LIFE × EARTH

The Cambrian Substrate Revolution

~521 to 509 million years ago

Phanerozoic · Paleozoic · Cambrian

The Cambrian substrate revolution is the change, between 521 and 509 million years ago, by which animals stopped grazing across the sea floor and began burrowing down into it.

The Cambrian substrate revolution is the change by which animals ceased to graze across the sediment surface and began working downward into it. The substrate is the bottom on which they live. This stirring of sediment by the organisms living in it is called bioturbation. Before the Cambrian, in the Precambrian, the seabed was firm, sealed under a microbial mat, a thin layer of microbes. This firm seabed gave way to a soft layer that burrowing mixes repeatedly. The mixed layer has covered the ocean floor ever since, and it determines what can be preserved in the sediment.

Why it matters

Burrowing carries oxygen downward into the sediment and buried compounds upward, and it converts a firm bottom into one that no longer supports an animal's weight. For every organism living on the bottom, the surface beneath it was altered. The change has lasted: the mixed layer persists and animals continue to rework it.

Dating & uncertainty

Burrowing had already begun in the latest Ediacaran, the period before the Cambrian. Trace fossils, marks left by animals in sediment, place the change in Cambrian Age 2, the second subdivision of the Cambrian, and not at the period boundary. The sea floor therefore did not change at the beginning of the Cambrian. Mat-covered bottoms persisted locally for millions of years.

Earth history as one day

21:14:442 h 45 min before midnight

if Earth’s 4.54 billion years were compressed into a single day