An iconic fossil of the Ediacaran biota, Dickinsonia costata.
Credit: Wikimedia
DUBLIN: Scientists have peered into the depths to see how life survived in the oxygen-poor oceans of the Ediacaran, the ancient period when mobile animals first evolved.
Studies in modern tropical lagoons in Venezuela suggest that 'biomats' of photosynthetic bacteria offer a refuge of oxygen-rich sediment in otherwise inhospitable waters for small animals. The conditions are comparable to those from the dawn of mobile life, said the scientists.
Ediacaran seafloors were covered by biomats, with little exposed sediment. "Slug and worm-like animals lived on, within and immediately beneath the mat," said lead author Murray Gingras of the University of Alberta, Canada, which "served as a food and oxygen resource for the early animals".
"By studying modern-day lagoons, we hoped to understand how animals evolved during the Ediacaran," said Gingras, who believes mobility in early animals was inspired and enabled by biomats.
Dawn of mobility
Complex animals first evolved during the Ediacaran period, between 635 and 542 million years ago, when the oceans were just becoming oxygenated. Oxygen levels may have been a mere 10% of today's levels.
The high-salinity Venezuelan lagoons are shallow, warm and low in oxygen. They are largely devoid of seafloor animals, except for small burrowing shore crabs and insect larvae that lived only in the mats. These qualities led the researchers to select them as a good match for Ediacaran waters.
For the study, published in the current issue of Nature Geoscience, Gingras and his colleagues examined an association of modern animals that looked similar. During the day, oxygen levels in the biomats were four times higher than in the overlying water columns, and the animals breathed and fed from the mats.
The early mobile animals may have evolved in such an environment, living in the biomats and forming burrows similar to those found in Ediacaran-aged rocks, said the scientists.
An ocean relic
The suggestion that the advent of mobility may have taken place within the oxygen oases appears to be consistent with the sedimentary record - the trace fossil record and evidence from Ediacaran ocean chemistry - said palaeontologist Aaron Sappenfield of the University of California.
"Among the abundant evidence of Ediacaran microbial life are simple horizontal trace fossils, providing robust evidence of the coexistence of benthic metazoans (a type of Ediacaran biota) and dense microbial communities at this time."
He noted that the current estimates for the average oxygen level of this seawater may have been insufficient to meet the demands of the majority of mobile animals. The microbial oases may have offered sanctuary, however.
