Spiders originated in the sea half a billion years ago

By Stephen Beech

Spiders originated in the sea half a billion years ago, suggests a new study.

Finely preserved brain features in a tiny marine arthropod fossil suggest that arachnids – spiders and their close kin – may have first evolved in the ocean rather than on land, say scientists.

A new analysis of the "exquisitely preserved" fossil that lived half a billion years ago challenges the widely held belief that their diversification happened only after their common ancestor had conquered the land.

Previous research has shown that spiders and scorpions have existed for some 400 million years, with little change.

Based on their fossil record, arachnids appeared to have lived and diversified exclusively on land.

For the new study, published in the journal Current Biology, researchers from Britain and the United States undertook a detailed analysis of the fossilised features of the brain and central nervous system of an extinct animal called Mollisonia symmetrica.

Until now, it was thought to represent an ancestral member of a specific group of arthropods known as chelicerates, which lived during the Cambrian period, between 540 and 485 million years ago, and included ancestors of today’s horseshoe crabs.

The research team was surprised to find that the neural arrangements in Mollisonia's fossilised brain are not organised like those in horseshoe crabs, as could be expected.

Instead, they were organised the same way as they are in modern spiders and their relatives.

Study leader Professor Nicholas Strausfeld, of the University of Arizona, said: "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.”

He says Mollisonia outwardly resembles some other early chelicerates from the lower and mid-Cambrian in that its body was composed of two parts: a broad, rounded "carapace" in the front and a sturdy segmented trunk ending in a broad, tail-like structure.

Some scientists have referred to the organization of a carapace in front, followed by a segmented trunk as similar to the body plan of a scorpion.

But nobody had claimed that Mollisonia was anything more exotic than a basal chelicerate, even more primitive than the ancestor of the horseshoe crab.

What Prof Strausfeld and his colleagues found, indicating Mollisonia's status as an arachnid, is its fossilised brain and nervous system.

As in spiders and other present-day arachnids, he said the anterior part of Mollisonia’s body - called the prosoma - contains a radiating pattern of segmental ganglia that control the movements of five pairs of segmental appendages.

Mollisonia also revealed an unsegmented brain extending short nerves to a pair of pincer-like "claws" - reminiscent of the fangs of spiders and other arachnids.

But the decisive feature showing arachnid identity was the unique organization of the mollisoniid brain, which is the reverse of the front-to-back arrangement found in present-day crustaceans, insects and centipedes, and even horseshoe crabs, such as the genus Limulus.

Strausfeld said: "It's as if the Limulus-type brain seen in Cambrian fossils, or the brains of ancestral and present-day crustaceans and insects, have been flipped backwards, which is what we see in modern spiders."

Co-author Dr. Frank Hirth, of King’s College London, says the latter finding may be a "crucial" evolutionary development, because studies of existing spider brains suggest that this back-to-front arrangement provides shortcuts from neuronal control centres to underlying circuits that coordinate a spider’s movements.

He believes the arrangement likely confers stealth in hunting, rapidity in pursuit and in the case of spiders, an exquisite dexterity for the spinning of webs to entrap prey.

Dr. Hirth said, "This is a major step in evolution, which appears to be exclusive to arachnids.

"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."

Strausfeld added: "The arachnid brain is unlike any other brain on this planet.

"And it suggests that its organization has something to do with computational speed and the control of motor actions.”

He says the first creatures to come onto land were probably millipede-like arthropods and probably some ancestral, insect-like creatures, an evolutionary branch of crustaceans.

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

He believes that the first arachnids on land may have contributed to the evolution of a critical defence mechanism: insect wings, hence flight and escape.

Strausfeld added: "Being able to fly gives you a serious advantage when you're being pursued by a spider.

"Yet, despite their aerial mobility, insects are still caught in their millions in exquisite silken webs spun by spiders."

The Mollisonia specimen is housed at the Museum of Comparative Zoology at Harvard University.