By James Ashworth
The Cambrian Explosion is a landmark moment in the history of life on Earth when many of the major groups of animals first appear in the fossil record.
New research, however, suggests that many of their key characteristics were already in place millions of years earlier during the Late Ediacaran Period.
The Cambrian Explosion may have been less of a burst of evolution, and more of a final flourish.
This event, which lasted from around 539 to 519 million years ago, is when the major animal groups alive today first appeared. Examples of these different categories, known by scientists as a phylum, include molluscs, arthropods and tardigrades.
While animals existed before the Cambrian Explosion, in a period known as the Ediacaran, they’ve been hard to study as they rarely fossilise. Generally, the only reminders of these animals are trace fossils, such as the tracks and burrows they left behind.
Our scientist Dr Zekun Wang has led new research to see what these traces can reveal about the animals which made them. The shape of an animal can be linked to the tracks that it leaves behind, while its route through an environment can be used to infer what senses it might have had.
“Life in the Ediacaran was no longer microscopic, but typically, it wasn’t able to move along the seafloor,” Zekun says. “By the Cambrian, however, animal life could explore the seabed. Something clearly changed, but the trace fossils that survive from this transition lack many anatomical details.”
“By studying their mathematical properties instead, we can infer what the animals that made the traces might have been like. We see a clear progression towards more advanced senses and ways of moving, as well as slender body profiles, at the end of the Ediacaran. This sets the stage for the Cambrian Substrate Revolution and the Cambrian Explosion.”
Zekun’s findings have been published across two papers in the journals Proceedings of the Royal Society B and Geology.
Life in the Ediacaran Period
The Ediacaran Period lasted for almost 100 million years from around 635 to 539 million years ago. It’s a time in Earth’s history when life evolved from more simple collections of cells to more complicated structures made up of different tissues.
While a few hardy organisms had already moved onto land, plants and animals lived exclusively in the sea. The sediment on the ocean floor occasionally trapped some of these soft-bodied animals, preserving species like Kimberella and Dickinsonia as body fossils.
The majority of the mobile animals from the Ediacaran Biota, however, are known from traces. Earlier in the period these are often simple horizontal trails and burrows, but change towards the end as the tracks start to become more complex.
Zekun’s research focuses on how smoothly and tightly the tracks turn to reveal new details about the animal which made them.
“Think of a snake, for example,” explains Zekun. “While these animals didn’t exist during the Cambrian, a snake will always make a long, curving path behind it because of its slender shape.”
“As a result, there are certain turns that can never be made by this body shape because they can only bend a certain amount. If there are abrupt changes in the angle of a track, then it must have been made by a shorter, even round, body shape.”
“We can also link the smoothness of a track to an animal’s perception, as animals with long range senses can plan a direct journey towards food. If they have limited sensory abilities, then the animal will only find food when it stumbles into it. In this case, the trajectory will be unsmooth and to some extent random as an animal wanders around its environment.”
Zekun, along with his co-authors Professor Gabriela Mangano, Professor Luis Buatois and Dr Nianzhi Hang, investigated over 170 different trace fossils from the Ediacaran–Cambrian transition. They compared the fossils to the tracks of modern arthropods, gastropods and worms to see what could be learned about wildlife more than 540 million years ago.
How did Ediacaran fauna develop?
After analysing the Ediacaran fossils, the team divided them into three different groups. The first, from around 550 million years ago, are relatively simple tracks with many abrupt turns. These are thought to have been made by simple animals with short, round bodies and limited senses that could turn on the spot.
The researchers suggest that these animals might have moved like amoeba, extending parts of their body to move forward, or had hair-like structures called cilia to help them move.
Around 545 million years ago, smoother tracks with fewer abrupt turns appear. The tracks resemble those left by living horseshoe crabs, slugs and snails, and suggest that short animals were gaining greater control of their movement to make more co-ordinated journeys.
At about the same time, entirely smooth trails similar to the tracks left behind by modern worms appear in the fossil record. While these animals can no longer turn abruptly, their long and streamlined bodies have reduced drag and a larger surface area for sensory organs to cover.
“By using the data from tracks made by known animals through deep time, my colleague Dr Olmo Miguez-Salas and myself were able to ultimately estimate the minimum body length of the trace maker,” Zekun says.
“Our results show that the length of the animals making these tracks gradually increases over the Ediacaran to Cambrian transition, moving from an ovoid shape towards more worm-like bodies.”
Together, the papers help to build the case that many of the key characteristics animals needed to thrive on Earth were developing millions of years before the Cambrian. These traits might have allowed them to survive the end of the Ediacaran, when many other unique species disappear after a presumed major extinction.
This provided a clean slate for the predecessors of modern animals to diversify during the Cambrian Explosion which followed, establishing much of life on Earth as we know it today.