Previously, Mollisonia symmetrica was believed to represent an ancestral member of a particular group of arthropods referred to as chelicerates, which lived throughout the Cambrian duration and consisted of forefathers these days’s horseshoe crabs.

To their surprise, University of Arizona’s Professor Nicholas Strausfeld and his associates discovered that the neural plans in the animal’s fossilized brain are not arranged like those in horseshoe crabs, as might be anticipated, however rather are arranged the exact same method as they remain in contemporary spiders and their loved ones.

“It is still strongly discussed where and when arachnids initially appeared, and what type of chelicerates were their forefathers, and whether these were marine or semi-aquatic like horseshoe crabs.”

Mollisonia symmetrica outwardly looks like some other early chelicerates from the lower and mid-Cambrian because its body was made up of 2 parts: a broad rounded carapace in the front and a tough segmented trunk ending in a broad, tail-like structure.

Some researchers have actually described the company of a carapace in front, followed by a segmented trunk as comparable to the body strategy of a scorpion.

No one had actually declared that Mollisonia symmetrica Was anything more unique than a basal chelicerate, even more primitive than the forefather of the horseshoe crab.

What Professor Strausfeld and co-authors discovered showing Mollisonia symmetrica‘s status as an arachnid is its fossilized brain and nerve system.

As in spiders and other contemporary arachnids, the anterior part of Mollisonia symmetrica‘s body (called the prosoma) includes a radiating pattern of segmental ganglia that manage the motions of 5 sets of segmental appendages.

In addition to those arachnid-like functions, Mollisonia symmetrica Exposed an unsegmented brain extending brief nerves to a set of pincer-like claws, reminiscent of the fangs of spiders and other arachnids.

The definitive function showing arachnid identity is the distinct company of the mollisoniid brain, which is the reverse of the front-to-back plan discovered in contemporary shellfishes, bugs and centipedes, and even horseshoe crabs, such as the genus Limulus

“It’s as if the Limulus-type brain seen in Cambrian fossils, or the brains of ancestral and contemporary shellfishes and bugs, have actually been turned in reverse, which is what we see in contemporary spiders,” Professor Strausfeld stated.

“The latter finding might be an essential evolutionary advancement, due to the fact that research studies of existing spider brains recommend that this back-to-front plan supplies faster ways from neuronal nerve center to underlying circuits that collaborate a spider’s (or its relative’s) incredible collection of motions,” stated Dr. Frank Hirth, a paleontologist at King’s College London.

“This plan most likely provides stealth in searching, rapidity in pursuit and when it comes to spiders, a charming mastery for the spinning of webs to allure victim.”

“This is a significant action in development, which seems special to arachnids.”

“Yet currently in Mollisonia symmetrica, we determined brain domains that represent living types with which we can forecast the hidden hereditary makeup that prevails to all arthropods.”

“The arachnid brain differs from any other brain on this world, and it recommends that its company has something to do with computational speed and the control of motor actions,” Professor Strausfeld stated.

“The very first animals to come onto land were most likely millipede-like arthropods and most likely some ancestral, insect-like animals, an evolutionary branch of shellfishes.”

“We may envision that a Mollisonia symmetrica-like arachnid likewise ended up being adjusted to terrestrial life making early pests and millipedes their day-to-day diet plan.”

“The very first arachnids on land might have added to the advancement of a crucial defense reaction: insect wings, thus flight and escape.”

“Being able to fly offers you a major benefit when you’re being pursued by a spider.”

“Yet, regardless of their aerial movement, bugs are still captured in their millions in elegant silken webs spun by spiders.”

The outcomes appear in the journal Existing Biology

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Nicholas J. Strausfeld et alCambrian origin of the arachnid brain. Existing Biologyreleased online July 22, 2025; doi: 10.1016/ j.cub.2025.06.063