Early and odd: A fossil of Dickinsonia a primitive animal from the Ediacara Hills of South Australia.
Credit: University of California Museum of Palaeontology
SYDNEY: Earth's earliest complex animals were confined to ancient estuaries before a change in ocean chemistry may have allowed them to spread around the globe, a new study suggests.
The authors argue that a lowering of oceanic salinity may have encouraged these primitive invertebrates to spread across the planet and replace the single-celled life which had dominated the Earth for two billion years.
The strange, soft-bodied animals first appeared 575 million years ago during the Ediacaran geological period. They lived in the ocean and some grew to two metres in length, but they disappeared from the fossil record when new animals with hard skeletons and shells evolved.
Ediacaran animals
Ediacaran fossils are preserved in only a few places in the world, and some of the best examples are found in the foothills of the Flinders Ranges near Adelaide in South Australia. Why such complex organisms, which left no progeny, appeared so suddenly is the centre of continuing scientific debate.
Theories on the factors that influenced the development of the Ediacaran biota include the erosion of a giant mountain range (see, Erosion of vast mountain range led to explosion of early life, Cosmos Online), which dumped nutrients into the oceans and led a rise in oxygen. Or, that the “sweetening” could be due to freshwater injected into the oceans from melting ice caps, as the world oscillated between ice ages and much warmer periods.
While many complex geological scenarios were playing out during the Ediacaran, something “acted as a trigger to allow the metazoans to take world stage,” said Pat Vickers-Rich, a palaeontologist with Monash University in Melbourne.
She has found evidence that the formation of massive salt deposits may have primed the seas, by lowering salinity and opening them up to become “global playgrounds” for the Ediacaran animals. “Taking the salt out of the oceans could have been one of the major drivers allowing [them] to develop and go global,” Vickers-Rich told Cosmos Online.
She presents her findings today at the Geological Society of Australia's Selwyn Symposium, at the University of Melbourne.
Much-needed oxygen
Discovering exactly when and how the salt deposits formed is part of ongoing research, however. “Some of the salt deposits could have resulted from conditions in small, widening seaways where circulation… was restricted,” said Vickers-Rich, something like the Red Sea today.
Astrobiologist Malcolm Walter, from the University of New South Wales in Sydney, the symposium’s plenary speaker, questioned the timing of the salt deposits, though.
There are some deposits dating from much earlier that don’t fit the theory, Walter said – for example a large salt deposit in central Australia that was 830 to 800 million years old, well before the evolutionary event.
Walter said the salt theory was a variation on another theory being presented at the symposium; that of the erosion of a giant mountain range contributing to a rise in oxygen.
“This is another version – it makes oxygen more soluble in the ocean by decreasing salt concentrations. Both ideas say that one of the key drivers [of the evolution] was passing a certain oxygen threshold,” he said.
