Mollisonia symmetrica, a primeval predecessor of arachnids and king crabs, possessed a “reversed” brain akin to that of present-day spiders, according to a fresh study.(Image credit: Nick Strausfeld)ShareShare by:
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An extraordinary fossilized brain infers that the forebears of spiders and allied arachnids might have previously crept about the ocean, rather than terra firma as was for a long time believed, a recent study has discovered.
The fossil reveals that some aspects of the brain of a currently extinct creature identified as Mollisonia symmetrica are structured in reverse in comparison to those of the majority of modern arthropods, a sizable category of invertebrates that encompasses creatures like insects, crustaceans, and myriapods. Nevertheless, the brain of M. symmetrica parallels those within an arthropod category: arachnids, a grouping that comprises spiders, scorpions, and ticks. This variance indicates that the sea-faring M. symmetrica constitutes an ancient progenitor of current arachnids, as researchers documented Tuesday (June 22) within the journal Current Biology.
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Chelicerates similar to M. symmetrica separated from other arthropods around the mid-Cambrian era. The Mollisonia genus, encompassing four acknowledged species, existed approximately 515 million to 480 million years prior. The M. symmetrica species possessed a segmented physique analogous to that of a scorpion, a circular carapace, and six sets of appendages utilized for motion and predation.
Even though scientists lack certainty regarding the specific timing when arachnids further diverged from additional chelicerates, arachnids have inhabited the world for about 400 million years. Until now, their fossil chronicle implied they solely existed terrestrially.
In this novel study, the researchers scrutinized the fossilized brain and central nervous system from an M. symmetrica specimen procured from the Burgess Shale formation situated within the Canadian Rockies. Their investigation revealed that the animal’s brain did not exhibit organization corresponding to that of a horseshoe crab from the Limulus genus. Instead, particular zones of its brain seemingly exhibit arrangement in the contrary orientation when juxtaposed against those of alternate arthropods, mirroring the arrangement observed within the brains of contemporary spiders. This proposes that arachnids underwent evolution and divergence from horseshoe crabs at a more remote period than previously hypothesized by scientists.
The brain of a horseshoe crab (left), Mollisonia symmetrica (center) and a contemporary spider (right). Take note of how the primeval creature’s tri-part brain (green, magenta and blue) is inverted when paralleled with that of the horseshoe crab. Conversely, it mirrors the brain of a modern spider.
“It’s as if the Limulus-pattern brain observed in Cambrian fossils, or the brains belonging to ancestral and modern-day crustaceans and insects, have been turned around, which is the spectacle we witness in current spiders,” Strausfeld remarked.
This reverse configuration is unique to arachnid brains among modern organisms, implying that M. symmetrica represented an early arachnid, and this distinctive arrangement evolved within oceanic settings as opposed to later on land. Investigations into the brains of extant spiders propose that the inverted arrangement empowers spiders to orchestrate a multitude of facets pertaining to predatory locomotion, including their stealth, swiftness, and nimbleness.
“The arachnid brain stands apart from every other brain across the planet, which alludes to the possibility that its organizational structure bears relevance to computational rapidity and the management of motor operations,” Strausfeld elucidated.
Detailed depictions of Mollisonia symmetrica’s brain spanning beyond 500 million years in the past.
The subtlety and velocity of spiders as terrestrial predators might have played a contributing factor towards the development of insect wings, which would have facilitated prey’s ability to escape, Strausfeld appended.
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“The capacity for flight bestows upon one an immense advantage when pursued by a spider,” Strausfeld expressed. “However, notwithstanding their airborne versatility, insects continue to be ensnared in massive numbers within delicate silken webs fabricated by spiders.”
To ascertain whether the resemblances between the brain of M. symmetrica and those of contemporary spiders originated from a mutual descent or pure chance, the researchers employed computer software for estimating the likelihood of a relationship between them. In undertaking this, they drew comparisons between the brain and bodily attributes belonging to several existing and extinct arthropods. The study pointed toward the potential evolution of the Mollisonia bloodline into the arachnid classification, signifying its potential to have engendered “the globe’s supreme arthropodan predators,” in the words of the researchers.
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Skyler WareSocial Links NavigationLive Science Contributor
Skyler Ware functions as a freelance science correspondent, tackling subjects encompassing chemistry, biology, paleontology, and Earth science. She participated as a 2023 AAAS Mass Media Science and Engineering Fellow affiliated with Science News. Her body of work has similarly been featured within Science News Explores, ZME Science, and Chembites, amongst others. Skyler possesses a Ph.D. in chemistry from Caltech.
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Minuscule spiders crafting monumental ‘puppet’ decoys from detached prey identified within Peru and the Philippines