Period· The columns of time

Ediacaran

635 Ma → 539 Ma

The first new period on the chart in a hundred and twenty-five years, and the one in which large, soft, unarmed bodies appear and then vanish.

What the evidence shows

Ratified in 2004, it was the first period added since the Ordovician in 1879, and the only Precambrian one whose base is a rock section rather than a number: the cap dolostone that ends the Marinoan glaciation, in the Flinders Ranges of South Australia. What follows is a world of bodies that resist classification, fronds anchored in mud, quilted discs, ribbed ovals grazing the microbial mats they lie on, some more than a metre long. Fossil cholesterol extracted from Dickinsonia settles that one, at least, as an animal. The ocean, meanwhile, records the largest carbon isotope excursion known, some twelve parts per thousand; in the last ten million years the assemblage collapses, while the first burrowers and the first skeletons arrive.

Bounds and conventions

Its base is a defined section and its top the base of the Cambrian at 538.8 million years, so both bounds are firm. What happens between them is much less secure: the cause of the Shuram excursion is still contested between a real change in ocean chemistry and later alteration of the rock, and whether the Ediacara biota died out or was displaced remains open, though the survivors are mostly thin, broad forms, which hints at falling oxygen rather than at competition.

Sources

1. Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
2. Bobrovskiy et al. (2018), Science · Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals
3. Busch et al. (2022), Earth and Planetary Science Letters · Global and local drivers of the Ediacaran Shuram carbon isotope excursion
4. Chen et al. (2014), Nature · Cell differentiation and germ-soma separation in Ediacaran animal embryo-like fossils
5. Evans et al. (2022), PNAS · Environmental drivers of the first major animal extinction across the Ediacaran White Sea-Nama transition
6. Wood et al. (2019), Nature Ecology & Evolution · Integrated records of environmental change and evolution challenge the Cambrian Explosion