Instead of looking for fossil at that age, the researchers looked for biomarkers:
Rather than searching for conventional body fossils, the researchers have been tracking molecular signs of animal life, called biomarkers, as far back as 660-635 million years ago during the Neoproterozoic Era. In ancient rocks and oils from Oman, Siberia, and India, they found a steroid compound produced only by sponges, which are among the earliest forms of animal life.
“Molecular fossils are important for tracking early animals since the first sponges were probably very small, did not contain a skeleton, and did not leave a well-preserved or easily recognizable body fossil record,” Zumberge said. “We have been looking for distinctive and stable biomarkers that indicate the existence of sponges and other early animals, rather than single-celled organisms that dominated the earth for billions of years before the dawn of complex, multicellular life.”
The biomarker they identified, a steroid compound named 26-methylstigmastane (26-mes), has a unique structure that is currently only known to be synthesized by certain species of modern sponges called demosponges. UCal Riverside, “Oldest evidence for animals found” at Phys.org
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See also: Researchers: The sponge is the oldest animal phylum after all, not the jellyfish (both around 600 mya)
Gunter Bechly: Dickinsonia is NOT likely an animal Rather, it “Rather, he thinks, it is an unknown type of life form, which belonged to “an alien clade” (558 mya) not certainly related to later life forms.” That’s, of course, an interesting consideration: Entire small kingdoms of life could have come and gone within a hundred million years.