Paleontologists have examined the fossilized characteristics of the brain and central nervous system of Mollisonia symmetrica, an extinct organism that existed during the mid-Cambrian period approximately 508 million years ago. Their findings indicate that the nervous system of Mollisonia symmetrica aligns with that of modern spiders and scorpions (arachnids). This revelation contests the long-standing theory that arachnid diversification occurred only after their common ancestors adapted to terrestrial life.
Mollisonia symmetrica is characterized by an unsegmented brain with short nerves extending into pincer-like appendages, resembling the fangs of spiders. Image credit: Strausfeld et al., doi: 10.1016/j.cub.2025.06.063.
Previously, Mollisonia symmetrica was thought to represent an ancestor of a specific group of arthropods known as Chelicerata, which thrived during the Cambrian period and included the forebears of today’s horseshoe crabs.
Surprisingly, Professor Nicholas Strausfeld and his team at the University of Arizona found that the organization of the nerve structure in the fossilized brain does not resemble that of horseshoe crabs but is instead more akin to that of contemporary spiders and scorpions.
“A lively debate continues regarding the origin of arachnids, the type of progenitor they emerged from, and whether these progenitors were horseshoe crabs,” Professor Strausfeld noted.
Mollisonia symmetrica shares physical features with other early chelicerates from the lower and middle Cambrian periods, possessing a body divided into two main segments.
Some researchers have highlighted the anterior shell followed by a segmented trunk reminiscent of scorpions.
However, no one has claimed that Mollisonia symmetrica was more closely related to horseshoe crabs than to more basal arthropods.
What Professor Strausfeld and his co-authors found is that Mollisonia symmetrica, identified as an arachnid, exhibits a fossilized brain and nervous system.
Similar to spiders and other modern arachnids, the anterior portion of Mollisonia symmetrica (known as the prosoma) features a pattern of segmental ganglia that governs the movement of five pairs of appendages.
In addition to these arachnid-like traits, Mollisonia symmetrica also possessed an unsegmented brain with short nerves extending into pincher-like structures, reminiscent of spider fangs.
Critically, the unique feature defining arachnids is the specific arrangement of the brain, which contrasts with the structure found in current crustaceans, insects, centipedes, and even horseshoe crabs like Limulus.
“It’s comparable to the Limulus type brains in Cambrian fossils, or the ancestral brains of modern crustaceans and insects, which are similar to those of contemporary spiders,” Professor Strausfeld remarked.
“These findings may signify a crucial evolutionary advancement, as studies of modern spider brains indicate this arrangement allows for quicker neural control pathways.
This configuration may enhance efficiency in hunting, quick pursuits, and stealth in arachnids.
“This is a significant evolutionary milestone, seemingly exclusive to arachnids.”
“In Mollisonia symmetrica, we identified brain regions corresponding to extant species, which could reveal the underlying genetic framework common to all arthropods.”
“The arachnid brain implies that, unlike other brains on Earth, its tissues are linked to rapid calculation and motor action control,” Professor Strausfeld explained.
“The earliest terrestrial creatures were likely arthropods that resembled insects, possibly ancestral to crustaceans.”
“We can envision Mollisonia symmetrica—like arachnids—adapting to land, which may have given rise to early insects and their feeding strategies.”
“The first land-dwelling spiders could have played a vital role in developing essential defensive traits, such as insect wings, leading to flight and evasion.”
“The ability to fly provides significant advantages when being pursued by spiders.”
“Nonetheless, despite the agility conferred by flight, insects remain ensnared in the intricate silk webs spun by spiders.”
The results will appear in the journal Current Biology.
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Nicholas J. Strausfeld et al. Cambrian origin of the spider brain. Current Biology Published online on July 22, 2025. doi:10.1016/j.cub.2025.06.063
Source: www.sci.news
