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Re: bipedal crocodylomorphs
DP: When you say "bringing the limbs beneath the body" that can mean two
different things. If you bring the knees beneath the hips and the feet
follow, that's one style. If you keep the knees sprawling and also bring
the feet beneath the hips, that's another style.
Does the former style even exist?
The hypothesis is that these animals should show similar features if they
developed their vertical limbs in a similar manner and thus should also
similar limbs if they developed bipedalism in the same manner.
It also greatly depends on their starting points. Look at any mammalian
pelvis (xenarthrans excepted -- they use the ischia as postacetabular alae
of the ilia). It's ridiculous. The ilia aren't parallel to the vertebral
column, but oblique. They don't even touch the vertebrae medially to the hip
joints, only far farther cranially. As a result, only 1, 2 or 3 sacral
vertebrae are actually connected to the pelvis! This is because a size
squeeze stands at the origin of Mammaliomorpha ( = Mammalia, Tritylodontidae
and the like). Mouse-sized animals don't need robust connections between
pelvis and vertebral column, so the number of sacrals and the sutures
between ilia and sacrum got _reduced_. As a result, in the mounted elephant
skeleton in the university here only 2 or so sacrals touch the ilia, which
are oriented transversely instead of longitudinally. In the giraffe skeleton
that number is 3 or 4 or something.
As a result, I'd say our ilia don't have postacetabular alae. They
consist mostly of preacetabular alae which lie _dorsally_ to the hip joints.
DP: alligators: bipedal ancestors.
If true at all, then _long_ ago.
chamaleons and drepanosaurids: bipedal at times on branches, at least
hypothetically in the latter case.
They don't move much in this stance, though.
DP: Silesaurus: bipedal ancestor. Paper coming out soon.
Sounds promising! :-)
Also noted elsewhere, *Sharovipteryx* doesn't
preserved a complete forelimb
DP: Forelimb has been found. Paper (not mine) out soon.
nor does it allow even a semi-accurate reconstruction of what's there.
Finally, forelimb anatomy of *Eoraptor* implies a bipedal posture with
possible quadrupedal locomotion (mesaxonal manus, for example).
DP: That's okay.
Not with me, though. Mesaxonic? It isn't symmetric about anything, so it
doesn't have an "axis". As for the possibility of a quadrupedal posture,
show me it was able to pronate its forearms. *Silesaurus* was,
*Herrerasaurus* wasn't... quadrupeds with vertical limbs must pronate,
except if they want to walk with their fingers pointing sideways instead of
forwards, a situation which AFAIK doesn't exist in the living world.
This all implies an elongated preacetabular ala means "jack squat"
about bipedalism, especially since it's present in so many more
"The cladogram will guide you Luke. Use the cladogram." - Obi Wan.
First of all, elongated preacetabular alae mean that some muscles were
enlarged. To find out whether this is useful for anything else than bipedal
locomotion is the next step. Secondly, the cladogram will tell you whether
an animal would "consider the idea" of elongating its preacetabular alae as
an adaptation to bipedal locomotion -- we didn't elongate ours, we shortened
it and twisted it into an even more vertical position, for biomechanical
reasons that are quite clear if we consider the configurations of other apes
as the starting point.