1. The appendicular skeleton consists of the limbs (fins) and the pectoral and pelvic girdles. Girdles brace limbs (fins) against the axial skeleton to offset forces for locomotion.The Appendicular Skeleton and Locomotion
2. Some modern vertebrates are limbless and lack a complete girdle system. These species have adapted mechanisms for limbless locomotion.
3. Limbs arise from embryonic limb buds; fins arise from fin folds.
4. Pectoral girdles: articulates with anterior fins or limbs; stabilized by axial and appendicular muscles.
Modern bony vertebrates have pectoral girdles that are variations of the primative girdle of bony Devonian fishes.
In more modern teleosts, the cleithrum becomes the major bone
of the pectoral girdle; the clavicle is lost. The scapula and coracoid
become smaller and/or fuse to form a coracoscapula.
Early tetrapods have pectoral girdles similar to bony fish but have gained an interclavicle and lost the posttemporal. During subsequent evolution, the supracleithrum and cleithrum are reduced and finally lost, leaving the interclavicle and clavicle as dermal bones. Replacement bones include the coracoid and scapula.
Fate of the interclavicle, clavicle, coracoid, and scapula:
Interclavicle: although absent in amphibians, it persists in
some reptiles, in birds, and lower mammals. In birds it is part of
the wishbone (furculum).
Clavicle: absent in amphibians (except anurans) and in some reptiles. Present in birds (long bones of furculum) and mammals.
Coracoids: substitute for the clavicle in lower tetrapods. Absent in mammals except for the coracoid process of the scapula.
Scapula: present in all tetrapods. Responsible for most (if not all) of the glenoid fossa which articulates with the humerus.
Dermal bone predominates in the pectoral girdle of bony fish; replacement bone predominates in tetrapods.
5. Pelvic girdles: begin in fish as ischiopubic plates (pelvic) to brace
the pelvic fins. Cartilaginous pelvic
plates also seen in tetrapod embryos (may remain cartilaginous
in urodeles): each ossifies in two centers to form a pubis and
an ischium..
An additional blastema (and ossification center) forms the ilium. These three bones ankylose to form an innominate bone. The acetabulum is formed at the junction and articulates with the femur.
The tetrapod ileum is braced dorsally against the transverse processes of the sacral vertebrae (variable number) forming a sacroiliac joint.
Except in birds, there is a ventral, pubic symphysis (or ischial symphysis...maybe both). The bones of the pelvic girdle enclose the pelvic cavity.
Between the ischium and pubis the obturator foramen transmits the obturator nerve and associated blood vessels.
6. Fins: serve as steering, stabilizing, and inclination devices.
Consist of two integumental surfaces reinforced by dermal fin
rays radiating from a skeletal base: lepidotrichia in bony fish and
ceratotrichia in cartilaginous fish. Actinotrichia are distal elements
in both groups. Pterygiophores form the skeletal base.
Paired fins brace against a girdle and may be group in three categories:
Lobed fins: fleshy proximal portion with a distal membranous component stiffened by fin rays. Dipnoans.
Fin fold fins: broad base and characteristic of cartilaginous fish.
Ray fins: reduction of basal skeletal elements makes these fins more flexible. Bony fish referred to as actinopterygians.
Median fins include several, median dorsal fins and a midventral anal fin which act as keels to prevent rolling. Anal fins may be modified as a gonopodium (clasper).
Caudal fins classified by the direction taken by the terminal portion of the notochord.
Five skeletal segments in every tetrapod limb: