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4w ago

Cambrian Fossil Reveals Marine Origin of Arachnids

Using high-resolution neuroanatomical imaging, researchers identified that Mollisonia’s reversed front-to-back brain architecture mirrors modern arachnids, overturning the theory that spiders first diversified on land.

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© Sergio Yoneda via Shutterstock
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Mollisonia symmetrica is characterized by a backward-folded unsegmented brain. Image credit: Strausfeld et al., doi: 10.1016/j.cub.2025.06.063.

Overview

  • The study analyzed the fossilized nervous system of the 500-million-year-old species Mollisonia symmetrica with high-resolution imaging.
  • It revealed Mollisonia’s brain has a reversed front-to-back organization identical to modern arachnids and distinct from horseshoe crabs, insects and crustaceans.
  • A statistical comparison of 115 neuronal and anatomical traits placed Mollisonia as a sister group to today’s spiders, scorpions and harvestmen.
  • Findings overturn the view that arachnids first diversified on land by supporting their emergence in marine environments during the Cambrian.
  • Researchers are now seeking similar neural evidence in other Cambrian specimens to further validate the ocean-origin theory for arachnids.