Trilobites are one of the first animals in the fossil record to develop complex eyes (as opposed to the light-sensitive spots that passed as early eyes). So understanding trilobite vision is also understanding the origins of eyes themselves. It has even been hypothesized that trilobite vision drove rapid changes in their prey’s body structure as prey evolved to escape sighted predators, thus fueling the Cambrian Explosion.
Trilobites had compound eyes, akin to those of today’s insects and crustaceans. We know that because trilobites’ lenses were made of calcite, so they often fossilized along with the rest of the trilobite’s exoskeleton.
But underneath the lenses were sensory cells, which wired vision up to the brain. Those sensory cells, like other soft tissue, rarely fossilize. Thus, seeing how trilobites’ eyes were wired into their brains has been impossible up to now.