Question: Is there any research corroborating my suspicion that vertebrates diverged from other deuterostomes via neoteny (the attainment of sexual maturity while still in larval stage)?
Context: Most echinoderms have pelagic, bilaterally symmetric larvae, while the adults are not bilaterally symmetric (except for sea cucumbers) and live on the sea floor. The same is true for tunicates, which are supposed to be the sister group to vertebrates.
Adult echinoderms also have very reduced nervous systems, although I am not sure whether the larvae have more advanced nervous systems than the adults. For tunicates, which are more closely related to vertebrates, the nervous system definitely does degenerate when transitioning from pelagic larvae to sessile adults.
Echinoderms are also one of the few clades of bilateria, except for vertebrates, which have endoskeletons. And of course, echinoderms, tunicates, and vertebrates are all deuterostomes.
In contrast to tunicates and echinoderms, the MRCA of vertebrates is thought to have resembled a lancelet, which is pelagic and which has a more advanced nervous system than adult tunicates or echinoderms. In other words, it seems like ancestral vertebrates retained larval features into adulthood, and then just grew bigger in size.