A surprising discovery about spiders changes existing theories about their origins.

The study was conducted by a team of American and British scientists from the University of Arizona, Lycoming College, and King's College London, led by Nicholas Strausfeld. The results were published on July 22 in the scientific journal Current Biology.

Scientists they analyzed a fossil from about 500 million years ago

Scientists have conducted a new, detailed analysis of fossilized features of the brain and central nervous system of an extinct animal called Mollisonia symmetrica . Previously, it was thought to represent an ancestor of a group of arthropods known as chelicerata, which lived during the Cambrian period (between 540 and 485 million years ago) and included the ancestors of today's horseshoe crabs. Mollisonia resembled some other early chelicerata from the Lower and Middle Cambrian. Its body consisted of two parts—a broad, rounded "armor" at the front and a robust, segmented trunk ending in a broad, tail-like structure. Some scientists believe this small marine arthropod resembled a modern-day scorpion.

Analysis Mollisonia fossils led to a surprising discovery concerning spiders

However, research has revealed that the arrangement of neurons in the fossilized Mollisonia brain is not organized like that of horseshoe crabs, or even crustaceans or insects. The pattern of radiating nerve centers is reversed—as in modern spiders and their relatives.

The fossilized brain and nervous system of this small marine arthropod thus resemble those of arachnids. The anterior part of Mollisonia's body, called the prosoma (cephalothorax), contains—as in spiders and other modern arachnids—radially arranged segmental ganglia that control the movements of five pairs of segmented legs.

But these aren't the only arachnid-like features. Mollisonia has been found to possess an unsegmented brain, from which short nerves run to a pair of pincer-like "claws," resembling the fangs of spiders and other arachnids.

– This is an important step in evolution, which seems to be unique to arachnids – emphasized Frank Hirth from King's College London, one of the co-authors of the study, quoted in the University of Arizona press release.

Until now, it was believed that arachnids evolved from a common ancestor that inhabited land. However, the discovery of the brain and nervous system of Mollisonia challenges the previously common belief that arachnid evolution occurred only after their common ancestor conquered land.

Scientists They examined a fossil from about 500 million years ago. They used advanced techniques imaging

For this study, Nicholas Strausfeld took dozens of photographs of a Mollisonia specimen housed at Harvard University's Museum of Comparative Zoology. He took these photographs under various lighting directions, light intensities, polarized light intensities, and magnifications.

David Andrew, co-author of the study, conducted a statistical analysis comparing 115 neural and related anatomical features in arthropods, both extinct and living. This was done to rule out the possibility that the similarity between the Mollisonia brain and the spider brain was a coincidence resulting from parallel evolution, rather than the result of a common developmental lineage. The analysis placed Mollisonia as a sister group to modern arachnids. This, in turn, further supports the theory that the Mollisonia lineage gave rise to the branch that now includes spiders and scorpions.

Using advanced imaging techniques, researchers were able to identify key anatomical features of the fossilized Mollisonia . Using a light microscope, they imaged its central nervous system. This led to a surprising discovery that arachnids may have evolved in the ocean, not on land.