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The Evolution of Avian Humeral Rotation and the Disappearance of Avian Manual Digits
Author(s) -
Parsons William,
Parsons Kristen
Publication year - 2015
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.29.1_supplement.867.17
Subject(s) - forelimb , anatomy , rotation (mathematics) , humerus , wrist , biology , mathematics , geometry
Avian forelimb rotation developed through a sequence of derived forelimb features starting from the rotation of the dromaeosaurid wrist and progressing upward to the formation of the triosseal canal. The morphology of the dromaeosaurid first metacarpal and semi‐lunate carpal allowed for a 190 degree rotation of the wrist and a broad lateral extension of the first manual digit. The differentiation of the sulci for the two tendons of the triceps brachii complex, Sulcus m. humerotricipitis and Sulcus m. scapulotricipitis in the distal end of the enantiornithine humerus and the straightening of the humeral shaft indicate that one of the possible functions of the separation of these two tendons was to create lateral rotation within the lower forelimb. This was an intermediate evolutionary state between the wrist rotation of the dromaeosaurid forelimb and the humeral rotation in the neornithine wing. This distal to proximal progression of forelimb rotation finally resolved into avian humeral rotation within the pectoral girdle which gave rise to the triosseal canal. The presence of robust manual digits, especially the first manual digit, would have enhanced dromaeosaurid wrist rotation by increasing the centrifugal inertia within the rotation of the manus, but that same mass/weight would have detracted from the efficiency of any shoulder‐based rotation. Thus, the loss of manual digits, the last major evolutionary dromaeosaurid/avian morphological modification, would have increased the efficiency of shoulder‐based rotation and correlated to the development of avian humeral rotation as well as the evolution of the triosseal canal. The sequential development of these features tracks the progressive evolution of the rotation of the dromaeosaurid/avian forelimb.