The Cambrian Substratum Revolution saw the first excavators burrowing into the seabed, creating a new niche and leading to the evolution of burrowing organisms. This may have contributed to the Cambrian explosion. Digging provided food, anchoring, and protection from predators, leading to the decline of microbial mats.
The Cambrian Substratum Revolution was a pivotal evolutionary event that occurred at the dawn of the Cambrian Period 542 million years ago. The revolution consisted of the first excavators digging deep into the substrate, instead of grazing on the surface or just below the microbial mats that dominated the seabed at the time. Indeed, the beginning of the Cambrian is internationally defined by the first appearance of Trichophycus pedum, a ubiquitous trace fossil with a distinctive ring pattern. The excavator who created the trace fossil is unknown, but it may have been a snail or a primitive arthropod.
Prior to the Cambrian substrate revolution, the seabed consisted of a microbial mat over a hard, stratified, sulphidic, anoxic substrate. The animals lived on the mat, attached to it by clips, grazed on the surface of the mat, nestled on it, or burrowed immediately beneath it. Because the underlying substrate was almost completely anoxic (without oxygen), it was filled with sulfate-reducing bacteria, which gave off hydrogen sulfide, poisonous to most other organisms, which discouraged them from digging deeply.
After billions of years of substrates dominated by sulfate-reducing bacteria, some pioneering organisms began digging deeper into the substrate, making it looser and more oxygenated. This initiated a feedback loop whereby the substrate became less hostile to animals and animals took the initiative to burrow into it more aggressively, consuming the bacteria and other animals that were colonizing the newly oxygenated environment. The end result has been the evolution of a wide range of burrowing organisms and the opening of a new oceanic niche. The oxygenation of the upper substrate layer may have contributed to the Cambrian explosion, a massive evolutionary radiation event that occurred soon after the Cambrian substrate revolution. Conversely, the Cambrian explosion may have had other causes, and the Cambrian substrate revolution may have been a side effect of the diversification of organisms.
There are three main benefits to digging that would have accelerated the Cambrian substrate revolution once it began: the availability of food, anchoring (which prevented animals from being swept away by currents), and avoiding predators. Since predators tend to be larger and less maneuverable than small animals, few of them would have burrowed into the muddy substrate. The Cambrian substrate revolution marked the beginning of the end for microbial mats, which are now found only in extreme environments inhospitable to burrowers, such as salt lakes and the deepest depths of the ocean floor.
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