What Were the Ediacaran Animals? Life Before Anything Had a Mouth

Reginald Sprigg, a young government geologist, noticed disc-shaped impressions in quartzite in the Ediacara Hills of South Australia in 1946 while assessing abandoned mines, and interpreted them as jellyfish. The problem was age: the rocks lay below the strata containing the oldest known fossils, and the scientific consensus of the day held that nothing complex existed before the Cambrian. His paper was rejected by Nature. Similar fossils had in fact been reported from Charnwood Forest in England decades earlier and ignored for the same reason, until a schoolboy, Roger Mason, drew attention to a frond-shaped specimen there in 1957, and it was described as Charnia, the first organism formally accepted from before the Cambrian. The period was given a name of its own in 2004, the Ediacaran, running from 635 to 539 million years ago, the first new geological period to be defined in more than a century.

The assemblage is unlike anything since, and it divides into a few recurring designs:

• •Rangeomorphs such as Charnia: fronds up to two metres tall, built entirely of a branching pattern repeated at four or five scales, anchored to the sea floor and with no opening of any kind
• •Dickinsonia: a flat oval quilted like a mattress, from a few millimetres to more than a metre, growing by adding segments at one end and known to have moved, since trails are preserved behind resting marks
• •Discs such as Aspidella, once read as jellyfish and now generally interpreted as holdfasts, the anchors of frond-like organisms whose upper parts rarely survive
• •Tribrachidium: a disc with three arms in a spiral, showing threefold symmetry, an arrangement no living animal uses
• •Kimberella: a slug-shaped organism found with scratch marks beside it, which look very like the scrapings of a mouth with a rasping structure, making it the best candidate in the whole assemblage for an animal in the ordinary sense

Interpretations have ranged widely and none commands full agreement. Early workers read them as ancestors of jellyfish, worms and arthropods. Adolf Seilacher argued from the 1980s that most belonged to an entirely separate experiment in multicellular life, the Vendobionta, built as fluid-filled quilted compartments with no organs, which failed completely and left no descendants. Others have proposed lichens, giant single-celled organisms, or a group of their own related to fungi. The evidence has moved somewhat towards animals for at least part of the assemblage: a 2018 study extracted cholesterol-type molecules from Dickinsonia specimens, and cholesterol is an animal marker, while the feeding trails of Kimberella indicate something with a front end and a gut. The frond-like rangeomorphs are harder, since their fractal branching is a design for absorbing dissolved nutrients across a surface, which suits an organism with no mouth and no known equivalent today.

Their world had two features that will not return. Oxygen had recently risen, after a long interval at low concentration, which is generally taken as the precondition for large bodies, and the seas may have been unusually rich in dissolved organic carbon that could be absorbed directly through a surface. More importantly, nothing ate anything. There were no predators, no burrowers and no mineralised skeletons, so the sea floor was covered by thick microbial mats, and most of these organisms appear to have lived on, in or just above that surface, immobile or nearly so. Preservation itself depended on the mats, since the fine sand pressed against a soft body and held by a bacterial film produced the death masks that survive. The absence of burrowing is the clearest signal of how different the world was, because once animals began to dig, they destroyed both the mats and the conditions that recorded anything.

The assemblage disappears at the boundary with the Cambrian, and the disappearance is quick in geological terms, over perhaps a few million years. Three explanations compete and are not exclusive. A mass extinction driven by a change in ocean chemistry would have removed them directly. Biological replacement is the more popular account: the first animals that could burrow, graze and hunt destroyed the microbial mats these organisms depended on and ate the organisms themselves, an argument sometimes called the agronomic revolution, and the appearance of predation is the strongest candidate for the trigger of the Cambrian diversification that followed. A third possibility is that they did not vanish but stopped being preserved, since the same burrowing that destroyed the mats destroyed the only conditions in which a soft body leaves a record. A handful of forms persist into Cambrian rocks, which supports the third at least in part, and the honest position is that the world changed in a way that removed both the organisms and the evidence at once.

The soft-bodied organisms of the period from 635 to 539 million years ago, found in South Australia in 1946 and in Leicestershire in 1957, were the first large complex life, built as fronds, quilted ovals, discs and three-armed spirals, mostly with no mouth, gut or limbs. Some, including Kimberella and probably Dickinsonia, look like genuine animals, while the fractal fronds have no modern equivalent. They lived on microbial mats in seas with no predators, and they vanished as burrowing and grazing animals destroyed both the mats and the conditions that preserved them.