Cambrian substrate revolution

Before this "widening of the behavioural repertoire",[3] bottom-dwelling animals mainly grazed on the microbial mats that lined the surface of the substrate, crawling above (like how freshwater snails still do) 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).

[7] 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;[8] however, their uneven width and tapering ends make it difficult to believe that they were made by living organisms,[9] and the original author has suggested that the menisci of burst bubbles are more likely to have created the marks he observed.

[10] The Ediacaran burrows found so far imply simple behaviour, and the complex, efficient feeding traces common from the start of the Cambrian are absent.

In ichnodiversity terms, the same proportions of these two modes (predominantly biomixing) are seen on each side of the Ediacaran-Cambrian boundary – even if bioirrigation occurrences become relatively more frequent in the Terreneuvian.

Precambrian burrows served a protective function, as the animals that made them fed above the surface; they evolved at the same time as other organisms began forming mineralised skeletons.

This exchange was made possible by the action of minute animals: Too small to produce burrows of their own, this meiofauna inhabited the spaces between sand grains in the microbial mats.

[6] Before the revolution, bottom dwelling organisms fell into four categories:[1] The "undermat miners" appear to have died out by the middle of the Cambrian period.

The helicoplacoids failed to adapt to the new conditions and died out; the edrioasteroids and eocrinoids survived by developing holdfasts for attachment to hard substrates, and stalks that raised their feeding apparatus above most of the debris that burrowers stirred up in the looser sea-floors.

Unfortunately, the oldest known fossils of polyplacophorans (molluscs with multiple shell plates) are from the Late Cambrian, when the substrate revolution had significantly changed marine environments.

[19] The revolution put an end to the conditions which allowed exceptionally preserved fossil beds or lagerstätten such as the Burgess Shale to be formed.

[5] The direct consumption of carcasses was relatively unimportant in reducing fossilisation, compared to changes in sediments' chemistry, porosity, and microbiology, which made it difficult for the chemical gradients necessary for soft-tissue mineralisation to develop.