"Basilosaurus" has an anguilliform (eel-like) body shape because of the elongation of the centra of the thoracic through anterior caudal vertebrae. In life, these vertebrae were filled with marrow, and because of the enlarged size, this made them buoyant. "Basilosaurus" probably swam predominantly in two dimensions at the sea surface, in contrast to the smaller "Dorudon", which was likely a diving, three-dimensional swimmer. The skeletal anatomy of the tail suggests that a small fluke was probably present, which would have aided only vertical motion.