Ancient fossils show that spiders may have come from the sea

Five hundred million fossils of a small marine predator are shaking long-standing assumptions about the origins of spiders and their relatives.

According to a new study Current Biologynervous system Mollisonia Symmetrica– Cambrian Arthropods – The brain characteristics of the Reveals are arachnoid features, indicating that they evolved in the oceans, rather than what was previously thought to be land.

Located at the Harvard Museum of Comparative Zoology, the fossil retains rare details of the creature’s brain and nerve rope. These neural patterns match those found in modern spiders and scorpions, rather than traditionally supposed marine relatives, such as horseshoe crabs. The results show that before eventually colonizing the land, spider webs may have developed unique brain tissue in aquatic environments.

Ancient brain, modern clues

Mollisonia’s body plan resembles early chelates: a dome-shaped cover, a segmented trunk and claw-like appendages. But its brain tells a different story. Using optical microscopes and statistical comparisons of 115 neural features across fossil arthropods, the researchers determined that Mollisonia’s brains lined up like modern spiders, and that the orientation of the key brain regions is the key area of orientation compared to crustaceans, insects, and even insect crabs.

“It seems like the Lilulus-type brain … has gone backwards, and that’s what we see in modern spiders,” explains Nicholas Strausfeld, a senior author at the University of Arizona.

Major findings from the study

– Mollisonia’s brain characteristics include radiation patterns of the ganglion and reverse sequences of brain regions.
– Its forebrain area is connected to the Chelicerae, a paw-like appendage, similar to spider fangs.
– Phylogenetic analysis considers Mollisonia as a close relative of modern spider webs, rather than horseshoe crabs.
– This supports land adaptation to the oceanic sources of spiderworms.

Why is brain arrangement important

Frank Hirth, co-author of King’s College in London, noted that reverse brain tissue may offer evolutionary advantages: “This is the main step in evolution…it’s very likely.” [conferring] The invisibility of hunting, the speed of pursuing speed and…the dexterity of website rotation. ”

Strausfeld added that this neural structure could also lead to the extraordinary predatory success of Arachnids, saying Arachnid’s brain is “different from any other brain on this planet.”

Rewrite the origin story of Arachnid

So far, it is believed that spider webs have evolved only after they migrate to land. Their signature brain tissue exists in marine life since 500 million years ago, and this discovery rewrites the narrative. Mollisonia, and possibly other similar Cambrian arthropods, may be the aquatic ancestors of today’s spiders, scorpions and their relatives.

If true, this challenges a single, land-based spider web and raises new questions about how many times these animals transition between the ocean and land.

Magazine: Current Biology
doi: 10.1016/j.cub.2025.06.063

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