Re: Arthropod phylogeny

On 6/2/25 10:58 AM, x wrote:

So there is the tadpole shrimp, the horseshoe crab, and the trilobite.
 There is Chelicerata and Mandibulata.
 There is Decapoda and Hexapoda.
 So basic questions.
 Are exoskeletons monophyletic?
 Is the trilobite more closely related to the tadpole shrimp,
the horseshoe crab, or are the tadpole shrimp and horseshoe
crab more closely related to each other than to the trilobite?
 Are sea spiders closely related to land spiders?  Do they have
too many joints in their legs to be closely related?  Are they
a crown group of the arthropods and they have eight legs by
coincidence?  Or are they actually pretty closely related to
land spiders?

If by exoskeletons you mean ecdysozoa, they are indeed a huge clade (see Wikipedia).
Trilobites are believed to be members of Artiopoda, a group of extinct critters, whose placement within Arthropoda is somewhat uncertain since the best way to order organisms in clades is genomic analysis, and for anything extinct that long (250 My) that isn't available.
Everything you mention specifically is a member of Arthropoda, and Wikipedia is your friend. Tadpole shrimp (Notostraca) have been separated from all other arthropods since the Devonian (360 MYa), and they look it.