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Appendicular Skeleton Pelvic Girdle The evolutionary history of the pelvic girdle is far simpler (fortunately!) than that of the pectoral girdle. Remember the pelvic girdle to attached to the vertebral column by the sacral ribs. This connection is considered an adaptation for transmitting the forces generated by the muscles of the hind limbs into forward movement of the body. The pectoral girdle, on the other hand, has a limited connection or no connection to the rest of the skeleton and a set of muscles (a ‘muscular sling’) suspends the anterior part of the body on land. Another difference between the pelvic and pectoral girdles is the fact that the pelvic girdle is only endoskeletal; that is, only endochondral bone and cartilage. The pelvic girdle is a simple rod of cartilage or bone (recall the shark skeleton in lab). Left and right pelvic bars of fish are often fused along the midventral line. The really interesting story begins with the terrestrial vertebrates. When we look at a primitive tetrapod, each half of the pelvis is a large plate-like structure made of three bones. The two ventral bones (pubis and ischium) of each half join along the midventral side of the body. The dorsal bone (ilium) is attached to the sacral ribs. Also, the epaxial muscles (dorsal muscles of the back and tail) attach to the ilium. Epaxial muscles are especially large in those tetrapods such as reptiles and salamanders because they are responsible for the lateral undulation of their body during locomotion. Below are two other bones, the pubis at the front and the ischium at the back. The hind limb articulates with the pelvic girdle in a cavity called the acetabulum. These three bones persist throughout the evolutionary history of vertebrates, but their shapes and relative sizes vary. The three bones of the pelvic girdle in mammals and birds fuse to form a composite bone called the innominate bone. The innominate bone of birds is fused to the synsacrum. Pectoral and pelvic appendages Fins Appendages of pectoral and pelvic girdles are the paired fins and paired limbs that project from these girdles. I don’t include other fins such as dorsals, anals, or the caudals. Paired fins act as steering devices and help to stabilize the vertebrate in the water. They can provide some propulsive force (the caudal fin is the main propulsive body region), but can also help to slow done or stop the fish. The fins of fish are made of two layers of epidermis stiffened internally by flexible rays that are either ossified dermal scales or modified keratinized scales that also develop in the dermis. Ossified dermal fin rays are known as lepidotrichia and are found in the fins of bony fish. Fin rays that are keratinized scales are known as ceratotrichia and are found in the cartilaginous fish such as sharks. The basic structure of a paired fin in a fish consists of a series of rod-like elements called pterygiophores that articulate with the girdle and a wide expanse of dermal fin rays. Two basic groups of osteichthyes (i.e. bony fish) can be recognized according to, among many other features, the structure of the fins. In the Actinopterygyii (“rayfinned” fish) that includes the vast majority of modern fish, fin rays reach the body wall. In the second group (Sarcopterygyii or “fleshy-finned” or “lobe-finned” fish) a fleshy portion of the fin at its base consists of musculature that extends from the body into the fin. The tetrapod limb, despite its many modifications that occur throughout the history of vertebrates, is a remarkably conservative structure. Proximally, the stylopodium has a single element that is either the humerus in the forelimb or the femur in the hindlimb. Following the stylopodium is the zeugopodium consisting of a pair of elements, the radius and ulna in the forelimb and the tibia and fibula in the hindlimb. A complex of small elements are next and comprise the carpus in the forelimb and the tarsus in the hindlimb. The digits (fingers and toes) are typically five in number although the earliest known tetrapods have six to eight digits and different lineages have reduced the number from five. Separate elements of each digit are the phalanges (singular phalanx). The autopodium consists of the carpus (or tarsus) and the digits. The autopodium of the forelimb is the manus (hand). In the hindlimb, it is the pes (foot).