Period· The columns of time

Calymmian

1.6 Ga → 1.4 Ga

In the Calymmian, from 1.6 to 1.4 billion years ago, organisms reached a size visible to the naked eye under an atmosphere holding at most a thousandth of today's oxygen.

Period in which organisms attain a size visible to the naked eye, under an atmosphere containing at most a thousandth of present-day oxygen.

What the evidence shows

Sediments spread as regular covers over stable continental platforms, an arrangement from which the period takes its name, formed on the Greek calymma, cover, and it is those platforms that preserved its fossils. In the Gaoyuzhuang Formation of North China, carbon compressions 1.56 billion years old preserve flat tongue-shaped bodies up to 30 centimetres long and nearly 8 wide, a billion years before the Cambrian. The rest of the record is microscopic and abundant: acritarchs, cysts of resistant organic wall, some smooth, others spined, many pierced by the single opening through which their occupant left. The atmosphere is poor in oxygen, at most a thousandth of the present level on chromium isotope evidence, and poor in ozone in consequence, so that ultraviolet reached the surface far more freely than today.

Bounds and conventions

Neither limit corresponds to an event; both proceed from a decision. The Gaoyuzhuang fossils are firmly dated but not firmly identified: their size and the regularity of their form argue for multicellular eukaryotes, their position within the group remains open, and an interpretation as bacterial colonies has not been formally excluded. The oxygen figure is a ceiling obtained from a single proxy, other proxies yielding higher values.