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Bipedal lizards and pterosaurs
Nick L. may not have seen postings months ago on this, but I refer him to
Shine and Lambeck 1989 for a field report about _Chlamydosaurus kingii_, the
Australian frillneck lizard. Frillnecks spend 95% of their life in trees,
but when terrestrial, walk about bipedally, resorting to all fours when
climbing on termite mounds and preparing for a bout of bipedal running, which
they, like a number of other lizards, can do from a standing start (contra
conventional wisdom). They are very territorial and get into a fight or
flight mode whenever they see humans (especially humans with cameras), which
is why this mode of progression has not been reported before (except in the
I agree with you about the footprint situation, but remember, those are
pterodactylids in a shoreline setting. Pterodactylids, with those long hand
bones (metacarpals) were certainly capable of dropping them a few inches to
touch the ground. Basal pterosaurs were also capable of quadrupedal
locomotion, both on the ground and on tree trunks, which is (getting back to
Chlamydosaurus) where they probably spent 95% of their time hanging around.
The proportions of _Chlamydosaurus_, as well as aspects of its tail and
illium are close to what one would expect in a basal pterosaur.
In lizards capable of bipedal progression two things stand out. The tail is
longer, thinner, stiffer and has reduced hemals arches and transverse
processes (see Snyder 1954, 1962). In addition, a tiny little anterior
process of the ilium is the source of a tendon that helps keep the body erect
over the hind limbs. In basal pterosaurs these features are taken to the
extreme. The tail is extremely long, thin and stiff and the hemal arches and
transverse processes are vestiges. In addition, the anterior process on the
ilium is greatly lengthened, especially so in the larger forms.
Now, if lizards can do it with rudimentary adaptations, I think, and I hope
this may convince you, that pterosaurs can stand, walk and run bipedally with
rather advanced adaptations.
Sacral count is another indicator of bipedalism. In bipedal lizards, the
sacral vertebrae are more robust than those of their quad cousins. In
pterosaurs, the sacrals are not only more robust, more vertebrae have become
sacralized, increasingly so in larger forms.
Sprawling limbs do not forbid parasagittal locomotion. I had a rhea skeleton
with a ilia so close together the sacrals between them literally disappeared
(contra most birds). The femora did not have a ninety-degree head on them.
Thus this poor cursor had to walk and run with knees bowed out to clear the
chest, as in pterosaurs.
What happens in pterosaurs is this, the femur rotates at whatever angle to
the pelvis (determined by the angle of the femoral head--which varies
considerably), but the tibia, due to its ninety degree articulation with the
femur, moves parasagittally beneath the body. I have articulated skeletons
of pterosaurs based on casts and original material on which this can be
One more thing. You do realize that when pterosaur wings are placed in the
quadrupedal position the wings open ventrally. If they are to open
laterally, that is, to extend for flight, the pterosaur has to get off its
manus to deploy them.
That's the short version.