plants, but also leaf litter on the forest floor and even the underlying soil containing roots and root nodules. The fossil plants that are preserved at different sites varies, indicating the presence of distinct plant communities."

One site included in the study, for example, preserved a rainforest plant community with dozens of fruit/seed-bearing species, as well as emergent conifers and lianas, a type of woody vine. Other sites appear to be dominated by a single species, indicating that different types of plant communities were buried and preserved.

Some sites also preserved layers of ferns, indicating that these plants were growing on older lava flows or ash fields before being buried by subsequent eruptions. Ferns are among the first living plants to colonize terrain affected by volcanic eruptions thanks to their tiny wind-borne spores.

The "exceptional" preservation state of the silcrete fossils enables the researchers to compare them with modern plants, helping to identify them and understand their evolutionary relationships.

"The preservation of these plants in Australia can often be compared with living plants at a cellular level. The better the preservation the more that you can do with the fossils," Rozefelds said.

The fossil plants all probably represent new species, apart from those that have already been described in the scientific literature, according to Rozefelds.

"The challenge is to work out their affinities, i.e. what are they related to," he said. "This often requires comparison with modern floras in Australia and from further afield in the Indo-Pacific region, and also South America."

These regions—along with others—were all once part of an ancient supercontinent known as Gondwana that had fully formed around 600 million years ago and began to fragment roughly 180 million years ago.

"So remarkably preserved, the silcrete plant fossils are now providing startling new insights into the history of some groups of Australian plants and the vegetation types in which they grew," Rozefelds wrote for The Conversation.