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Re: the dinosaur's rear-facing shoulder socket
Concerning shoulder glenoid orientation in dinosaurs -
It is odd that the shoulder joint was oriented strongly ventro-posteriorly in
the primitive and bipedal herrerasaurs (see Sereno etc papers in J. Vert
Paleo), since it would seem to merely limit lateral mobility of the arm for
no good purpose. In later theropods the shoulder joint faced more to the
side, allowing considerable arm motion (see my fig. in Pred. Dino. World).
Marasuchus may have been at least partly quadrupedal, so its posterior
orientation is explained.
Erect legs are not more energy efficient than sprawling legs, a Komodo
monitor walks a mile using about the same amount of energy as a mammal or
bird of the same size. It is clear why reptiles tend to have nonerect legs -
they produce the broad, stable foot fall pattern needed when walking at the
slow speeds their limited aerobic capacity limits them to. By the same token,
the fast walking speeds forced by pendulem action erect limbs seem to compel
mammals and birds (& probably dinos) to have higher aerobic scopes.