Abstract
We investigated differences in displacement of the diaphragmatic vertebra between Otariidae, the forelimb swimmers, and Phocidae, the hind limb swimmers. The diaphragmatic vertebra was displaced in the caudal direction on vertebrae in Otariidae, whereas it was displaced in the cranial direction in Phocidae. The displacement occurred in samples that had abnormal vertebral counts as well as those with normal counts. This vertebra occurs at the boundary between the thoracic type vertebra (or prediaphragmatic) and lumber type vertebra (or postdiaphragmatic). When the diaphragmatic vertebra is displaced in the cranial direction, the lumber type area increases whereas the thoracic type area decreases. On the other hand, when it displaces in the caudal direction, the thoracic type area increases whereas the lumber type area decreases. Because a long thoracic region might be more advantageous for otariids and a long lumber region might be more advantageous for phocids when they swim, caudal displacement of diaphragmatic vertebra might tend to occur in otariids and cranial displacement might tend to occur in phocids.
