The adaptive radiation of tetrapods from tetrapodomorph fish was initially marked by a series of morphological transitions as they gradually left their ancestral aquatic habitat. Studies of the fin-to-limb transition have yet to be applied to numerous unresolved problems in studies of tetrapod posture and locomotion. Here I show that a functional study analyzing the degree of forearm pronation in a sample of major tetrapod clades can be combined with a biomechanical model to provide a new view concerning the broad phylogenetic transitions in posture and locomotion from basal tetrapods to amniotes, the derived land-dwelling tetrapods.
Permanently semi-pronated wrist and finger joints constrained the tetrapod forelimb to retain a sprawling gait with considerable axial rotation at the shoulder and elbow. This limited the forelimb’s role in any form of terrestrial locomotion relative to the hindlimb. In contrast, the hindlimb, with its functionally uniplanar joint alignment, became the dominant propulsive organ.
This study shows that selection pressures acting to retain perpendicular forelimb joints have prevented the majority of dominant terrestrial amniotes from achieving full pronation of the hand. Only a few select diapsid lineages, such as dinosaurs, were able to circumvent the effects of semi-pronation. Small mammals and chameleons overcame the semi-pronation constraints convergently, by evolving full pronation for arboreal locomotion on narrow branches.
|Advisor:||Gebo, Daniel L.|
|Commitee:||Parrish, J. Michael, von Ende, Carl N.|
|School:||Northern Illinois University|
|School Location:||United States -- Illinois|
|Source:||MAI 49/01M, Masters Abstracts International|
|Subjects:||Morphology, Paleontology, Biomechanics|
|Keywords:||Functional morphology, Limbs, Posture, Pronation, Radius, Tetrapods|
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