Where to place the fossil in the geologic record has been a mystery since the first species and specimens of this fossil organism were first discovered in the Flinders Ranges in South Australia. A few specimens have also been found in the Mogilev Formation in the Dniester River Basin of Podolia, Ukraine and in the Central Urals, in Russia.
Reg Sprigg, the original discoverer of the Ediacaran biota in Australia in 1947 described Dickinsonia, naming it after Ben Dickinson, then Director of Mines for South Australia, and head of the government department that employed Sprigg.
Sprigg, in writing of his discovery, suggested the fossils were from the early Cambrian period and could be early jellyfish. He wrote:
“The fossils occur as impressions on surfaces of flaggy quartzite. The five genera and species described are almost certainly all pelagic Coelenterates, and while several forms are referred to the class Scyphozoa, it is possible that one or more species may be more correctly assigned as Hydromedusae. The more problematical forms may prove to be pneumatophores or swimming bells.”
What all this means is that Dickinsonia has been variously described as a jellyfish, coral, polychaete worm, turbellarian, mushroom, xenophyophoran protist, sea anemone, lichen, and even a close ancestor of the chordates.
In April 2011, Gregory J. Retallack conducted an extensive study of Dickinsonia, concluding that growth, decay and burial compaction of Dickinsonia were more like those of plants, lichens, and fungi than of worms, jellyfishes or anemones.
Dickinsonia is not a plant – but an animal
Scientists with The Australian National University (ANU) and overseas have discovered molecules of fat in the ancient Dickinsonia fossil – revealing it to be the earliest confirmed animal in the geological record that lived on Earth 558 million years ago. The results of the study were published in the journal Science on September 21, 2018.
The strange oval-shaped creature with rib-like segments running along its body ranged in size from a few millimeters to about 1.4 meters (4 ft 7 in) in length, and from a fraction of a millimeter to a few millimeters thick. It was part of the Ediacara Biota that lived on Earth 20 million years before the ‘Cambrian explosion’ of modern animal life or about 575 million years ago.
ANU Ph.D. scholar Ilya Bobrovskiy discovered a Dickinsonia fossil so well preserved in a remote area near the White Sea in the northwest of Russia that the tissue still contained molecules of cholesterol, a type of fat that is the hallmark of animal life.
How did Bobrovskiy find out the fossils contained cholesterol? He developed a method to test for fossil sterols in the remnants of the Ediacarans, comparing the results to biomarkers extracted from the surrounding rock.’
“The molecular composition of the fossil and the molecular composition from the seafloor around the fossil was black and white,” said co-author Jochen Brocks, an associate professor at the Australian National University (ANU) in Canberra. “An abundance of ancient cholesterols—up to 93 percent—in the trace remains pointed to an animal. Whereas the surrounding seafloor had very little and instead contained what’s known as ergosteroids, suggesting the presence of green algae.”
“The fossil fat molecules that we’ve found prove that animals were large and abundant 558 million years ago, millions of years earlier than previously thought,” he added. “Scientists have been fighting for more than 75 years over what Dickinsonia and other bizarre fossils of the Ediacaran Biota were,” he explained, adding: “The fossil fat now confirms Dickinsonia as the oldest known animal fossil, solving a decades-old mystery that has been the Holy Grail of paleontology.”
The Cambrian Explosion – rise of modern animal groups
The discovery of Dickinsonia being one of the very earliest of Earth’s animals helps to explain the Cambrian Explosion, said to have occurred approximately 541 million years ago in the Cambrian period when most major animal phyla appeared in the fossil record.
This period only lasted about 20 to 25 million years, but It resulted in the divergence of most modern metazoan phyla. The event was also accompanied by a major diversification of other organisms.
For the longest time, scientists were perplexed at the seemingly rapid appearance of fossils in the “Primordial Strata,” as noted by William Buckland in the 1840s. Even Charles Darwin talked about the lack of earlier fossils as one of the main difficulties for his theory of descent with slow modification through natural selection.
The Ediacaran species largely disappeared when the Cambrian explosion occurred. As such, they do straddle an ancient age when the Earth was dominated by bacteria and a later age of dominance by animals.
