The unusual fossil brain suggests that spiders and other arachnids may have once cut off the ocean, rather than land as long thought, new research finds.
Fossils show that certain features of the brains of now monster animals known as Morrison symmetric rings are located posteriorly compared to the features of most modern arthropods, a large group of invertebrates, including animals such as insects, crustaceans and fir-like. But M. Mymmetrica’s brain resembles the brain of one arthropod group: Arachnids, a class that includes spiders, scorpions and mites. This difference suggests that ocean-based M. symmetrica is the early ancestor of modern Arknids, researchers reported on Tuesday (June 22) in Current Current Biology.
“It is still hotly debated where Arknids first appeared, what kind of chelicalates their ancestors appeared, and whether these were semi-aquatic, horseshoe-shaped crab-like,” co-author Nicholas Strausfeld, a professor specializing in royal neuroscience at the University of Arizona, said in a statement. Celicelates form a major group of arthropods, including arcnides and horseshoe-shaped crabs.
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Celicerates like M. symmetrica diverged from other arthropods by the mid-Cambrian period. The genus Morrison, which has four known species, lived around 515 to 480 million years ago. The species M. symmetrica had segmented bodies like scorpions, round shells, and six sets of appendages for movement and hunting.
Scientists are not sure exactly when ahinides branched off from other cyriates, but arcnids have been around for about 400 million years. Up until now, their fossil records suggested that they lived only on the land.
In a new study, the researchers investigated the fossilized brain and central nervous system of M. symmetrica specimens from the Burgess Shale Formation in the Canadian Rockies. They discovered that the animal’s brains were not organized like horseshoe-shaped crabs of the genus Limulus. Instead, certain areas of that brain appear to be positioned in opposite directions compared to other arthropod regions, similar to the way modern spider brains are laid out. This suggests that it evolved and branched out from horseshoe-shaped crabs earlier than scientists thought.
“It’s like the square-shaped brain found in Cambrian fossils, or the ancestor and present brain [day] The crustaceans and insects are turned backwards, which is what you see in modern spiders,” says Strausfeld.
Related: Why do spiders have eight legs?
This opposite arrangement is exclusive to the arcnid brains of modern animals, M. Mymmetrica is an early kunido, suggesting that this unique arrangement evolved in the ocean rather than on land. Existing spider brain studies suggest that reversed setups allow spiders to adjust many aspects of predatory movement, including stealth, speed, and dexterity.
“The Arachnid’s brain suggests that unlike the rest of the brains on this planet, its tissues are associated with calculation speed and control of motor actions,” says Strausfeld.
The spider’s stealth and speed as land hunters may have contributed to the evolution of the insect’s wings that would have allowed prey to escape, Strausfeld added.
“Being able to fly gives you serious benefits when you’re being chased by spiders,” Strausfeld said. “But despite their aircraft mobility, insects are still caught up in millions of people in exquisite silk nets spun by spiders.”
To understand whether the similarity between the brains of M. symmetrica and modern spider brains came from a general strain or mere coincidence, researchers used computer programs to estimate the possibility that the two might be related. To do this, they compared the brain and body characteristics of several living arthropods. This analysis suggested that the Morrison lineage eventually evolved into the arcnid group. It means that it could have led to “the planet’s most successful arthropod predator,” the researchers wrote in their study.
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