A fossilized creature that lived more than 500 million years ago is turning the origin story of spiders and scorpions on its head. Scientists now say that its brain structure indicates arachnids may not have evolved on land, as was previously believed, but rather in the oceans.
This bold idea comes from a new study by researchers at the University of Arizona and King’s College London. The team examined an ancient arthropod fossil and found surprising similarities to modern spiders.
Fossil indicates arachnid origins
Spiders, scorpions, and their kin – collectively known as arachnids – have barely changed in the past 400 million years.
Arachnids have reigned as top arthropod predators on land, and their fossil record strongly suggested that they evolved and diversified in terrestrial ecosystems.
The team studied a rare fossil of Mollisonia symmetrica, which has long been considered an early marine chelicerate in the same broader group that includes horseshoe crabs (Limulidae) and is closely related to the arachnids.
The assumption had always been that Mollisonia was part of an ocean lineage that led to land-based arachnids, but only much later on.
Ocean fossil had a spider’s brain
According to the researchers, this didn’t hold up under closer scrutiny. Instead of looking like the nervous system of a horseshoe crab, Mollisonia’s brain was like that of a modern spider.
Its nervous system included a distinct, radiating layout of ganglia – clusters of nerve cells – and an unsegmented brain that sendt signals to pincer-like front limbs. This unexpected configuration is a clue.
“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,” said Professor Nicholas Strausfeld, lead author of the study.
Modern arachnid brain structure
Mollisonia had a two-part body: a broad carapace at the front and a segmented tail-like trunk at the back.
Some researchers have compared this body shape to that of a scorpion. But nobody expected this creature to have anything close to a modern arachnid brain structure.
The Mollisonia nervous system had five pairs of ganglia in the prosoma (the front part of the body) for controlling the five limb pairs. That same layout is seen in spiders today.
The fossil also revealed short nerves that extend from the brain to a set of claw-like appendages – similar to the fangs in modern arachnids.
While the nerve connections to the chelicerae come from the rear of the brain in crabs, these connections come from the front of the brain in Mollisonia (and all modern spiders).
“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,” said Professor Strausfeld.
Advantages of the spider brain
This flipped brain may be the reason that spiders are so good at what they do – stalking, pouncing, and spinning intricate webs.
“This is a major step in evolution, which appears to be exclusive to arachnids,” said Frank Hirth of 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.”
Hirth suggested that this reversal creates neural shortcuts, making a spider’s brain better at coordinating fast and precise movements.
“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,” said Professor Strausfeld.
From ocean predators to land hunters
Arachnids may have had a head start when it came to surviving on land.
The first land animals were probably insect-like and millipede-like arthropods. But a Mollisonia-like ancestor may have made the transition too – and then turned its attention to those early pioneers.
“We might imagine that a Mollisonia-like arachnid also became adapted to terrestrial life making early insects and millipedes their daily diet,” said Professor Strausfeld.
These early arachnid predators may have helped push insects to evolve wings to fly away from danger.
“Being able to fly gives you a serious advantage when you’re being pursued by a spider,” noted Professor Strausfeld. “Yet, despite their aerial mobility, insects are still caught in their millions in exquisite silken webs spun by spiders.”
A fossil under the microscope
To capture the tiny details of Mollisonia’s nervous system and brain, Professor Strausfeld spent time at the Museum of Comparative Zoology at Harvard University.
The fossil was photographed under different lighting conditions, using polarizing filters and magnification to capture features invisible to the naked eye.
Still, the researchers needed to be sure this wasn’t just a coincidence – a case of two unrelated animals evolving similar traits.
To rule that out, David Andrew, a former University of Arizona graduate student now at Lycoming College, ran a statistical comparison. He analyzed 115 neural and anatomical traits across both living and extinct arthropods.
The results were clear: in terms of its nervous system, Mollisonia sits right next to modern arachnids on the evolutionary tree.
Other Mollisonia-like fossils haven’t been preserved well enough to check for similar spider brain features. But if they had the same nervous system structure, their descendants could explain the many arachnid species we see today.
The full study was published in the journal Current Biology.
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