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?

Yes; synapomorphy of Ecdysozoa.

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?

Current thinking is that trilobites are arachnomorphs and so more closely related to horseshoe crabs.

Are sea spiders closely related to land spiders?

No. But both are probably arachnomorphs.

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?

All "crown group" means is a clade bounded by living representatives. Not sure just how 8 legs maps onto the tree.

 Or are they actually pretty closely related to
land spiders?