Molluscs could have been the first animals to venture on land

The colonisation of land by animals was a landmark event in the history of life on Earth.

The first animals to set foot out of the oceans were believed to be arthropods, a highly successful group of invertebrates whose living members include insects and spiders.

But as many of these ancient arthropods had hard outer shells, this increased their chances of being preserved in the fossil record compared to other animal groups. The rich supply of their remains has led to arthropods being named the earliest land pioneers, but new research reveals this may not be the full picture.

Scientists have been studying the preserved tracks made by animals that were moving around on land over 500 million years ago and believe they were made by molluscs. This is another large group of invertebrates whose living members include animals such as limpets, snails and squid.

Dr Zekun Wang, a palaeontologist at the Natural History Museum and lead author of the study, says, “The most exciting part of this research is discovering that arthropods may not have been the first animals to explore these environments that are exposed to air in the inter-tidal region.”

“Instead, mollusc or mollusc-like animals might be the pioneers of these early excursions onto land which date back to the Cambrian. These animals could endure at least 15 minutes of air exposure, which shows they weren’t just coming onto land and immediately collapsing, they were able to spend a reasonable amount of time out of the water.”

The findings of this study were published in the journal Proceedings of the Royal Society B.

The early land pioneers

Animals evolved in the ocean over half a billion years ago and were initially restricted to marine environments for millions of years.

By around 420 million years ago, fossils show that animals were living out of the water. But how the transition from sea to land occurred is not fully understood and has fascinated scientists.

Th first movements likely started when animals living close to the shore temporarily ventured onto land before dying or retreating back into the water. But trying to understand more about this crucial period is challenging as fossils don’t often preserve well in these coastal environments as they were exposed to wave action and weather events.

With a lack of animal remains, scientists instead looked to trace fossils found in rocks that were exposed to air around half a billion years ago. Trace fossils are impressions of the biological activity of animals, such as tracks recording where they crawled or burrowed.

“Our first step was to identify what sort of animals were making these early trace fossils that were potentially formed on sediments exposed to air.” Says Zekun.

“We examined the traces of living and fossilised animals, looking for patterns in their movement that were characteristic of a particular group. We then applied this to the trace fossils and were able to identify that these indentations were likely produced by molluscs.”

“We then carried out some computer simulations to see how the ancient animals moved through sediment in environments that were in water and exposed to air.”

This allowed the team to distinguish between trace fossils which formed underwater and those that formed on land. This helped to confirm that the fossils they were looking at were formed by a mollusc moving on land. But it also allowed the scientists to figure out how long they were spending out of the water.

“We could also estimate how fast these organisms were moving, which helped us calculate how long they actually spent on land,” Explains Zekun.

The colonisation of land

The movement of animals onto land paved the way for life today as we know it.

At first, the terrestrial environment would have provided an opportunity for marine animals to escape predators or access new food sources with little competition. This could have been an essential lifeline which may have benefited individuals that were able to survive the longest out of water.

Over time, molluscs, arthropods and other animals evolved more features allowing them to spend more time out of the water. Eventually, after tens of millions of years, they were able to survive on land permanently.

Dr Imran Rahman, a palaeontologist at the Natural History Museum and co-author of the study, says, “The movement of these animals onto land would have had really important effects for the Earth’s system more broadly.”

“By coming onto land, they would have started to move and mix the sediment, which would have affected the distribution and availability of nutrients. If they died in this environment, their remains would provide another source of nutrients.”

“The effects of this ecosystem engineering could have paved the way for other animals to colonise the land in subsequent times. So it was a critical episode in the history of life, and we are now beginning to understand it more.”