The Worms That Turned

Research shows first burrowing animals helped engineer explosion of life

This article is reproduced with kind permission from the New Memorial University Gazette

The Cambrian Explosion was the rapid diversification of life on Earth, which began in the seas about 542 million years ago. One of the most important aspects was the evolution of animals that could burrow into sand and mud. This 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 on fossils from Newfoundland and Labrador, published in Geological Society Special Publication ‘Earth System Evolution and Early Life: A Celebration of the Work of Martin Brasier’, provides new insights into exactly which burrowing animals were the most important engineers.

The paper was written by Dr. Duncan McIlroy, a professor with Memorial University’s Department of Earth Sciences, Faculty of Science; Dr. Liam Herringshaw, a former post-doctoral fellow at Memorial and current lecturer at the University of Hull in the U.K.; and Dr. Richard Callow, also a former post-doctoral fellow in the Department of Earth Sciences currently of Statoil in Norway.

“For the first few billion years of life on Earth, life in the oceans was pretty simple,” said Dr. McIlroy.

“It was mostly microscopic, and most of the organic matter produced in the water column by photosynthesis ended up on the seafloor, and became permanently buried. With the evolution of large, immobile organisms known as the Ediacaran fauna, around 585 million years ago, some of the seafloor nutrients 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 in search of buried nutrients.”

Fundamental change

The researchers argue that this evolution of burrowing caused a fundamental change in the interactions between the Earth’s chemistry, geology and biology, helping to trigger the Cambrian Explosion.

The act of burrowing allowed access to buried organic matter, the creation of habitats, and introduced oxygen and other elements that increased the activity and diversity of micro-organisms in the seafloor sediments. The 2D Precambrian seafloor became the much more modern 3D seafloor of the Cambrian.

“The evolution of burrowing completely changed the Earth, irreversibly creating seafloor conditions so that they became ideally suited to complex animal life.” — Dr. Liam Herringshaw

The paper’s authors think the additional microbial food resources engineered by burrowing may have literally fuelled the Cambrian-era explosion of animal life by providing a rich new energy source for the newly evolved animal groups with their high metabolic demands.

“The evolution of burrowing completely changed the Earth, irreversibly creating seafloor conditions so that they became ideally suited to complex animal life,” said Dr. Herringshaw.

“The process by which animals change an environment to their own benefit is known as ecosystem engineering. By studying the fossilized burrows from the earliest Cambrian rocks preserved in Newfoundland and Labrador, Canada, we could apply modern ecological methods to try to better understand what these pioneer burrowers were doing, and how their behaviour affected the surrounding sediment and water column.”

Biosphere engineering

Using these methods, the researchers assigned “ecosystem engineering impact” values to fossil burrow types from the earliest Cambrian rocks in Newfoundland and Labrador.

“One might even call it biosphere engineering!” — Dr. Duncan McIlroy

The most famous fossil burrow from the early Cambrian, Treptichnus pedum, which is used to define the base of the Cambrian, was found to be important, but probably not as important as other deeper, more complex fossil burrows in the same rocks. This is because deeper burrows create more habitat within the sediment for other animals, and increase microbial productivity, which is food for many seafloor-dwelling microbivorous animals.

“The creation of the more 3D Cambrian world with positive feedback loops between microbial productivity in the sediment and the burrowing organisms that feed on those microbes is an example of ecosystem engineering at the grandest scale,” said Dr. McIlroy.

“One might even call it biosphere engineering!”

• Journal reference: Liam G. Herringshaw, Richard H. T. Callow and Duncan McIlroy, Engineering the Cambrian explosion: the earliest bioturbators as ecosystem engineers Geological Society, London, Special Publications, 448, first published on January 23, 2017, doi:10.1144/SP448.18