Two things: first, horseshoe crabs almost certainly have changed/evolved considerably over four hundred million years, just in aspects of their physiology that aren’t fossilized. Second, horseshoe crabs have like nine different types of eyes; even that tail is essentially one big eye, covered in photoreceptive cells. We humans consider ourselves the “dominant” species, but I don’t think we could handle crawling around in slime and mud for four hundred million years quite as well as they have.

Why aren’t these things considered Trilobites exactly? They look like Trilobites and I’m pretty sure they fill some of the same general niches? Is it just a taxonomical thing? Are they just not in the right clade?