Half-a-billion-year-old fossil could be earliest known arachnid

The most ancient known arachnid could be more than 500 million years old.

While spiders and scorpions are some of the Earth’s most recognisable animals today, discovering their ancestors is much harder. Fossils of the earliest arachnids are few and far between, making it difficult to understand the key moments in their evolution.

New research into an ancient fossil from Canada called Mollisonia reveals that it might be an early arachnid. Scientists were able to decipher the structure of its brain from an exceptionally well-preserved fossil, showing that its structure is similar to spiders and scorpions living today.

Professor Nicholas Strausfield, the lead author of the study, says that Mollisonia also highlights the many questions that remain over how arachnids evolved and moved from the sea onto the land.

“It is still vigorously debated where and when arachnids first appeared, what their ancestors were and whether these animals were marine, or semi-aquatic like horseshoe crabs,” Nicholas explains.

"We might imagine that a Mollisonia-like arachnid could have later become adapted to terrestrial life by making early insects and millipedes their daily diet.”

Dr Richie Howard, our curator of fossil arthropods, described the new paper as “really interesting”, but said that some of its findings will be controversial.

“A 505-million-year-old arachnid is much older than anything that’s been found before, and there are strong similarities between the brain tissues in Mollisonia and living arachnids. However, we’ll need to compare other aspects of their biology beyond the brain to confirm how closely these animals are related.”

“It’s not unreasonable that Mollisonia could be a close fossil relative of arachnids, but there are other important fossils to consider like sea scorpions – whose brains we know very little about.”

The findings of the study were published in the journal Current Biology.

What does Mollisonia reveal?

Mollisonia has previously been described as a fossil chelicerate. This is the broader animal group that includes arachnids as well as horseshoe crabs and sea spiders, which despite their names are entirely separate from the crabs and spiders we’re familiar with.

Researchers examining a Mollisonia fossil in Harvard University’s Museum of Comparative Zoology had the opportunity to look inside its carapace because the specimen was split into two pieces. By taking a variety of microscope images at slightly different levels of focus, the scientists were able to make out the structure of its soft tissue.

The front of its body, known as the prosoma, contained a fan-like structure which is similar to the one found in the nervous systems of living scorpions, spiders and their relatives. This structure is involved with controlling an animal’s legs as well as its other appendages.

More important, however, was Mollisonia’s recurved brain. This structure is only found in arachnids and is thought to give spiders and their relatives better control over their many appendages for tasks such as spinning silk, running and feeding.

“The arachnid brain is unlike any other brain on this planet,” Nicholas says. “It’s as if the brains of ancestral and present-day crustaceans and insects have been flipped backwards”.

“It suggests that its organisation has something to do with an arachnid’s computational speed and the control of its motor actions.”

By comparing Mollisonia’s features to other arthropods, the researchers believe that it is a stem arachnid. This means it’s part of the same taxonomic group as spiders and scorpions but doesn’t have any living descendants.

Where do arachnids come from?

If Mollisonia’s identity is confirmed, then it would replace a 430-million-year-old scorpion known as Dolichophonus as the oldest known arachnid. The ancestors of living arachnids must have evolved at some point in time between these fossils, but evidence from these animals are yet to be found.

“The exoskeleton of arachnids isn’t mineralised, meaning that it doesn’t fossilise as often as those of trilobites or crabs,” explains Richie. “Plus, fossils of land-living animals are much rarer than those of marine ones, as most kinds of fossiliferous rock are formed from marine sediments.”

“The ideal candidate would be a fossil with the key characteristics of arachnids, such as six pairs of appendages on the front part of the body, but also having features shared by multiple living arachnid groups.”

Fossils from this time would also help to answer how arachnids moved onto land. Some scientists think that modern land-based arachnids all descend from a single group that climbed out of the sea, while others suggest that different groups became terrestrial independently.

As to why arachnids first left the water, that’s also unclear at this stage. It’s possible that protecting young from marine predators might have been one driver. Another possibility is that arachnids followed their prey onto the land.

Until we have more fossils, we might never fully know the cause or reasons. Richie suggests that another group of extinct arthropods, known as the sea scorpions, might provide clues about the arachnids’ transition.

“I think sea scorpions are an important part of the arachnids’ story, as they appear in the fossil record between Mollisonia and the first modern groups,” Richie adds. “They have characteristics that suggest they could be the transitional group between ancient marine chelicerates and land-based arachnids.”

“As more fossils are described, new information about arachnid origins will be revealed.”