At one time, no one supposed that ancient brains would leave any evidence but it turns out that sometimes they do. And what did the researchers find?:
One of the challenges of interpreting ancient arthropod anatomy is the lack of close modern relatives available for comparison. But luckily for us, Euproops can be compared to the four species of living horseshoe crabs.
Even to the untrained eye, a comparison of the fossil’s nervous system with that of a modern horseshoe crab (below) leaves little question that the same structures are found in both species, despite them being separated by 310 million years.
The fossil and living nervous systems match up in their arrangements of nerves to the eyes and appendages, and show the same central opening for the oesophagus to pass through.John Paterson et al., “Ancient brains: a look inside the extraordinary preservation of a 310-million-year-old nervous system” at The Conversation (July 26, 2021) The paper is open access.
So there was no significant evolution of the nervous system for 310 million years… That’s called stasis.
See also: Stasis: Life goes on but evolution does not happen