Today’s spiders spend most of their time on land, scuttling through fields, forests, deserts, and just about every other terrestrial habitat that’s out there. But that doesn’t mean that their ancestors always lived terrestrially.
A new study in Current Biology suggests that the arachnids — the group of arthropods that includes spiders, scorpions, and their close relatives — might have originally been ocean dwellers. Based on the finely preserved brain and nerve structures of a fossilized marine arthropod from around 500 million years ago, the study shows that the first arachnids may have emerged in the oceans and then transitioned to land later on.
“It is still vigorously debated where and when arachnids first appeared,” said Nicholas Strausfeld, a study author and a neuroscientist at the University of Arizona, in a statement. But the new research provides some answers, hinting that the ancestors of spiders and scorpions may have had a wetter start than traditionally thought.
Read More: This 444 Million-Year-Old Arthropod Was Fossilized Inside Out
An Spider Ancestor From the Sea?
Advanced imaging techniques allowed the research team to identify key anatomical features in the fossilized remains of the Mollisonia specimen. (Image Credit: Nick Strausfeld)
Arachnids have been around for around 400 million years, and according to the fossil record, they appear to have spent most of that time on land. But what about the first arachnids? Did they emerge on water or on land?
To answer that question, Strausfeld took a variety of photographs of a finely preserved fossil of Mollisonia symmetrica, a marine arthropod from the Cambrian period, approximately 500 million years ago, under various lighting conditions and magnifications. Separated into two segments, including a broad carapace at the front of its body and a segmented trunk and tail at the back, the fossil species was traditionally thought to be an ancestor of the horseshoe crab today.
But by revealing the fossilized traces of the specimen’s nervous system, the photographs illustrate that Mollisonia’s brain and nerves were arranged less like those in a modern horseshoe crab and more like those in a modern spider or scorpion, instead.
According to their analysis, Strausfeld and his co-authors suggest that the species was likely an ancestor of the first arachnids, implying that spiders and scorpions originated from a marine arthropod, rather than a terrestrial one.
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Origins of the Backwards Brain
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. (Image Credit: Nick Strausfeld)
So, what was it about the Mollisonia nervous system that indicated it was a relative of today’s spiders and scorpions?
First, Mollisonia had a network of radiating nerves that controlled the movement of its multiple appendages, much like the network of radiating nerves in arachnids today. Second, Mollisonia had a series of short nerves that controlled its pair of front-facing “claws,” similar to the nerves in modern arachnids, which control their own pairs of fangs and pincers. Third, Mollisonia’s brain had an inverted structure, which was not organized from front to back, as in horseshoe crabs, but from back to front, as in spiders, scorpions, and other arachnids.
Strausfeld and his co-authors say that this backwards arrangement may have been highly advantageous for the first arachnids, shortening the distance between their neuronal control centers and their nerves. In fact, this set-up might have helped them with their speed, stealth, and agility, allowing the first arachnids to exercise the impressive control over their own movement that’s still seen in spiders and scorpions today.
“The arachnid brain is unlike any other brain on this planet,” Strausfeld said in the release. “It suggests that its organization has something to do with computational speed and the control of motor actions.”
Read More: Scientists Find Fossilized Brains From 500-Million-Year-Old Arthropods
A Hope For Future Fossils
Of course, the similarities between the Mollisonia nervous system and the arachnid nervous system could represent nothing more than a mere coincidence. So, to confirm that they were the result of common ancestry, the researchers ran a statistical analysis of 115 neuronal traits from species throughout the arthropod family tree. Their analysis corroborated that Mollisonia was closely related to the first arachnids, supporting the theory that the species was an ancestor to modern spiders and scorpions.
Similar specimens may help researchers retrace more of the arachnids’ evolutionary history in the future. Though it is rare to find specimens with finely preserved nervous systems, the hope is that other species like Mollisonia could show how different arthropod lineages led to the different arachnids that skitter across the terrain today.
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Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.
