Period· The columns of time

Silurian

443 Ma → 420 Ma

The Silurian, a short period from 444 to 419 million years ago, brought jaws to vertebrates, the first fossils of land animals and the ozone layer.

Short period marked by the acquisition of jaws in vertebrates, by the first fossils of terrestrial animals and by the establishment of the ozone layer.

What the evidence shows

Barely twenty-five million years long, one of the shortest periods on the chart, it carries three first records. In South China, rocks 439 to 436 million years old yield the oldest teeth of a jawed vertebrate and then, three million years later, complete jawed fishes; the jaw derives from a modified gill arch, and every later jaw is a variant of it. On land, a millipede from the Scottish island of Kerrera, dated to about 425 million years, is the oldest body fossil of an animal living out of water, molecular clocks placing the transition earlier. The atmosphere changes at the same time: land plants build tissue far poorer in phosphorus, for a given quantity of carbon, than marine organisms, so that the same phosphorus supply buries more carbon and releases more oxygen, whose level then reaches its present value, sufficient for an ozone layer to filter ultraviolet.

Bounds and conventions

Two golden spikes bound the period. The internal dates are less settled: the oldest body fossil of a land animal gives 425 million years, molecular clocks a markedly older date, and the gap between them, narrower than often presented, is not closed. The establishment of the ozone screen is inferred from models of the oxygen and phosphorus cycles rather than measured, and a recent estimate substantially lowers the Proterozoic ozone column that preceded it.