Arthropods: Can you explain this evolutionary anomaly

Actually, this was not meant as a solicitation to arthropods, just in cast that was misread. Anyway. . .

Most arthropods have compound eyes, the exceptions being the arachnids, who seem to have simple eyes, when they have them at all.

Horseshoe crabs are generally thought to be more closely related to arachnids than to other arthropods. However, they have a neat combination of compound and simple eyes, with the compound eyes being the “main” eyes.

So, the question. . . Does the fossil record show that merostomes and arachnids have common ancestors more “recently” than they do with other arthropods.

I’m not a biologist, and I’m willing to buy their closer relationship. The “interesting” point to me is that of all arthropods, only arachnids seem to have developed simple eyes as the “primary” eyes. So, is there any record of when arachnids and merostomes diverged?

First off, I wouldn’t call it an “anamoly”. It more like… just a fact.

Anyhow, various eye types have evolved independently more than once. For example, the “camera eye” style used by vertebrates also evolved in the mullosks, as seen in squid and octopodes. The last common ancestor of the vetebrates and cephalpods is believe to pre-date eyes, so there you go.

Just to throw more fuel on the fire… trilobites, an extinct category of anthropods, evolved eyes made with calcite lenses in both their compound and single eye varieties. Trilobites had represensatives with both sorts, sometimes co-existing in the same species, as in the horseshoe crabs you mention.

And the trilobites and horseshoe crabs are thought (at present, by many, though not all) to be more closely related than horseshoe crabs and other athropods.

So there is precedent for the combination in the athropods, and in the lineage most closely related to the horseshoes than any other.

Does that help?