The tiny fossil of a creature which lived half a billion years ago shows that arachnids formed in Earth’s ancient oceans, rather than on land.

About 500 million years ago, the “Cambrian explosion” saw a massive diversification leading to the evolution of all the basic body plans of animals which still exist today. This includes the evolution of arthropods – animals with external skeletons which today includes spiders, crabs, insects and scorpions.

Spiders, scorpions and horseshoe crabs – belonging to the group called chelicerates – have been virtually unchanged for more than 400 million years.

The main difference with extinct species is that they could be massive. The largest ever scorpions were about 70cm in length. Meanwhile, ancient relatives of spiders and scorpions called eurypterids (sometimes called “sea scorpions”) could grow to 2.5m long.

But the evolutionary origin of spiders and scorpions has long been a mystery. It was previously thought that arthropods diversified into spiders and scorpions after they had transitioned from the sea onto land.

New research published in Current Biology suggests the opposite – that the arthropod group which became scorpions and spiders first emerged under water.

“It is still vigorously debated where and when arachnids first appeared, and what kind of chelicerates were their ancestors, and whether these were marine or semi-aquatic like horseshoe crabs,” says lead researcher Nicholas Strausfeld from the University of Arizona, USA.

Strausfeld’s team analysed the fossil specimen from the species Mollisonia symmetrica.

Illustration of what Mollisonia would have looked like. Credit: Nick Strausfeld.

Mollisonia was first discovered in 1912 in Canada’s Burgess Shale. There are 4 Mollisonia species found in North America and China, all just a few centimetres in length.

Mollisonia is from the Cambrian period (540–485 million years ago) and resembles other chelicerates from that time. But Strausfeld and colleagues found something in the fossilised nervous system which set this little, armoured creature apart.

The front part of Mollisonia’s body has radiating segmental ganglia (neuron clusters) that control the movements of 5 pairs of segmented appendages. This same pattern is seen in spiders and other present-day arachnids. Mollisonia also has unsegmented brain which extends short nerves to a pair of pincer-like “claws” like the fangs of spiders. 

The clincher which puts Mollisonia firmly in the arachnid camp is the organisation of its brain. It is the opposite arrangement to modern crustaceans, insects and centipedes, and even horseshoe crabs like those of the genus Limulus.

Advanced imaging techniques allowed the research team to identify key anatomical features in the fossilized remains of the Mollisonia specimen. Credit: Nick Strausfeld.

“It’s as if the Limulus-type brain seen in Cambrian fossils, or the brains of ancestral and present days crustaceans and insects, have been flipped backwards, which is what we see in modern spiders,” Strausfeld says.

“This is a major step in evolution, which appears to be exclusive to arachnids,” adds co-author Frank Hirth from the UK’s King’s College London. “Yet already in Mollisonia, we identified brain domains that correspond to living species with which we can predict the underlying genetic makeup that is common to all arthropods.”

“The arachnid brain is unlike any other brain on this planet and it suggests that its organisation has something to do with computational speed and the control of motor actions,” Strausfeld explains.

A side-by-side comparison of the brains of a horseshoe crab (left), the Mollisonia fossil (center) and a modern spider (right) reveal the surprising findings of this study. The organization of Mollisonia’s three brain regions (green, magenta and blue) are inverted when compared to the horseshoe crab, and instead resemble the arrangement found in modern spiders. Credit: Nick Strausfeld.

The first land animals were probably millipede-like and insect-like arthropods. 

“We might imagine that a Mollisonia-like arachnid also became adapted to terrestrial life making early insects and millipedes their daily diet,” Strausfeld says.

The first arachnids on land may even have contributed to the evolution of a critical defence mechanism, insect wings, spurring an evolutionary arms race between predator and prey that is hundreds of millions of years old.

“Being able to fly gives you a serious advantage when you’re being pursued by a spider,” Strausfeld explains. “Yet, despite their aerial mobility, insects are still caught in their millions in exquisite silken webs spun by spiders.”