Download Appendicular Skeleton

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).