Fossil leaves provide clues to ancient Australian habitat

Fossil leaves provide clues to ancient Australian habitat

by Adityarup “Rup” Chakravorty Tuesday April 19th, 2016

Heather and scrubland cover this part of Stirling Range National Park in southwestern Australia. In the Upper Cretaceous, central Australia may have looked alike, with a mosaic of low, open vegetation and wooded thickets. Credit: Raymond Charpentier.

Researchers have long believed that Australia’s scrublands developed over the past 25 to 30 million years as part of a global trend towards colder, drier climates in which rainforests have given way. to more open and fire-prone environments.

However, results from recent fossil pollen studies suggest that open habitats were common throughout Australia much earlier. And now the findings of a new study based, in part, on the first detailed analysis of Australian leaf fossils from the Late Cretaceous support the idea that open vegetation and the influence of fire on the landscape were widespread in Australia il over 66 million years ago. since.

“Our findings force us to question the idea that rainforests were the dominant biome across Australia 80 to 60 million years ago,” says Raymond Carpenter, a paleontologist at the University of Tasmania and the University of Adelaide, both in Australia, and lead author. of the new study published in the American Journal of Botany.

Carpenter and his colleagues extracted both microfossils – pollen and spores, for example – and macrofossils, such as leaves, from a soil core taken from the Bundey Basin in central Australia. Many species represented by microfossils found in the core had previously been observed and dated using stratigraphy at other locations. The team was thus able to use these known microfossils to determine the age of specific sediment layers and leaf fossils in the core studied.

The presence of fossil fragments of leaves in the core allowed them to draw up a detailed morphological picture of the plants growing in this region 70 million years ago. The fossil leaves of at least three of the plants identified were “very small” – less “than 2.5 millimeters wide – and several of the leaf fragments examined” had stomata [pores used for respiration] located on both sides,” says Carpenter — two characteristics of plants suited to grow in open environments, not rainforests.

In addition to fossil leaf features that indicated plants adapted to open environments, Carpenter’s team also found charcoal in the same sediment layers, indicating that the ecosystem experienced episodes of burning.

Additionally, more than half of the fossil pollen has been identified as belonging to various plants in the family Proteaceae, a large southern hemisphere flowering plant family with over 1,700 living species. Although modern members of the Proteaceae “live in different habitats, several members live in open areas and are adapted to fire,” says Carpenter, another indication that scrub, not rainforest, likely dominated Cretaceous vegetation. superior in Australia.

Fossils can be a keystone when building a picture of ancient ecosystems, says Michael Crisp, a researcher at the Australian National University in Canberra who was not involved in the study. “Fossils provide the only tangible, concrete evidence of the past,” he says, “but the fossil record is sparse and often quite patchy.”

It is rare to find the combination of micro- and macrofossils analyzed by Carpenter and his colleagues in one place; that is why their findings are compelling,” says Maria A. Gandolfo, a paleobotanist at Cornell University who was not involved in this study. “This is probably the first article in which [leaf] cuticle remains are used in combination with fossil pollen information to infer the presence of an entire biome,” she says.

While the microfossils and leaf fossils discovered and analyzed in this study support the idea that scrubland vegetation was widespread across Australia much earlier than previously thought, both Crisp and Gandolfo say they remain cautious about extrapolating an entire ecosystem from these fossils alone.

“It is, after all, a data point,” says Crisp: the fossils come from a single nucleus.

Gandolfo shares this concern: “We only know about 5% of the fossil record, and as we discover more and more fossils, we tend to change our perceptions and therefore our conclusions,” she says.

Carpenter and his colleagues are working on examining materials from other nuclei, he says. But, he adds, the distribution of pollen and spores in the nucleus examined in this study matches the pollen-spore assemblies [in other cores] of similar age, which he says gives him confidence that “in Late Cretaceous Australia, instead of huge tracts of rainforest, there was probably a mosaic of rainforest-like plants and open habitats influenced by fire”.