Cambrian substrate revolution
Encyclopedia
The "Cambrian substrate revolution" or "Agronomic revolution", evidenced in trace fossil
s, 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 mat
s 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 fossil
isable 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 . 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 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.
Some simple pre-Cambrian horizontal traces could have been produced by large single-celled organisms; equivalent traces are produced by protists today.
and Skolithos
, and traces normally attributed to arthropod
s, such as Cruziana
and Rusophycus
. The vertical burrows indicate that worm-like animals acquired new behaviours, and possibly new physical capabilities. Some Cambrian trace fossils indicate that their makers possessed hard (although not necessarily mineralised) exoskeletons.
However, it appears that organisms did not feed upon the sediment itself until after the Cambrian.
first appeared over , 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.
was abundant. The free exchange of the pore waters with oxygenated ocean water was essential to make the sediments inhabitable. 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 them up, allowing water and chemicals to mix on both sides of the mat.
.
s that had covered the Ediacaran sea floor became increasingly restricted to a limited range of environments:
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.
Before the revolution, bottom dwelling organisms fell into four categories:
The "undermat miners" appear to have died out by the middle of the Cambrian
period. "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.
Early sessile
echinoderm
s 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.
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 monoplacophora
n ("single-plated") molluscs date from the Early Cambrian, where they grazed on microbial mats. Most modern monoplacophorans live on soft subtrates 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 stromatolite
s (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. No fossils have been found of aplacophora
ns (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.
fossil beds
such as the Burgess shale
to be formed. 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. 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. Post-Cambrian lagerstatte 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.
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.
.
Trace fossil
Trace fossils, also called ichnofossils , are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings , urolites , footprints and feeding marks, and root cavities...
s, is the diversification of animal burrowing
Burrow
A burrow is a hole or tunnel dug into the ground by an animal to create a space suitable for habitation, temporary refuge, or as a byproduct of locomotion. Burrows provide a form of shelter against predation and exposure to the elements, so the burrowing way of life is quite popular among the...
during the early Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
period.
Before this "widening of the behavioural repertoire", bottom-dwelling
Benthos
Benthos is the community of organisms which live on, in, or near the seabed, also known as the benthic zone. This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths.Many organisms...
animals mainly grazed on the microbial mat
Microbial mat
A microbial mat is a multi-layered sheet of micro-organisms, mainly bacteria and archaea. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces but a few survive in deserts. They colonize environments ranging in temperature from –40°C to +120°C...
s that lined the surface, crawling above or burrowing just below them. These microbial mats created a barrier between the water and the sediment underneath
Substrate (marine biology)
Stream substrate is the material that rests at the bottom of a stream. There are several classification guides. One is:*Mud – silt and clay.*Sand – Particles between 0.06 and 2 mm in diameter.*Granule – Between 2 and 4 mm in diameter....
, which was less water-logged than modern sea-floors, and almost completely anoxic
Hypoxia (environmental)
Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...
(lacking in oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
). As a result the substrate was inhabited by sulfate-reducing bacteria
Sulfate-reducing bacteria
Sulfate-reducing bacteria are those bacteria and archaea that can obtain energy by oxidizing organic compounds or molecular hydrogen while reducing sulfate to hydrogen sulfide...
, whose emissions of hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
(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 fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
isable 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.
Burrowing before the Cambrian
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
(bilaterally symmetrical animals). Putative "burrows" dating as far back as 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.
Some simple pre-Cambrian horizontal traces could have been produced by large single-celled organisms; equivalent traces are produced by protists today.
The early Cambrian diversification of burrow forms
From the very start of the Cambrian period (about ) many new types of traces first appear, including well-known vertical burrows such as DiplocraterionDiplocraterion
Diplocraterion is an ichnogenus describing vertical U-shaped burrows having a spreite between the two limbs of the U. The spreite of fainter U-shaped traces appears above and below the final tunnel, made as the organism moved up and down through the sediment.- Further reading :* Šimo V. &...
and Skolithos
Skolithos
Skolithos is a common trace fossil ichnogenus whose original form consisted of approximately vertical cylinders. One well-known occurrence of Cambrian trace fossils is the famous 'Pipe Rock' of northwest Scotland...
, and traces normally attributed to arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s, such as Cruziana
Cruziana
Cruziana is a trace fossil consisting of elongate, bilobed, approximately bilaterally symmetrical burrows, usually preserved along bedding planes, with a sculpture of repeated striations that are mostly oblique to the long dimension...
and Rusophycus
Rusophycus
Rusophycus is a trace fossil allied to Cruziana. Rusophycus is the resting trace, recording the outline of the tracemaker; Cruziana is made when the organism moved...
. The vertical burrows indicate that worm-like animals acquired new behaviours, and possibly new physical capabilities. Some Cambrian trace fossils indicate that their makers possessed hard (although not necessarily mineralised) exoskeletons.
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.However, it appears that organisms did not feed upon the sediment itself until after the Cambrian.
Anchorage
An advantage to living within the substrate would be protection from being washed away by currents.Protection
Organisms also burrow to avoid predation. Predatory behaviourPredation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
first appeared over , 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.
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 sulfideHydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
was abundant. The free exchange of the pore waters with oxygenated ocean water was essential to make the sediments inhabitable. 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
Bioturbation
In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...
— movement that dislodged grains and disturbed the resistant biomats— broke them up, allowing water and chemicals to mix on both sides of the mat.
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 CambrianCambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
.
Effects on ecosystems
After the agronomic revolution, the microbial matMicrobial mat
A microbial mat is a multi-layered sheet of micro-organisms, mainly bacteria and archaea. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces but a few survive in deserts. They colonize environments ranging in temperature from –40°C to +120°C...
s 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.
- Rocky substrates which the burrowers could not penetrate.
- The depths of the oceans, where burrowing activity today is at a similar level to that in the shallow coastal seas before the revolution.
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.
Before the revolution, bottom dwelling organisms fell into four categories:
- "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
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
period. "Mat encrusters" and "mat stickers" either died out or developed more secure anchors
Holdfast
A holdfast is a root-like structure that anchors aquatic sessile organisms, such as seaweed, other sessile algae, stalked crinoids, benthic cnidarians, and sponges, to the substrate. ...
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.
Sessility (zoology)
In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
echinoderm
Echinoderm
Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone....
s were mostly "mat stickers". The helicoplacoids
Helicoplacus
Helicoplacus is the earliest well-studied fossil echinoderm. Fossil plates are known from several regions. Complete specimens were found in Lower Cambrian strata of the White Mountains of California....
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.
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 monoplacophora
Monoplacophora
Monoplacophora, meaning "bearing one plate", is a polyphyletic class of mollusks with a cap-like shell, living on the bottom of deep sea. Extant representatives were unknown until 1952; previously they were known only from the fossil record.- Definition :...
n ("single-plated") molluscs date from the Early Cambrian, where they grazed on microbial mats. Most modern monoplacophorans live on soft subtrates in deep parts of the seas, although one genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
lives on hard substrates at the edges of continental shelves
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
. 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 stromatolite
Stromatolite
Stromatolites or stromatoliths are layered accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms of microorganisms, especially cyanobacteria ....
s (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. No fossils have been found of aplacophora
Aplacophora
Aplacophora is a monophyletic group of small, deep-water, exclusively benthic, shell-less marine mollusks found in all oceans of the world. The group comprises the two clades Solenogastres and Caudofoveata , which between them contain 28 families and about 320 species...
ns (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.
Palaeontological significance
The revolution put an end to the conditions which allowed exceptionally preservedBurgess shale type preservation
The Burgess Shale of British Columbia is famous for its exceptional preservation of mid-Cambrian organisms. Around 40 other sites have been discovered of a similar age, with soft tissues preserved in a similar, though not identical, fashion...
fossil beds
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
such as the Burgess shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
to be formed. 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. 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. Post-Cambrian lagerstatte 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
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
represents a real diversification, and is not a preservational artefact - even if its timing did not coincide directly with the Agronomic revolution.
The rise of burrowing represents such a fundamental change to the ecosystem
Paleoecology
Paleoecology uses data from fossils and subfossils to reconstruct the ecosystems of the past. It involves the study of fossil organisms and their associated remains, including their life cycle, living interactions, natural environment, and manner of death and burial to reconstruct the...
, that the appearance of the complex burrow Treptichnus pedum
Treptichnus pedum
Treptichnus pedum is regarded as the earliest widespread complex trace fossil...
is used to mark the base of the Cambrian period.
Geochemical significance
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 isotopic tracers, and by quantifying the abundance of the sulfate mineral gypsumGypsum
Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...
.