Cambrian Substrate Revolution

The "Cambrian substrate revolution" or "Agronomic revolution", evidenced in trace fossils, is the diversification of animal burrowing during the early Cambrian period.

Before this "widening of the behavioural repertoire", bottom-dwelling animals mainly grazed on the microbial mats that lined the surface, crawling above or burrowing just below them. These microbial mats created a barrier between the water and the sediment underneath, which was less water-logged than modern sea-floors, and almost completely anoxic (lacking in oxygen). As a result the substrate was inhabited by sulfate-reducing bacteria, whose emissions of hydrogen sulfide (H₂S) made the substrate toxic to most other organisms.

Around the start of the Cambrian, organisms began to burrow vertically, forming a great diversity of different fossilisable burrow forms as they penetrated the sediment for protection or to feed. These burrowing animals broke down the microbial mats, and thus allowed water and oxygen to penetrate a considerable distance below the surface. This restricted the sulfate-reducing bacteria and their H₂S emissions to the deeper layers, making the upper layers of the sea-floor habitable for a much wider range of organisms. The upper level of the sea-floor became wetter and softer as it was constantly churned up by burrowers.