The "Cambrian Substrate Revolution," also called the "Agronomic Revolution," is shown by trace fossils and marks a sudden increase in the variety of animals that burrowed during the early Cambrian period.
Before this change, bottom-dwelling animals mostly fed on thin layers of microbes that covered the seafloor. These animals either moved along the surface, like freshwater snails do today, or dug only slightly below the microbial layers. These microbial layers acted as a barrier between the water above and the sediment below. The sediment was less wet than modern seafloors and mostly lacked oxygen. This environment was home to bacteria that use sulfate, which released hydrogen sulfide (H₂S), making the seafloor harmful to most other life.
At the start of the Cambrian period, animals began digging deep into the sediment, creating many different types of burrows and fossil traces. These burrowing animals damaged the microbial layers, allowing water and oxygen to reach deeper into the seafloor. This limited the sulfate-using bacteria and their harmful hydrogen sulfide emissions to deeper layers. The upper layers of the seafloor became wetter and softer because burrowing animals constantly mixed the sediment, making it easier for more organisms to live there.
Burrowing before the Cambrian
Fossil evidence from the Ediacaran period, about 565 million years ago, shows traces of organisms moving on and just below microbial mats that covered the seafloor. These burrows are horizontal, near the surface, and were likely made by animals that fed above the surface but burrowed to avoid predators. If these burrows were made by living things, they suggest the presence of moving organisms with heads, which may have been bilaterians (animals with symmetrical body plans). Some possible burrows from 1,100 million years ago may have been made by animals feeding on the undersides of microbial mats, which could have protected them from harsh ocean conditions. However, these burrows have uneven widths and tapering ends, making it unlikely they were made by living organisms. The original researcher suggested these marks might instead have been caused by the remains of broken bubbles. Ediacaran burrows found so far suggest simple behaviors, while the more complex and efficient feeding traces common in the Cambrian period are not seen in Ediacaran fossils. Some simple horizontal traces from before the Cambrian period may have been made by large single-celled organisms, similar to how protists create similar traces today.
The early Cambrian diversification of burrow forms
During the Cambrian period (about 538.8 million years ago), many new types of traces first appeared. These include common vertical burrows such as Diplocraterion and Skolithos, as well as traces usually linked to arthropods, like Cruziana and Rusophycus. The vertical burrows suggest that worm-like animals developed new behaviors and possibly new physical abilities. Some Cambrian trace fossils show that their creators had hard (but not always mineralized) exoskeletons.
It is important to understand the difference between burrowing that adds oxygen to sediment—called bioirrigation—and burrowing that moves sediment grains around—called biomixing. Biomixing can lower oxygen levels in sediment by bringing organic material to deeper layers, where it is broken down without oxygen. In terms of ichnodiversity, similar amounts of these two types of burrowing (mostly biomixing) are found on both sides of the Ediacaran-Cambrian boundary. However, bioirrigation becomes more common in the Terreneuvian period.
Advantages of burrowing
Many animals dig into the ground to find food, such as other animals that also dig or organic material. When tiny sea creatures called plankton die, their remains fall to the ocean floor, providing food. If this organic material mixes into the mud or sand, it can be eaten by other animals. However, before the Cambrian period, plankton may have been too small to sink, so there was not enough organic material reaching the ocean floor. It seems that animals did not start eating the sediment itself until after the Cambrian period.
Living inside the sediment offers protection from strong ocean currents that could wash animals away.
Animals also dig to avoid being eaten by predators. Hunting behavior began more than 1 billion years ago, but hunting large animals became important just before the Cambrian period started. Burrows from the Precambrian era served as protection because the animals that made them lived above the surface. These burrows developed at the same time as other animals started to form hard shells or bones.
Enabling burrowing
Microbial mats created a layer that separated the sediments below from the ocean water above. This separation caused the sediments to lack oxygen, and hydrogen sulfide (H₂S) became common there. For the sediments to support life, the pore water needed to mix with oxygen-rich ocean water. This mixing happened because of tiny animals called meiofauna. These animals lived in the spaces between sand grains in the microbial mats. Their movement, which moved sand grains and disrupted the thick mats, allowed water and chemicals from above and below to mix.
Effects of the revolution
The Cambrian substrate revolution was a long and uneven process that happened at different speeds in different places during the Cambrian period.
After the agronomic revolution, microbial mats that covered the Ediacaran seafloor became limited to only certain environments.
The first burrowers likely ate the microbial mats and dug beneath them for protection. This digging caused the mats to decline.
Before the revolution, bottom-dwelling organisms were divided into four groups:
The "undermat miners" likely disappeared by the middle of the Cambrian period. "Mat encrusters" and "mat stickers" either disappeared or developed stronger anchors to attach to soft or hard surfaces. "Mat scratchers" were limited to rocky areas and deep ocean regions, where both they and the mats could survive.
Early sessile echinoderms were mostly "mat stickers." Helicoplacoids could not adapt to new conditions and died out. Edrioasteroids and eocrinoids survived by growing holdfasts to attach to hard surfaces and stalks to raise their feeding parts above loose seafloor debris. Mobile echinoderms, such as stylophorans, homosteleans, homoiosteleans, and ctenocystoids, were not greatly affected by the substrate revolution.
Early molluscs likely grazed on microbial mats, so it is reasonable to think they lived in areas where mats still existed. The oldest known fossils of monoplacophoran molluscs (those with a single shell plate) date to the Early Cambrian, where they grazed on microbial mats. Most modern monoplacophorans live on soft seafloor areas in deep ocean regions, though one type lives on hard surfaces near continental shelves. The oldest known fossils of polyplacophorans (molluscs with multiple shell plates) are from the Late Cambrian, when the substrate revolution had changed marine environments significantly. Since these fossils are found with stromatolites (stubby pillars made by microbial mats), it is thought that polyplacophorans grazed on microbial mats. Modern polyplacophorans mainly graze on mats on rocky coastlines, though some live in the deep sea. No fossils of aplacophorans (shell-less molluscs) have been found. These are considered the most primitive living molluscs. Some burrow into deep seafloor sediments to eat microorganisms and detritus, while others live on reefs and eat coral polyps.
The revolution ended the conditions that allowed exceptionally preserved fossil beds, such as the Burgess Shale, to form. Direct consumption of dead organisms had little effect on fossilization compared to changes in sediment chemistry, porosity, and microbiology, which made it harder for chemical gradients needed for soft-tissue mineralization to develop. Like microbial mats, environments that supported this type of fossilization became limited to harsher, deeper areas where burrowers could not live. Over time, burrowing became widespread enough to make this type of preservation impossible. Post-Cambrian fossil beds of this kind are usually found in unusual environments.
The increase in burrowing is important because burrows provide clear evidence of complex organisms. They are also more likely to be preserved than body fossils. The absence of trace fossils has been used to suggest that large, mobile bottom-dwelling organisms were not present. This helps scientists understand the early Cambrian period and shows that the Cambrian explosion was a real diversification of life, not just a result of fossil preservation conditions, even if it did not happen at the same time as the agronomic revolution.
The rise of burrowing represents a major change in ecosystems. The appearance of the complex burrow Treptichnus pedum is used to mark the beginning of the Cambrian period.
The increased bioturbation (mixing of seafloor sediments by burrowing organisms) meant that sulfur, which enters the ocean from volcanoes and rivers, was more likely to be oxidized rather than buried as sulfide. Burrowing organisms exposed sulfur to oxygen, allowing it to form sulfate. This activity is linked to a sudden increase in sulfate levels near the start of the Cambrian. This change can be seen in the geological record through sulfur isotope measurements and the abundance of the sulfate mineral gypsum.