So, I've been page-hopping on wikipedia pages about arthropods, as one does, and learned the difference between cheliceriforms sea spiders, limules and arachnids) and mandibulates (crustaceans, myriapods and insects) is that the prior have cheliceres where the latter have mandibles. Splendid, but these seem like the same things and both take extremely different forms, so how can the distinction be so clear cut? Now, of course, I followed the white rabbit deeper in its hole, and this page told me cheliceres evolved from the second pair on antennae of other arthropods. I already had a book about insects that told me their mandibles evolved from limbs, so I thought I had the whole picture: A common ancestor with two pairs of antennae, both having round tardigrade-like mouths and two of its descendants developping articulated mouthpieces separately, one from its antennae giving rise to cheliceriforms, and the other from its front legs, birthing the mandibulates, and everyone's happy.

But the next day, I was bugged again when I wondered where trilobites fit into this, and I found this cladogram that groups them with the mandibulates within antennulata. So now I get that cheliceriforma and mandibulata are not sister groups, but also, cheliceriforma is outside of the group caracterized by having antennae. Am I to conclude that none of their ancestors had antennae? Then what did their cheliceres evolve from? Is it also legs, and then they're only different from mandibles in the sense that they evolved separately? Or is it yet something else?