Period· The columns of time

Cambrian

539 Ma → 487 Ma

The Cambrian, from 539 to 485 million years ago, is the period in which nearly all animal body plans appeared, and later radiations explored them without adding new ones.

Period in which nearly all animal body plans appear, later radiations having explored those plans without adding new ones.

What the evidence shows

Between about 540 and 518 million years, the rocks fill with arthropods, molluscs, annelids and the first chordates. The rate can be measured: the speed at which trilobite anatomy changes gives the radiation a duration of about twenty million years, which is rapid for evolution and slow for an explosion. It begins with an accumulation of millimetre-scale shells, cones, tubes and plates released from limestone by acid dissolution, mostly elements of animals whose bodies remain unknown. Eyes appear very early and are already effective: a shale in South Australia deposited about 515 million years ago has yielded a compound eye two to three centimetres across carrying at least sixteen thousand facets, comparable to the best compound eyes alive. This documentation depends on a few deposits of exceptional preservation, at Chengjiang and in the Canadian Rockies, where gills, guts and nerve cords are preserved as films of carbon.

Bounds and conventions

A golden spike in Newfoundland, at 538.8 million years and placed on the first appearance of a trace fossil rather than a body, makes its base among the best defined on the chart. What gives it its reputation is far less so: whether the explosion corresponds to a real acceleration of evolution or to an improvement in fossilisation associated with hard parts is not settled; molecular clocks place the origin of many animal groups before it, although for trilobites clocks and fossils now agree on a Cambrian beginning.