The Ediacaran biota are the earliest known complex, many-celled organisms: soft-bodied living things that lay on or stood up from the sea floor between about 575 and 538.8 million years ago, at the very end of the Precambrian. Some were feathery fronds up to 1.85 m long, some flat, ribbed ovals more than a metre long, others worms the size of a grain of rice. Named after the Ediacara Hills in South Australia, they are now known from at least 52 rock formations worldwide. They show what large life looked like before the animals of the Cambrian took over, and most of them are still hard to place on the tree of life.
Mistaken for jellyfish
The first finds were not recognised for what they were. In 1868 Murray found small discs in Newfoundland, later named Aspidella and taken for jellyfish. In 1933 Gürich collected similar fossils in Namibia but assigned them to the Cambrian, and in 1946 Sprigg noticed what he took for fossil jellyfish in the Ediacara Hills. When a 15-year-old schoolgirl, Negus, spotted a fern-like fossil in Charnwood Forest in England, her teacher dismissed the idea of Precambrian fossils. A year later, in 1957, Mason and his friends found it while rock climbing, and it was named Charnia. In 1959 Glaessner linked the English finds with the earlier ones.
The jellyfish idea has not survived: not one is now confidently recognised as a jellyfish. Many of the discs are holdfasts, sand-filled bulbs that anchored the stems of upright fronds.
Deep fronds and shallow mats
The fossils fall into three groups of different ages. The Avalon assemblage, about 575 to 565 million years old, is defined by Mistaken Point in Newfoundland, a UNESCO World Heritage Site since 2016. Its fronds lived in water too deep for sunlight to support photosynthesis, which is one reason Charnia is not thought to be a plant or alga. Fine volcanic ash buried them and kept details that coarser sand would have lost.
The White Sea assemblage, about 560 to 550 million years old, takes its name from Russia’s White Sea coast. The Nama assemblage, about 550 to 542 million years old, is best represented in Namibia, where many of its fossils lie in sand bars formed at the mouths of deltas. Almost everywhere the sea floor was covered by microbial mats, which left a wrinkled “elephant skin” texture in the rock. At Nilpena in South Australia, a single bed has yielded more than 400 fossils of 16 genera. Storms and underwater flows buried these communities in sand, sometimes forming mineral “death masks” over the bodies.
The oldest large fossils appear after the Gaskiers glaciation, which lasted about 250 000 years around 580 million years ago. Unlike the Snowball Earth glaciations before it, it is thought not to have covered the globe.
Animal or something else
How most Ediacaran organisms fed is unclear. The most widely accepted idea is that the quilted forms absorbed nutrients directly from seawater. Charnia grew as a frond 1 to 66 cm long, built from repeated branching units, and its relative Trepassia reached 1.85 m. From 1966 it was read as a sea pen, but that idea has since been discredited.
Dickinsonia was round to oval, ribbed on either side of a central groove, from a few millimetres to about 1.4 m long but at most a few millimetres thick. It moved over the sea floor and fed on microbial mats through its underside, leaving chains of feeding marks. In 2018 cholesterol, made only by animals, was reported from Russian specimens, although not everyone accepts this as proof.
Others look more like animals. Kimberella, up to 15 cm long, had a stiff but unmineralised shell, a muscular foot and a frilled fringe, and scratch marks around it resemble the feeding traces of molluscs. Ikaria, 2 to 7 mm long, had a body with a distinct front and back end, and its size matches small burrows found in the same rocks. Quaestio, described in 2024, is the earliest known animal with a left-right asymmetry.
Gone by the Cambrian
The Ediacaran communities vanish at the base of the Cambrian, 538.8 million years ago, and Cambrian animals almost completely replaced them. In the youngest, Nama assemblage, extinctions outpaced new arrivals throughout. Changing environments, new predators, competition and gaps in preservation have all been proposed as causes.
