Before the Cambrian, Earth's first big organisms looked like quilted mattresses
Around 600 million years ago, seafloors filled with strange soft creatures shaped like fronds, discs, tubes and padded mattresses. They were the earliest known complex multicellular life, and they vanished as the Cambrian began. Scientists still argue whether most of them were animals at all.
These organisms lived during the Ediacaran Period, roughly 635 to 538.8 million years ago, after the planet thawed from the great Cryogenian glaciations. Most stayed fixed in one place, and they survive mainly as impressions in sandstone. A burst of diversification called the Avalon explosion may have happened around 575 million years ago. Well-known communities come from the Ediacara Hills of South Australia, Russia's White Sea coast and a site in Sonora, Mexico, found in 1995 and about 555 million years old.
Placing them on the tree of life is notoriously hard. Suggestions include lichens, algae, foraminifera, fungi, microbial colonies, or something between plants and animals. A few forms hint at sponges or cnidarians, and Kimberella may resemble molluscs. Adolf Seilacher proposed a separate group, now called Vendobionta, for forms that match nothing alive, which would make them a failed experiment in multicellularity. A 2018 study, however, found cholesterol preserved in Dickinsonia, one of the period's most famous fossils.
Recognition came slowly. Alexander Murray found disc-shaped Aspidella in Newfoundland in 1868, but because they lay beneath the Cambrian layers then thought to hold the first animal life, Elkanah Billings's idea that they were living things was brushed aside. Georg Gürich's Namibian specimens of 1933 and the jellyfish-like shapes Reg Sprigg spotted in the Ediacara Hills in 1946 were both misdated as Cambrian. The breakthrough was Charnia, a frond from Charnwood Forest in England, first found in 1956 by 15-year-old Tina Negus, who was not believed, and again a year later by schoolboys including Roger Mason. Careful geological maps left no doubt the rock was Precambrian, and in 1959 Martin Glaessner linked all the finds.
Fine detail arrived in 1967, when S.B. Misra found fossils in volcanic ash at Mistaken Point, Newfoundland, the first from deep-water sediments. After decades of rival names such as Vendian and Sinian, geologists formally named the period after the Australian site in March 2004. Why the biota disappeared is still debated; ideas include new predators, competition, environmental change and simple gaps in preservation.
Source: Ediacaran biota
