On the development of Cetacean extremities: II. Morphogenesis and histogenesis of the flippers in the spotted dolphin (Stenella attenuata).

Externally, the flippers of Cetacea resemble fish fins, but their internal structure is entirely mammalian. They show, however, some adaptative deviations from the typical pattern of the mammalian extremities, the most striking of which is an increased number of phalanges. The aim of this study is to describe the course of the development of flippers in the spotted dolphin (Stenella attenuata) and compare its features with other similar species from an evolutionary perspective. Early stages of flipper development were studied histologically. Differentiation of cartilaginous anlagens of the skeleton progresses proximodistally, condensation in digital rays being evident sooner than chondrogenesis in the carpal region. In one specimen, the temporary presence of cartilaginous rudiments of two carpal elements, which are not found in adults, was observed. At all examined stages, phalangeal number progressively increases up to (radial to ulnar) 3, 7, 7, 5, 3 in the most advanced stage. The reason for this condition is the specialised function of these limb-like structures. It is a classical example of convergence, in which mammalian extremities change their form to emulate the fin function. A similar condition is found in another group of originally terrestrial animals secondarily fully adapted to the aquatic mode of life-Ichyosauria (Reptilia).