A fossil estimated to be more than 500 million years old is offering a new perspective on origins of spiders and scorpions. Researchers say the discovery suggests arachnids, including spiders, come from the sea, challenging the long‑accepted belief that these creatures evolved only on land.
The study, conducted by scientists from the University of Arizona and King’s College London, focused on Mollisonia symmetrica, an early marine animal. The structure of its preserved nervous system closely resembles that of modern spiders, suggesting a shared ancestry that may have begun in the sea rather than dry environments. The findings were published in Current Biology.
Fossil suggests spiders come from the sea
Arachnids — which include spiders, scorpions, mites, and ticks — have shown little physical change for hundreds of millions of years. Fossil evidence has traditionally placed their evolution on land, but this new research paints a different scenario.
The brain structure of Mollisonia revealed five nerve clusters that matched the arrangement seen in present‑day arachnids. These clusters controlled five pairs of limbs at the front of the animal’s body. Its nervous system also connected to claw‑like appendages similar to spider fangs — but in a position opposite to what is found in crabs and horseshoe crabs. This arrangement points to a closer link with spiders rather than other marine relatives.
Lead author Nicholas Strausfeld explained that the fossil helps address long‑debated questions about when arachnids first appeared and whether they emerged from marine or semi‑aquatic environments.
Ocean ancestry could explain arachnid skills
The unique wiring in the fossil’s nervous system may help explain the exceptional hunting abilities of spiders, such as stalking prey and weaving webs. Co‑author Frank Hirth of King’s College London noted that some brain regions in Mollisonia align with those of modern arachnids, hinting at shared genetic patterns across these creatures.
Researchers believe early arachnids may have adapted to life on land earlier than previously thought, feeding on primitive insects and millipedes. This predator‑prey relationship may have even influenced the evolution of wings in insects, offering them a way to escape ground‑dwelling hunters.
The team studied the fossil using specialized imaging tools at Harvard University’s Museum of Comparative Zoology and compared it with more than 100 traits from living and extinct arthropods. Their analysis placed Mollisonia close to modern arachnids on the evolutionary tree, though incomplete fossils of related species leave questions still unanswered.