What can we learn from fossils? We can estimate the shape and size of an extinct animal. Anything else? Well, if soft tissues (like the brain or muscles) fossilize, it could tell us how the animal functioned or behaved. Unfortunately, soft tissue decomposes quickly after an animal dies. They aren't preserved as fossils very often.
That's why we felt really lucky when we came across a fossil of an extinct horseshoe crab with a preserved central nervous system (CNS). We discovered that the organization of the CNS in our fossil is the same as in horseshoe crabs living today. It hasn't changed in over 300 million years! We also figured out how our unique fossil might have formed. This could help others discover similar fossils in the future.