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Ornithischian Dinosaurs and the Origin of Birds

Peter M. Galton
Evolution
Vol. 24, No. 2 (Jun., 1970), pp. 448-462
DOI: 10.2307/2406818
Stable URL: http://www.jstor.org/stable/2406818
Page Count: 15
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Ornithischian Dinosaurs and the Origin of Birds
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Abstract

The postpubic rod of ornithischians is homologous with the pubis of birds and other reptiles and the prepubic process, that was not specifically developed for abdominal support, was probably formed after the main part of the pubis came to lie parallel and close to the ischium. This change in position of the pubis of ornithischians and birds resulted in a posterior shift of the center of gravity that was advantageous to a bipedal animal. The change was possible once the musculature on the pubis was no longer necessary for protracting the femur because it was functionally replaced by the musculature on the anterior process of the ilium. The posterior reorientation of the pubis may only have occurred once but birds cannot be derived from any known ornithischian. The common ancestor of ornithischians and birds may have been a cursorial biped of the Middle Triassic with a long anterior process to the ilium and a backwardly directed pubis. The lack of similarity between the two groups is attributed to the specializations of ornithischians associated with the great adaptive shift to a herbivorous diet. Several features of birds, including the backwardly directed hallux, were probably developed during a cursorial "Preproavis" stage that subsequently took to the trees to give the arboreal "Proavis" that gave rise to Archaeopteryx and the other birds.

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