Cambrian substrate revolution

Many organisms burrow to obtain food, either in the form of other burrowing organisms, or organic matter. The remains of planktonic organisms sink to the sea floor, providing a source of nutrition; if these organics are mixed into the sediment they can be fed upon. However, it is possible that before the Cambrian, plankton were too small to sink, so there was no supply of organic carbon to the sea floor.[14] However, it appears that organisms did not feed upon the sediment itself until after the Cambrian.[15]

An advantage to living within the substrate would be protection from being washed away by currents.

Organisms also burrow to avoid predation. Predatory behaviour first appeared over 1 billion years ago, but predation on large organisms appears to have first become significant shortly before the start of the Cambrian. 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.[6]

After the agronomic revolution, the microbial mats that had covered the Ediacaran sea floor became increasingly restricted to a limited range of environments:

• Very harsh environments, such as hyper-saline lagoons or brackish estuaries, which were uninhabitable for the burrowing organisms that broke up the mats.[5]
• Rocky substrates which the burrowers could not penetrate.[1]
• The depths of the oceans, where burrowing activity today is at a similar level to that in the shallow coastal seas before the revolution.[1]

Ironically, the first burrowers probably fed on the microbial mats, while burrowing underneath them for protection; this burrowing led to the downfall of the mats they were feeding on.[6]

Before the revolution, bottom dwelling organisms fell into four categories:[1]

• "mat encrusters", which were permanently attached to the mat;
• "mat scratchers", which grazed the surface of the mat without destroying it;
• "mat stickers", suspension feeders that were partially embedded in the mat; and
• "undermat miners", which burrowed underneath the mat and fed on decomposing mat material.

The "undermat miners" appear to have died out by the middle of the Cambrian period.[5] "Mat encrusters" and "mat stickers" either died out or developed more secure anchors that were specialised for soft or hard substrates. "Mat scratchers" were restricted to rocky substrates and the depths of the oceans, where both they and the mats could survive.[1]

Crinoid holdfasts on a hard substrate from the Upper Ordovician of northern Kentucky

Early sessile echinoderms were mostly "mat stickers". 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. Mobile echinoderms (stylophorans, homosteleans, homoiosteleans, and ctenocystoids) were not significantly affected by the substrate revolution.[1]

Early molluscs appear to have grazed on microbial mats, so it is natural to hypothesize that grazing molluscs were also restricted to areas where the mats could survive. The earliest known fossils of monoplacophoran ("single-plated") molluscs date from the Early Cambrian, where they grazed on microbial mats. Most modern monoplacophorans live on soft substrates in deep parts of the seas, although one genus lives on hard substrates at the edges of continental shelves. Modern monoplacophorans have less diverse shell forms than fossil genera. 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. Since they are found with stromatolites (stubby pillars built by some types of microbial mat colony), it is thought that polyplacophorans grazed on microbial mats. Modern polyplacophorans mainly graze on mats on rocky coastlines, although a few live in the deep sea.[1] No fossils have been found of aplacophorans (shell-less molluscs), which are generally regarded as the most primitive living molluscs. Some burrow into the sea-floors of deep waters, feeding on micro-organisms and detritus; others live on reefs and eat coral polyps.[17]

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.[18] Just like microbial mats, environments which could produce this mode of fossilisation became increasingly restricted to harsher and deeper areas, where burrowers could not establish a foothold; as time progressed, the extent of burrowing increased sufficiently to effectively make this mode of preservation impossible.[18] Post-Cambrian lagerstätten of this nature are typically found in very unusual environments.

The rise in burrowing is of further significance, for burrows provide firm evidence of complex organisms; they are also much more readily preserved than body fossils, to the extent that the absence of trace fossils has been used to imply the genuine absence of large, motile bottom-dwelling organisms. This furthers palaeontologists' understanding of the early Cambrian, and provides an additional line of evidence to show that the Cambrian explosion represents a real diversification, and is not a preservational artefact - even if its timing did not coincide directly with the Agronomic revolution.[5]

The rise of burrowing represents such a fundamental change to the ecosystem, that the appearance of the complex burrow Treptichnus pedum is used to mark the base of the Cambrian period.[12]

The increased level of bioturbation meant that sulfur, which is steadily supplied to the oceanic system from volcanoes and river runoff, was more readily oxidised - rather than being rapidly buried and sitting in its reduced form (sulfide), burrowing organisms continually exposed it to oxygen, allowing it to be oxidised to sulfate. This activity is suggested to account for a sudden rise in sulfate concentration observed near the base of the Cambrian; this can be recorded in the geochemical record both by using δ³⁴S isotopic tracers, and by quantifying the abundance of the sulfate mineral gypsum.[19]