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Re: New paper on fish fingers
Here's the way I see it... One hypothesis for the evolution of the turtle
shell posits that the carapace evolved via expansion of the ribs and
vertebrae. The other hypothesis proposes that the carapace evolved from
fusion of osteoderms, which fused with the underlying skeleton.
_Odontochelys_ has a plastron, but the dorsal armor is composed only of
neural plates (no osteoderms), and so has been cited in support of the
But wait. It can easily be argued that the costal plates are there and fused
to the ribs, rather than the ribs being broadened (as they are in
*Eunotosaurus* and *Pumiliopareia*); that would mean it has the same
carapace morphology as *Chinlechelys*, except that it lacks the peripherals,
the nuchal and the pygal (which are undoubted osteoderms) as well as the
neck & tail spikes.
Li et al. take this possibility into account and then dismiss it based on
the ontogeny of modern turtles, where (in the known cases) the costals start
to ossify (intramembraneously) from the ribs, rather than having a separate
ossification center. But just a few lines above, they explain that there's
variation in how the neurals are formed in the ontogeny of extant turtles:
in some, the neurals start to ossify from the tips of the neural spines
rather than having a separate ossification center, while in others they have
separate ossification centers and later fuse to the vertebrae; the latter,
they say, must have been the way *Odontochelys* did it, because the neurals
are disarticulated from the vertebrae in some places. Looks like the
vertebrae reach the ossification centers of the neurals before ossification
of the neurals starts in some turtles but not in others. My question is why
the same can't apply to the costals: the ribs may reach the ossification
centers of the costals before ossification of the costals starts, but that
doesn't mean the costals are homologous to a part of the ribs.
In other words, I interpret *Odontochelys* to have the same condition as
*Chinlechelys*, except without sutures between the costals, without
peripherals/nuchal/pygal, without neural arches that sit between two centra
instead on top of just one, and apparently with a bit more fusion between
the costals and the ribs.
And besides, ontogeny can evolve. Vertebrae are endochondral
ossifications -- except in crown-group salamanders, where they ossify
intramembraneously and are not preformed in cartilage. Surely salamander
vertebrae are homologous to everyone else's vertebrae?