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Question: Are arachnids and insects so distantly related that they were already diverging before their (independent) emergence(s) on land?

I am curious to know what the fossil record and molecular evidence has had to say so far about such questions.

To the extent that arachnids and insects might not be any more closely related to one another than they are to crustaceans, and crustaceans are mostly aquatic, it does not seem impossible/implausible to me that the terrestrial transitions of insects and arachnids could have been independent of one another. In fact, considering that wood lice are crustaceans, one can conclude that there have already been at least two independent terrestrial transitions for arthropods, so postulating a third seems less of a stretch to me.

I suppose the thought process behind this is related to this question. Specifically, to the best of my knowledge, flight emerged several times among distinct insect lineages after appearing on land, in cases of convergent evolution. The fact that no lineage of arachnids ever evolved winged flight while many lineages of insect evolved flight independently of one another suggests that the "fundamental recipe" for insects and arachnids is substantially different, i.e. that they are not as closely related as one might have otherwise thought.

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    $\begingroup$ Arachnids and Insects are exceedingly distantly related. Although both are arthropods, there are other Chelicerates, Myriapods, and all of Crustacea separating them on the tree. Both diversified on land, independently $\endgroup$ – Karl Kjer Dec 8 '17 at 19:16
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Those are two very separate questions. Arachnids include terrestrial groups such as spiders, scorpions, and other arthropods. Arachnids are members of Chelicerata, which includes horseshoe crabs, Sea scorpions (extinct), and probably Trilobites (extinct). Chelicerates evolved in the marine environment, and invaded land afterwards. Arachnids and Insects evolved on land. Hexapods are nested within Crustacea, in a group called Pancrustacea. The likely sister taxon of Hexapoda (Insects, including Protura, Collembola and Diplura) are Remipedia, which are aquatic. See Misof et al. 2014 for the most complete molecular phylogeny, with dates. http://science.sciencemag.org/content/346/6210/763

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Wikipedia appears to contain at least some part of the answers to this question, specifically here and here. Specifically, it mentions that arachnids are chelicerates, which diverged from other arthropods during the Cambrian (according to molecular evidence?) or at latest during the late Ordovician (according to the fossil record). Spiders and other tetrapulmonates (not including scorpions) apparently evolved from trigonotarbids, whose (terrestrial?) fossil record dates back to the late Silurian. So apparently not only arthropods, but even arachnids in particular, have had multiple terrestrial transitions. Chelicerates are also part of the larger clade Arachnomorpha, which is perhaps more speculative because it includes several extinct classes of arthropods for which there is only a fossil record and no molecular evidence.

Regardless of the validity of the Arachnomorpha grouping, however, their appears to be a consensu that the chelicerates are a "sister group" (i.e. separate branch) from the Mandibulata, containing crustaceans and insects. (I.e. regardless of which group trilobites and other types of extinct arthropod are more closely related to.) In fact, since insects apparently evolved from crustaceans, crustaceans are most likely paraphyletic (like monkeys or reptiles), such that Mandibulata is essentially the "monophyletic version" of crustaceans. Also apparently my belief that flight evolved multiple times independently among insects appears to be false, inasmuch as the group of all winged (or formerly winged) insects is believed to be monophyletic. The fossil record for insects apparently also consists of terrestrial specimens (well, this is also true for arachnids, since they evolved from the terrestrial trigonotarbids), and dates back to the Ordovician.

But anyway, to the extent that arachnids are chelicerates and insects are crustaceans, and that the chelicerates and crustaceans diverged from each other either during or before the Cambrian period, indeed arachnids and insects are de facto very distantly related and must have emerged on land independently of one another (in fact, both the chelicerates and crustaceans must have both had multiple terrestrial transitions, it seems, which seems quite an accomplishment). In particular, this appears to be another case where major evolutionary divergences were occurring long before the non-expert layperson would be able to identify them based on the fossil record.

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  • $\begingroup$ Don't forget Myriapoda, which are also part of Mandibulata. But yes, Chelicerates are the likely sister group of Mandibulata. Insects evolved flight only once. $\endgroup$ – Karl Kjer Dec 8 '17 at 19:00

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