Fossils of some of Earth’s first macroscopic animals – the roughly 550-million-year-old Ediacara biota – have been identified in rocks that record Australia’s ancient shorelines.
Scientists had thought that these archaic lifeforms lived out to sea in calmer waters. But the new study, published in the Journal of Sedimentary Research, has identified thriving communities of Ediacaran fossils preserved in the Flinders Ranges, South Australia, where the rocks were laid down in shallow coastal waters.
The discovery shows that these early organisms had already evolved traits to deal with waves and tidal currents on what was a turbulent coastline half a billion years ago. It also helps scientists understand how these early organisms may have lived, and gives us more information on the timing of their ultimate demise.
Ediacaran fossils are found in only a handful of places on Earth. One of the youngest sites is in Namibia, where species diversity is far reduced compared with the older Australian examples and others like it. Scientists had linked this to the fact that the rocks in Namibia were deposited in shallow, turbulent waters, which most organisms at the time weren’t equipped to handle.
The new finding from Australia shows that Ediacaran organisms were in fact able to inhabit harsh, near-shore conditions. The reduced diversity observed in the younger Namibian deposits might therefore archive genuine evolutionary change, for example a mass extinction, potentially the first such event on Planet Earth.
“Ediacaran macrofossils offer valuable information about the early history of large and complex organisms on Earth. Now we know these organisms were able to live in water depths shallow enough to paddle in, we can start to reassess how and where the early ancestors of several living animal groups originally evolved and diversified” said lead author William McMahon, a postdoctoral researcher at the University of Hull.
By studying the different layers of rocks in South Australia, McMahon and co-authors were able to ‘time-travel’ to a period when the region was inhabited by otherworldly and long-extinct organisms like Dickinsonia and Spriggina - dinner-plate or worm-shaped creatures, respectively, that lived and fed on a layer of algal and bacterial slime.
“Our discovery that the Australian Ediacaran fossils are found in rocks also preserving widely recognisable coastal sedimentary features like ripples and dunes proves that these early animals were active and abundant in very shallow waters” said Alex Liu, lecturer at the University of Cambridge and co-author on the study.
The team spent several weeks searching for well-preserved sequences of fossil-bearing rocks. Careful documentation of different sedimentary structures preserved in the rocks allowed them to build a picture of how the environment changed through time, and how the organisms that lived within them adapted to and interacted with those changes.
Studies of Ediacaran fossils in the Flinders Ranges can be traced back to 1947 and Reginald Sprigg’s original report of fossil “jellyfish” from the Ediacara Hills, a name subsequently given to the “Ediacaran System” of geological time. Research from the region has thrown significant light on the early history of multicellular life. The recognition that the organisms had established a foothold in coastal waters opens up new avenues for research into their evolution and ultimate disappearance.
Animal life would not make it fully onto the land until a later period of time known as the Silurian. However, the new discovery in Australia suggests Earth’s coastal waters were already home to a diverse range of creatures over 100 million years earlier.
McMahon, William, et al. "Ediacaran life close to land: coastal and shoreface habitats of the Ediacaran macrobiota in the central and southern Flinders Ranges, South Australia." Journal of Sedimentary Research (2020). DOI: 10.2110/jsr.2020.029