Comparative Anatomical Study on the Relationships between the Vestigial Pelvic Bones and the Surrounding Structures of Finless Porpoises (Neophocaena phocaenoides)

Abstract

ABSTRACT. Morphology of the modern cetaceans represents the results of adaptation of the ancestral terrestrial mammals to aquatic life through their evolutional processes. Some of the primitive fossil cetaceans are known to have both fore and hind limbs, whereas the pelvic bones of modern cetaceans are, in general, a pair of slender rod-like structures within the abdominal wall muscles just anterior to the anus with no articulations to the axial skeleton in poth sexies. It is interesting and important to consider the causes and processes of how the hind limbs were lost and how the pelvis was reduced during the process of adaptation. In the present study, we tried to evaluate the topography and function of rudimentary pelvic bones of the finless porpoise (Neophocaena phocaenoides), one of the members of the odontocete cetaceans, with special references to the structures around the pelvic bones. Some soft tissues such as M. ischiocavernosus relating to the pelvic bone are transformed following the drastic reduction of the pelvis. This transformation tells us that the cetaceans adapted to the aquatic life during evolutional processes chose the tail flukes driven by the powerful trunk muscles for locomotion, instead of modifying the hind limbs into hind flippers as seen in pinnipeds. On the other hand, it is evident that a function of the pelvic bones of the male finless porpoise was supporting the penis as those of terrestrial mammals. It is noteworthy that the morphological features of the ancestral terrestrial mammals can be traced when they are carefully compared with those of the finless porpoise. KEY WORDS: comparative anatomy, finless porpoise, pelvic bone.J. Vet. Med. Sci. 66 (7): [761][762][763][764][765][766] 2004 It is evident that the cetacean and the terrestrial artiodactyls are sister taxa based on the recent studies, e.g. [19]. Characteristic morphologies such as the modified forelimbs and the streamlined body shape of the modern cetaceans represent the results of adaptation of the ancestral terrestrial mammals to the aquatic way of life through evolutionary processes. Above all, it is interesting and important to consider the causes and processes of how the hind limbs were lost and how the pelvis was reduced during the process of adaptaion. On the other hand, existence and function of the hind limb are discussed in some of the primitive fossil cetaceans. For example, Ambulocetus natans [25] recovered from lower to middle Eocene beds in Pakistan had functional fore-and hind limbs. However, Dordon atrox (formerly Prozeuglodon atrox) [26] found from more recent lower Eocene beds in Egypt, had vestigial hind limbs with no bony connection with the vertebral column and were used possibly for very limited purpose such as mating [11]. What are the differences in anatomical features we can find on the soft tissues around the pelvic bones in modern cetaceans during the processes of losing the hind limbs and reducing the pelvis through the evolutional processes mentioned above?In terrestrial mammals, e.g. th...

Sexual dimorphism and morphometric relationships in pelvic bones of Commerson's dolphins (Cephalorhynchus c. commersonii) from Tierra del Fuego, Argentina

“…They are embedded within the abdominal wall muscles on both sides, cranial to the anus (Tajima et al . ). Additional pelvic limb bones corresponding to the femur, and rarely to the tibia, have been reported in some cetaceans (Omura , ; Gingerich et al .…”

Hip bone morphometrics of bottlenose dolphins (Tursiops truncatus) from Adriatic Sea: Sex determination and postnatal development

“…Additionally, their reduction in size and changed position may facilitate dorso‐ventral undulations of the peduncule (Tajima et al . ). In contrast to the extensive bony connection between the hip bones and vertebral column in terrestrial mammals, cetacean hip bones in extant species associate only loosely with the vertebral column by soft tissues.…”

“…, Tajima et al . ), and Commerson's dolphin ( Cephalorhynchus c. commersonii ) (Cáceres‐Saez et al . ).…”

“…Arvy (, ) strongly posited that each should be called “the abdominal bone.” And phrases like “vestigial pelvic bone” (Tajima et al . ), “pelvic rudiments” (Perrin ), “pelvic vestiges” (Rommel ) persist, even though the small bones in question are homologous not to the pelvis of terrestrial mammals but to their hip bones. The bony pelvis refers to a substantial bony ring that comprises the left and right hip bones (ossa coxarum), the sacrum, and the first few caudal vertebrae (Getty , Dyce et al .…”

Abdominal fat pads act as control surfaces in lieu of dorsal fins in the beluga (Delphinapterus)

“…Further study should be done to assess the role of the paired ventrolateral fat pads in locomotor control. In their study of finless porpoise ( Neophocaena ) abdominal myology, Tajima et al . (2004) argued that muscles attached to the pelvis likely played a key role in cetacean evolution by contributing to dorsolateral undulation that replaced pelvic limb motion in locomotion.…”