Ardea
Official journal of the Netherlands Ornithologists' Union

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Harrison C.J.O. (1977) The limb obsteology of the diving petrels and the Little Auk as evidence of the retention of characters in morphologically convergent species. ARDEA 65 (1-2): 43-52
The variation in minor structural characters on the bones of fossil and subfossil birds is used in assigning them to known taxa. If the occurrence of evolutionary convergence in general appearance, of the type known in recent birds, also affected these smaller differences in structure it might challenge the basis of such identifications. In order to test this possibility the details of structure of limb bones of the strongly convergent Little Auk Plotus alle and diving petrels of the genus Pelecanoides were compared. Although these limb bones had become similar in shape and relative length, they retained the details of structure diagnostic of the parental taxa, even on small elements. Convergence in these species involves a modification of the procellariiform wing for underwater swimming in Pelecanoides. This has produced very close similarity to Plotus in the structure of the proximal end of the humerus, and changes in the ulna and distal end of the humerus. The entepicondyle of the humerus is much enlarged, and a peculiar projecting process on the proximal end of the ulna articulates with a corresponding hollow on the former. These structures strengthen the muscle system for wing extension and impart greater rigidity to the wing.


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