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.
The traces of organisms moving on and directly underneath the microbial mats that covered the Ediacaran sea floor are preserved from the Ediacaran period, about 565 million years ago. The only Ediacaran burrows are horizontal, on or just below the surface, and were made by animals which fed above the surface, but burrowed to hide from predators. If these burrows are biogenic (made by organisms) they imply the presence of motile organisms with heads, which would probably have been bilaterans (bilaterally symmetrical animals). Putative "burrows" dating as far back as 1,100 million years may have been made by animals that fed on the undersides of microbial mats, which would have shielded them from a chemically unpleasant ocean; however, their uneven width and tapering ends make it difficult to believe that they were made by living organisms, and the original author has suggested that the menisci of burst bubbles are more likely to have created the marks he observed. The Ediacaran burrows found so far imply simple behaviour, and the complex, efficient feeding traces common from the start of the Cambrian are absent.
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