Thursday, February 11, 2010

Cetacean Skeletal System


Cetaceans trace their evolutionary history back to the archicetes, and even further back to a common ancestor with the Artiodactyla (even-toed Ungulates such as deer, camels and cattle). Formerly land mammals, whales and porpoises have gone through extensive morphological and physiological changes to adapt to their aquatic environment. Looking at their fusiform shape, complete with (in many cases) a dorsal fin, it is easy to see why these mammals were once thought of as fish. Water imposes the same environmental constraints upon the fish and dolphin, hence they have convergently evolved to the morphology that is best suited for aquatic locomotion. The skeletons of cetaceans are so striking because they are so strikingly different from land-based organisms, yet distinctly mammalian. It is as if every possible mammalian bone has been morphed, while still remaining identifiable.

The general shape of these whales, as mentioned briefly before, is designed to offer the least resistance to movement in water. The head, which usually tapers off towards the snout, passes into the trunk without a definable neck. The dorsal fin is positioned near the center of the body in most odontocetes, occurring towards the lower third section of the back in larger whales. The dorsal fin is entirely cartilaginous, held up by strong ligamentous fibers and a fibrous core. The flukes provide primary movement, and is also absent of bone, except for the distal end of the backbone which lies between them.

Forelimbs are flattened and shaped into fin-like paddles most often referred to as flippers. The scapula (shoulder blade) is broad and flattened, and has an acromion and coracoid process which articulate from the scapular body in a flat projection. These two processes may occur in different shapes, or be absent from some species of cetaceans. An example of this is the humpback whale (Megaptera novaeangeliae) which is wanting of both the acromion and coracoid processes. Similarly, the clavicle is missing in all cetaceans.

One of the oddest structures of the cetacean skeleton is the lower flipper. Digits are still clearly demarcated and are found all within the same integument. The metacarpals and phalanges are indistinguishable from each other, but the phalanges occur more distally and can be identified in that sense. The first digit, known as the pollex, may be missing from certain species such as the blue whale (Balaenoptera musculus) resulting in a more slender fin. The phalanges are equipped with epiphysial cartilage at each end which is replaced by bone as the animal grows. The phalanges and metacarpals are attached to the wrist bones (carpalia, or carpals), which are cartilaginous in the juveniles of some species. These bones are in turn connected to the elbow joint, which does not allow much motion between the upper arm and the fore-arm, and is often fused together in elderly whales. The fore-arm is comprised of the ulna and radius, while the upper arm refers to the humerus. The humerus is a short bone whose head articulates with the scapula. The distal end of this bone is comparatively flattened and terminates in two facets to meet the fore-arm bones. These bones are also short and flattened, and reside parallel to each other. The proximal end of the ulna is known as the olecranon, and is a a hatchet-edge projection that occurs proximally.

External hind-limbs are completely absent in all cetaceans, but a vestigial pelvis remains. This pair of pelvic bones has no contact with the vertebral column, and the femur and tibia range in presence throughout species. Sperm whales (Physeter macrocephalus) are the only odontocetes to possess a femoral vestige, the rest of the toothed whales lacking all hind-leg bones.

The cranium is strongly compressed from front to back and appears globular. The rostrum is often elongated and serves as a beak in various delphinids. The seven cervical vertebrae are highly compressed, and may be fused in differing order in various species. Transverse processes are well developed in the lumbar and caudal regions, and diminish in size towards the tip of the tail. The chevron bones, which are "ventral V-shaped elements faceted to the under surfaces of contiguous vertebral bodies," articulate between the hind end of one vertebra and the fore-end of the next.

Rib count is also variable between species, 12-16 pairs the normal amount. The Southern Bottlenosed Whale (Hyperoodon planifrons) has the least number of pairs of all mammals, with only 8. The sternum is composed of several fused elements, with species-variable numbers of ribs attached. For example, the breast-bones of Mysteceti are only large enough to attach to the first pair of ribs.

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