(Inside Science) -- When the ancient supercontinent Gondwana took shape around the beginning of the Cambrian Explosion 541 million years ago, the kingdom of life known as Animalia burst into a kaleidoscope of new species. The ancestors of today's organisms developed blueprints that have subsequently led to our familiar tree of life, and paleontologists have found large deposits of them in fossils, most famously in a deposit in Canada known as the Burgess Shale.
But remnants of the mysterious period of time that led up to this explosion of life, known as the Ediacaran, are harder to find in the fossil record. Over the past few decades, scientists have slowly pieced together pictures of some of the period’s enigmatic life forms. These include the disc-shaped fossils known by their genus as Aspidella, as well as Dickinsonia, the oval-shaped creatures thought to be the first terrestrial animal. But scientists are still uncertain about how these organisms fit into evolutionary history.
In new research, Brazilian paleontologists unveiled a robust collection of fossils from the Ediacaran, all found in southeastern Brazil’s Itajai Basin. They published their findings online in the journal Gondwana Research. The researchers revealed fossils dated to 563 million years old, including animals from at least three genuses -- Aspidella, Palaeopascichnus and Nimbia -- along with others that haven't been fully identified. This makes them the oldest Ediacaran fossils found in any landmass that was part of Gondwana -- including Africa, Australia, South America, Antarctica, the Indian subcontinent and the Arabian Peninsula -- before the supercontinent broke up about 180 million years ago.
The new fossils from Brazil come from organisms that lived a few million years after the oldest Ediacaran fossils, which were found in present-day Newfoundland about 574 million years ago. Those older fossils are known as the Avalon assemblage. The Avalon, together with a pair of somewhat younger assemblages that include some of the same fossils found in the Itajai Basin, provide a glimpse into the environments where these ancient organisms lived and evolved. But the Brazilian fossils represent an important new vantage point of ancient life.
The finding represents the most robust and reliable evidence of fossils of soft-bodied Ediacaran organisms from South America, said Alexander Liu, a paleobiologist at the University of Cambridge who was not involved with the study. This new research, led by paleobiologist Bruno Becker-Kerber at the Federal University of Sao Carlos in Brazil, confirms that Aspidella and Palaeopascichnus survived at localities all across the ancient Earth and across a range of environments from the deep sea to shallower waters where sunlight could reach.
These two unmistakably Ediacaran fossils have also been found in the deep-sea environments recorded in the Avalon assemblage. Finding them in the shallower settings of the Itajai Basin, along with ancient, well-preserved dense communities of small, ocean sediment-dwelling microorganisms called “microbial mats,” suggests that they persisted across a significant environmental transition. These mats have lasted through many environmental changes and are indeed still abundant across the planet.
“When you look into the classic Avalon assemblage you see that the fossils occur in really deep marine settings and you have no evidence of microbial mats,” said Becker-Kerber. “In Itajai Basin, the fact that you have [multiple organisms] associated with microbial mats means that this could be evidence of some of the first macroscopic life transitioning to shallower settings.”
One Itajai fossil, known as Aspidella, has a complicated history. This disc-shaped fossil was discovered in 1872, long before scientists could confirm that it existed prior to the Cambrian Explosion. For nearly 150 years, scientists have attempted to make sense of their origins. Some scientists suggested the discs were jellyfish fossils and others have claimed they represent the byproducts of high-pressure gas flowing out from the seafloor. Recently, the consensus has been that they formed the disc-shaped bases of ancient fronds. Becker-Kerber and his colleagues found an Aspidella fossil in the Itajai Basin with a “stalk-like” structure emanating from it, supporting a biological origin. But he is still cautious when determining the origin of these disc-shaped organisms.
“There is always a geologic process that can cause these morphologies,” he said. By cutting, polishing and analyzing the specimens closely, Becker-Kerber and his colleagues were able to distinguish between fossilized living organisms versus structures caused by natural processes occurring on the seafloor.
For example, some of the disc-shaped structures had spikes radially emanating from them, which excited the researchers due to their similarity to Cambrian sponges. But upon further inspection it became clearer and clearer that the chemistry and shape of the structure resembled discs of the mineral pyrite, rather than ancient organisms.
Other mysterious forms perplexed the scientists. But it is possible that many of them are remnants of microbial mats that were preserved to varying degrees and inconsistently across what may have once been a vast quilt of tiny organisms. Ediacaran organisms could have thrived within these rich communities and later been preserved under the pyrite “death masks” left by decomposed microbes.
“Becker-Kerber and his colleagues extend the paleogeographic range of some Ediacaran fossils and together this helps us put together a more complete picture of the Ediacaran period,” said Frances Dunn, a paleontologist at the Oxford Museum of Natural History who was not involved with the research.
With more fossils potentially hiding away in remote locations, the full picture of the Ediacaran has yet to be revealed. And perhaps one day, scientists will know how these mysterious organisms will fit into the epic story of life. “This is a really interesting time as we try to unravel how the assemblages are related and whether or not the changes represent a true evolutionary signal,” said Dunn.
Editor's Note (August 14, 2020): The story was changed after publication to paraphrase and clarify the quote from Alexander Liu.