Green History – southern beeches and the role of vulcanicity in shaping the modern Australian flora

Fossil rainforest floras from northeastern Australia range in age from 35-20 million years and are a treasure trove of undescribed plants that remain largely unstudied. We wanted to answer some key questions about them. How were the fossil plants preserved? What plants were growing at these sites? How similar were these floras? What do these floras tell us about the origins and history of the modern Australian plant communities.

Perhaps, most surprisingly, all of the sites we studied are directly associated with the regional volcanism that was occurring at that time. In a high-altitude site in the Eungella region, near Mackay, 28-30 Ma sediments were deposited in a shallow lake and evidence of concurrent volcanism is seen in the masses of volcanic ash-fall pellets that occur in the oil shale. The flora is preserved as a compression flora and ash-fall pellets were responsible for the downing of flying insects that are found in the shale.

The Capella flora, in contrast, is the most diverse silicified fruit and seed flora known from Australia. The fossil plants were buried by volcanic activity (28-32Ma) and subsequently permineralised and the plants are typically preserved in remarkable detail (https://theconversation.com/a-botanical-pompeii-we-found-spectacular-australian-plant-fossils-from-30-million-years-ago-222512). Our studies demonstrated that these sub-basaltic silcretes occur commonly with volcanics throughout eastern Australia from Tasmania to Central Queensland. These studies recognized that two distinct types of silcretes occur in Australia, sub basaltic silcretes and silcretes preserved in fluvial sediments. The Eungella and Capella floras, while of similar age, differ significantly in the plant communities represented.

The Eungella high-altitude fossil forests are dominated by Nothofagus (Southern Beeches) and fossil pollen and leaf fossils at the site indicate that a number of Southern Beech species occurred in this region. Today only a single species, the Antarctic Beech (Nothofagus moorei) occurs on the Queensland/New South Wales border at high altitude, but the fossil record indicates that they were more widespread in Australia in the past.

Nothofagus is largely a southern Hemisphere genus that, today, occurring in southern Australia, New Zealand, South America, New Guinea and New Caledonia. There are four groups recognized in Nothofagus which can be identified using the characteristics of the modern pollen. At least three of these groups are recognized in the fossil pollen flora at Eungella which demonstrates that different groups of Nothofagus co-occurred together in northern Australia. One particular group, Brassospora, was particularly abundant and the dominant pollen type at Eungella. It is commonly recorded throughout the Cenozoic in pollen studies in northern Australia and no longer occurs in Australia as is now restricted to New Caledonia and New Guinea.

The Capella flora lacks Nothofagus but consists of a diverse rainforest assemblage with emergent conifers, an upper storey of rainforest trees and lianas that often have affinities with the World Heritage Wet Tropics rainforest communities of northeastern Queensland. In the Capella region we discovered the first fossil evidence of Pandanus and Melastomataceae in Australia. Rhizomes and leaves of bracken ferns (Pteridium) were also recorded from four sites in Eastern Australia.

Our studies demonstrate that these groups of plants are of southern (Gondwanic) origin as they were present in Australia prior to contact with the southeast Asian Island Arc. The silcrete floras because of their exceptional, three-dimensional preservation offer unique opportunities for comparative research with the contemporary Australian flora as evidenced by our current projects examining silicified wood of the Banksia family (Proteaceae) in Australia.

Our studies of Eungella and Capella floras show that distinct plant communities occurred in the region at this time and they provide the first macrofossil records of Nothofagus and other plant groups from northern Australia. We are revisiting the known fossil records of Nothofagus, including newly discovered material from a fossil site in Kingaroy, will allow us to document the history of the southern beeches in Northern Australia. These studies and ongoing work on the sub-basaltic silcrete floras provide unique opportunities to understand the history and evolution of the modern Australian flora, in a time of active volcanism.