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.

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    $\begingroup$ Based on morphology some hold a conceptually related - but opposing -suspicion that ( within protostomia ) caterpillars evolved from onychophorans by hybridogenesis ( rather than being larvae ) pnas.org/content/106/47/19901.full $\endgroup$
    – tsttst
    Aug 17 '16 at 4:59
  • $\begingroup$ @tsttst This is a very interesting article and concept, which I had never heard of before -- thank you for the reference! I am looking forward to reading all of it $\endgroup$ Aug 17 '16 at 17:06

echinoderms are sort of bilaterally symmetrical, imagine if you took a velvet worm and then twisted it around in a big loop with the legs sticking out until the head was stuck to the tail end but still pointed away from each other. That is basically how echinoderms went from bilateral to radial. of course in practice this is done by rearranging hox genes but it is a good way to visualize it. https://evodevojournal.biomedcentral.com/articles/10.1186/2041-9139-5-22

as for chordates look at hemichordates (which also have endoskeletons) and you can see why the general belief is that tunicates evolved that life stage after branching off(kinda like barnacles) instead of vertebrates losing that life stage.

  • $\begingroup$ Thank you for the interesting reference - I look forward to reading it later! The point about hemichordates is a good one which I failed to consider previously. $\endgroup$ Nov 28 '16 at 13:51

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