It’s very rare to find an intact fossil brain but a rare combination of minerals preserved one from 310 million years ago:
Finding an intact brain that is just like a modern horseshoe crab’s brain enables paleontologists to make some guesses about the ancient animal’s behavior:
“The preserved central nervous system lends insight into the ancient crab’s behavior, the researchers say. Because the fossil brain is so similar to the brains of modern horseshoe crabs, Bicknell says, it’s safe to say the ancient animal’s walking, breathing and even feeding habits were probably similar to horseshoe crabs’ today, including eating with their legs. “Imagine eating a hamburger with your elbows,” Bicknell says. – Rebecca Dzombak, “How Fossilization Preserved a 310-million-year-old Horseshoe Crab’s Brain” at ScienceNews”
To the extent that the horseshoe crab is more closely related to the spider, it may have a similar type of intelligence. But we can’t be sure just now. For example, the nautilus is a cousin of the octopus but, while the octopus is sometimes considered a “second genesis” of intelligence, on account of its braininess, no such thing is said of the nautilus.News, “Do brains really evolve? The horseshoe crab’s brain didn’t ” at Mind Matters News
Takehome: Researchers found that the 310 million-year-old brain was almost identical to that of a modern horseshoe crab. The behavior was likely similar too.
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If we find life on exoplanets, some of it might be “crabs”. Over millions of years, many crustaceans gradually grew to look more and more like crabs, a process called convergent evolution. In an environment similar to Earth’s, we might expect life forms to converge on similar solutions. “Crabbiness” might be one of them.