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Re: bauplan convergence
I don't think dinosaurs were primitively bipedal, but they probably were
primitively scansorial. It is fairly straightforward to evolve a
form (e.g., sauropods) as well as a strictly bipedal form (e.g., theropods)
from a scansorial form, much more difficult to evolve a quadrupedal form
a strictly bipedal form.
I'm not certain it's that difficult for quadrupeds to evolve from bipeds.
In certain dinosaur groups the changes in the skeleton during growth
strongly suggests that the juveniles were fully bipedal and the adults were
mostly (if not entirely) quadrupedal. We see this in prosauropods and
certain ornithopods. In _Tenontosaurus_, for example, the sprite little
juveniles are built much like adult hypsilophodontids, whereas the adults
are bigger and heavier (no surprises there) with limb proportions consistent
with a quadruped.
If the transition from biped to quadruped can happen in the growth series of
a single species, I see no reason why it should be so difficult to happen
within an evolutionary lineage. Why should it be necessarily "more
difficult" than switching from scansorial to quadrupedal.
At any rate, it's like comparing apples and oranges. Creatures evolve
according to need and not to in accordance to any perceived "level of
difficulty". It's like that bugbear of an argument that flight in birds
probably evolved in an arboreal ancestor because it is "easier" to evolve
flight from the trees down than the ground up. Maybe it is, but it's
totally irrelevant to any discussion of phylogeny.
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