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Re: Paleontology and Dinosaurology 101: What layperson MUST to know about?
Juvenile se[y]mour[...]iamorphs, temnospondyls, and others have external
gills and were fully aquatic: "tadpoles" in the broad sense.
Of course there's a story behind this.
The plesiomorphic state is to have a gradual ontogeny from a
salamander-larva-like larva to a more or less terrestrial adult. In
lissamphibians, many of the events of ontogeny -- loss of gills, closure
of gill slits, appearance of many skull bones, dramatic restructuring of
the palate -- happen at the same time (that's called metamorphosis), and
the frogs took this to the next level after greatly extending and
elaborating the earliest larval stage* at the expense of all others,
concentrating even more events (appearance of more skull bones, sexual
organs**, lungs... and the loss of the tail) in metamorphosis.
Amphibamid temnospondyls concentrated some of these events. That's the
only known case of even partial metamorphosis outside Lissamphibia,
though I must caution that the ontogeny of most limbed vertebrates other
than amphibamids, a handful of other temnospondyls like
*Sclerocephalus*, the seymouriamorph *Discosauriscus*, lissamphibians
and amniotes is very poorly known. This holds all the way down to
*Eusthenopteron*, of which there are long and detailed growth series,
but the smallest individuals already have a completely ossified skull...
People do talk about "metamorphosis" in *Discosauriscus*, but this is
arbitrarily defined as the disappearance of the external gills; no other
event happens at the same time.
* Tadpoles have for instance taken the embryonic cartilage skeleton of
the head and run with it. They're also mostly herbivorous, while
postmetamorphic frogs are all carnivorous (in the broad sense); this
means the gut changes drastically in metamorphosis.
** That must be why there are so many salamanders without metamorphosis
or with incomplete metamorphosis, but no frogs without complete