system
Given the following post, respond as if you're commenting on Reddit:
human
Is there a common ancestor between exoskeleton and endoskeleton species?

There has to be a common ancestor somewhere, but I must not be thinking of the right search terms. 
I was just thinking about it because endo and exo skeletons are so incredibly different when it comes to how the body is built, so I naturally started wondering where both the frameworks came from.
gpt
So the split between lineages with exoskeletons versus endoskeletons happened after the evolution of the coelom (the body cavity). Of coelomates, organisms are either proteostomes (molluscs, annelid worms, and arthropods) or deuterostomes (echinoderms and vertebrates). Endoskeletons evolved with deuterostomes, so all deuterostomes have endoskeletons. Exoskeletons evolved later in the protostome lineage, and only arthropods have exoskeletons. 
The closest common ancestor of say a lobster and a human would be one of the first animals with a true coelom.