Paleontologists have analyzed the fossilized options of the mind and central nervous system of Mollisonia symmetrica, an extinct animal that lived within the mid-Cambrian seas round 508 million years in the past. Their outcomes present that Mollisonia symmetrica’s nervous system corresponds to that of residing spiders and scorpions (arachnids). This discovery challenges the extensively held perception that the diversification of arachnids occurred solely after their widespread ancestor had conquered the land.
Mollisonia symmetrica is characterised by a backward-folded unsegmented mind. Picture credit score: Strausfeld et al., doi: 10.1016/j.cub.2025.06.063.
Till now, Mollisonia symmetrica was thought to characterize an ancestral member of a particular group of arthropods referred to as chelicerates, which lived through the Cambrian interval and included ancestors of immediately’s horseshoe crabs.
To their shock, College of Arizona’s Professor Nicholas Strausfeld and his colleagues discovered that the neural preparations within the animal’s fossilized mind will not be organized like these in horseshoe crabs, as may very well be anticipated, however as a substitute are organized the identical approach as they’re in fashionable spiders and their relations.
“It’s nonetheless vigorously debated the place and when arachnids first appeared, and what sort of chelicerates had been their ancestors, and whether or not these had been marine or semi-aquatic like horseshoe crabs.”
Mollisonia symmetrica outwardly resembles another early chelicerates from the decrease and mid-Cambrian in that its physique was composed of two elements: a broad rounded carapace within the entrance and a sturdy segmented trunk ending in a broad, tail-like construction.
Some scientists have referred to the group of a carapace in entrance, adopted by a segmented trunk as just like the physique plan of a scorpion.
However no person had claimed that Mollisonia symmetrica was something extra unique than a basal chelicerate, much more primitive than the ancestor of the horseshoe crab, for instance.
What Professor Strausfeld and co-authors discovered indicating Mollisonia symmetrica’s standing as an arachnid is its fossilized mind and nervous system.
As in spiders and different present-day arachnids, the anterior a part of Mollisonia symmetrica’s physique (referred to as the prosoma) incorporates a radiating sample of segmental ganglia that management the actions of 5 pairs of segmental appendages.
Along with these arachnid-like options, Mollisonia symmetrica additionally revealed an unsegmented mind extending brief nerves to a pair of pincer-like claws, harking back to the fangs of spiders and different arachnids.
However the decisive function demonstrating arachnid id is the distinctive group of the mollisoniid mind, which is the reverse of the front-to-back association present in present-day crustaceans, bugs and centipedes, and even horseshoe crabs, such because the genus Limulus.
“It’s as if the Limulus-type mind seen in Cambrian fossils, or the brains of ancestral and present-day crustaceans and bugs, have been flipped backwards, which is what we see in fashionable spiders,” Professor Strausfeld stated.
“The latter discovering could also be an important evolutionary improvement, as a result of research of present spider brains recommend that this back-to-front association offers shortcuts from neuronal management facilities to underlying circuits that coordinate a spider’s (or its relative’s) wonderful repertoire of actions,” stated Dr. Frank Hirth, a paleontologist at King’s School London.
“This association seemingly confers stealth in looking, rapidity in pursuit and within the case of spiders, an beautiful dexterity for the spinning of webs to entrap prey.”
“This can be a main step in evolution, which seems to be unique to arachnids.”
“But already in Mollisonia symmetrica, we recognized mind domains that correspond to residing species with which we are able to predict the underlying genetic make-up that’s widespread to all arthropods.”
“The arachnid mind is in contrast to some other mind on this planet, and it means that its group has one thing to do with computational velocity and the management of motor actions,” Professor Strausfeld stated.
“The primary creatures to come back onto land had been most likely millipede-like arthropods and possibly some ancestral, insect-like creatures, an evolutionary department of crustaceans.”
“We’d think about {that a} Mollisonia symmetrica-like arachnid additionally turned tailored to terrestrial life making early bugs and millipedes their each day weight loss program.”
“The primary arachnids on land could have contributed to the evolution of a vital protection mechanism: insect wings, therefore flight and escape.”
“With the ability to fly provides you a severe benefit once you’re being pursued by a spider.”
“But, regardless of their aerial mobility, bugs are nonetheless caught of their tens of millions in beautiful silken webs spun by spiders.”
The results seem within the journal Present Biology.
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Nicholas J. Strausfeld et al. Cambrian origin of the arachnid mind. Present Biology, revealed on-line July 22, 2025; doi: 10.1016/j.cub.2025.06.063
