Cambrian substrate revolution

Before:

After:

Sessile organism
anchored in mat

Animal grazing
on mat

Animals embedded
in mat

Animals
burrowing
just under
mat

    =Microbial mat

Firm, layered, anoxic, sulphidic substrate

Animals moving on / in
surface of sea-floor

Loose,
oxygenated
upper substrate
with
burrowing
animals

Before and after the Cambrian substrate revolution

Part of a series on
The Cambrian explosion
Fossil localities Burgess Shale Chengjiang Sirius Passet Doushantuo
Key organisms Ediacaran biota Dickinsonia Kimberella Kimberichnus Vernanimalcula Burgess-type Marrella Radiodonts Halwaxiids Opabinia Odontogriphus Small shelly fauna Helcionellids
Evolutionary concepts Trends Cambrian substrate revolution Themes Cladistics Convergent evolution Stem and crown groups
v t e

The "Cambrian substrate revolution"^[1] or "Agronomic revolution",^[2] evidenced in trace fossils, is a sudden diversification of animal burrowing during the early Cambrian period.

Before this "widening of the behavioural repertoire",

Around the start of the Cambrian, organisms began to burrow vertically, forming a great diversity of different fossilisable burrow forms and traces as they penetrated the sediment for protection or to feed.^[5] These burrowing animals broke down and weakened the microbial mats, thus allowing 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.^[1]

Burrowing before the Cambrian

The traces of organisms moving on and directly underneath the microbial mats that covered the Ediacaran sea floor are preserved from the

Some simple pre-Cambrian horizontal traces could have been produced by large single-celled organisms; equivalent traces are produced by protists today.^[11]

The early Cambrian diversification of burrow forms

From the very start of the Cambrian period

It's important to make a distinction between burrowing that introduces oxygen into the sediment – bioirrigation – and those that simply move sediment grains around – biomixing. The latter can actually reduce the amount of oxygen available in the sediment, by bringing organic material down to depths where it will be anaerobically respired. 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.[15]

Advantages of burrowing

Feeding

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.^[16] However, it appears that organisms did not feed upon the sediment itself until after the Cambrian.^[17]

Anchorage

An advantage to living within the substrate would be protection from being washed away by currents.^{[verification needed]}

Protection

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]

Enabling burrowing

Microbial mats formed a blanket, cutting off the underlying sediments from the ocean water above. This meant that the sediments were anoxic, and hydrogen sulfide (H₂S) was abundant. The free exchange of the pore waters with oxygenated ocean water was essential to make the sediments habitable. 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. Their bioturbation – movement that dislodged grains and disturbed the resistant biomats – broke the mats up, allowing water and chemicals above and below to mix.^[5]

Effects of the revolution

The Cambrian substrate revolution was a long and patchy process that proceeded at different rates in different locations throughout much of the Cambrian.^[18]

Effects on ecosystems

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]

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]

Early

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

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.^[21]

See also

• Matground

Further reading

Callow, R. H. T.; Brasier, M. D. (2009). "Remarkable preservation of microbial mats in Neoproterozoic siliciclastic settings: Implications for Ediacaran taphonomic models". Earth-Science Reviews. 96 (3): 207–219.