The worms that turned

Our new research from Newfoundland identifies how the first burrowing animals helped engineer the explosion of life as we know it. It took me about 7 years to finally write it up for publication, but let's not worry too much about that. It's the blink of a geological eye.

The Cambrian Explosion was the (geologically) rapid diversification of life on Earth, which began in the seas about 542 million years ago. One of its most important drivers was the evolution of animals that could burrow into sand and mud. These pioneering worms and other invertebrates turned the seafloor from a 2D to a 3D world, creating new habitats and ecosystems and completely changing the movement of nutrients and resources, over the period of a few million years. New research I carried out with my dear friends Duncan McIlroy and Rich Callow on the earliest Cambrian rocks from Fortune Head, Newfoundland, published by the Geological Society of London, provides insights into exactly which burrowing animals were the most important engineers. 

The spiralling trace fossil Gyrolithes from Fortune Head, Newfoundland. We interpret this type of burrow as having had major ecosystem engineering impact in the Cambrian Explosion.

For the first few billion years of life on Earth, life in the oceans was extremely simple. It was mostly microscopic, mostly produced in the water column by photosynthesis, and most of this organic matter ultimately ended up on the seafloor and became permanently buried. Around 585 million years ago, with the evolution of large, immobile organisms known as the Ediacaran Fauna, some of these nutrients locked up in the sediments of the seafloor began being recycled. However, it was not until the start of the Cambrian period, around 542 million years ago, that animals like worms and arthropods evolved, and started burrowing into the seafloor.

The Ediacaran fossil Charniodiscus from Bonavista, Newfoundland.

Whether they did this to escape newly evolved predators, in search of buried nutriment, or for some other reason, is still debated, but whatever the primary driver, it was evolutionarily revolutionary. The act of burrowing created habitats, allowed access to buried organic matter, and would have introduced oxygen and other elements that increased the activity and diversity of micro-organisms in the seafloor sediments. Our research considers that this evolution of burrowing caused a fundamental change in the linked biosphere-geosphere system, helping to trigger the Cambrian Explosion. The 2D Precambrian seafloor became the much more modern 3D seafloor of the Cambrian. The additional microbial food resources engineered by burrowing may have literally fuelled the Cambrian explosion of animal life, by providing a rich new energy source for the newly evolved animal groups with their high metabolic demands.
The process by which animals create, modify or maintain a habitat, either through their physical presence (e.g. a coral reef) or their behaviour (such as burrowing) is known as ecosystem engineering. By applying modern ecological methods to the fossilized burrow types preserved in the earliest Cambrian rocks at Fortune Head in eastern Newfoundland, we were able to to try to better understand what these pioneer burrowers were doing, and how their behaviour affected the surrounding sediment and water column.

Delegates from Ichnia 2012 inspect the Ediacaran-Cambrian boundary at Fortune Head.

When the fossil burrows were assigned 'Ecosystem Engineering Impact' values in accordance with modern ecological methods, such as those developed by researchers Martin Solan and Richard Bambach, we found that some structures were much more important than others. The most famous fossil burrow from the early Cambrian, Treptichnus pedum, which is used to define the base of the Cambrian, was shown to be an important engineered structure. However it was probably not as important as other deeper, more complex fossil burrows in the same rocks, such as Gyrolithes. This is because these deeper burrows would have created greater volumes of habitat within the sediment for other animals, and helped increase microbial productivity, which would then have provided food resources and fluxes for other marine animals.

Once the burrowing had begun, it developed and diversified in all sorts of ways. The beginning of the more three-dimensional Cambrian world created positive feedback loops between the burrowing organisms' behaviour and the microbial productivity in the sediment. New habitats, new behaviours and new food sources continued to evolve in an example of ecosystem engineering at the grandest scale. One might even call it biosphere engineering. Or, as my old Newfoundland office mate Dario would have had it:


An explosion, but not Cambrian (image from Wikimedia Commons).

The published paper, Engineering the Cambrian Explosion: the earliest bioturbators as ecosystem engineers, is freely available to everyone via Gold Open Access thanks to the generosity of Duncan McIlroy and the Memorial University of Newfoundland. It is part of a special publication of the Geological Society of London, Earth System Evolution and Early Life: A Celebration of the Work of Martin Brasier.