The External, Musculoskeletal and Neuro-Anatomy of Sea Turtles
53
attaches to the carapace (Figure 2.7) and is limited in its range of movement. The
vertebral arches of the successive cervical vertebrae have articulating processes with
sliding joints that allow limited dorsal–ventral bending of the neck, but little twisting.
Each dorsal (or trunk) vertebra is composed of a separate dorsal arch and
ventral vertebral body, which articulates bilaterally with a pair of ribs. Rib heads
are aligned with the junctions of adjacent vertebral bodies. In Dermochelys,
there is no hypertrophy of intermembranous bone between the ribs of the carapace or formation of peripheral bones. The bony carapace skeleton remains
composed solely of an expanded nuchal bone, ribs, and vertebrae. Cheloniid
vertebrae and ribs are connected via intermembranous bone to produce the
carapacial bones. Neural bones are incorporated into the vertebral arch elements;
the ribs and their intermembranous expansions form pleurals, and peripheral
bones form the margins of the carapace. In L. kempii, the peripheral bones also
widen with age and increasing body size. The spaces between the ribs and the
carapace, the fontanelles, are closed by a thick membranes of cartilage and
fibrous connective tissue underlying the scutes. The anterior-most bone is the
nuchal, and the posterior-most of peripheral bones are the pygals. Between the
last neural bone and the pygal are the suprapygals (one to three; usually two),
which lack any vertebral fusion (Figure 2.7). The lateral processes of the sacral
vertebrae, deep to the suprapygals, are not fused to the carapace. These lateral
processes are formed by rib-like processes that articulate with the ilium. The
caudal vertebrae of females are short and decrease in size distally; those of
mature males are large with robust lateral and dorsal processes (Figure 2.12).
The cheloniid plastron is composed of four pairs of bones: epiplastron, hyoplastron, hypoplastron, and xiphiplastron, and one unpaired bone, the entoplastron (Figure 2.8). The leatherback plastron is formed from a peripherally located ring of
reduced, paired plastron bones. No entoplastron is present (Deraniyagala, 1939;
Pritchard, 1997).
2.3.3 APPENDICULAR SKELETON
The appendicular skeleton of turtles differs from that of other reptiles in that the
shoulder and pelvic bones are located inside the ribs and ventral to the vertebrae.
This morphological difference results in rotation and repositioning of many bony
structures in the limbs and of the associated soft tissues (muscles, nerves, blood
vessels). The anterior appendicular skeleton is composed of flippers (Figure 2.13)
and triradiate pectoral girdles (Figure 2.14). The latter are formed of two bones, the
scapula, with its ventral acromion process, and the procoracoid (also called a coracoid). The scapula is aligned dorsoventrally and attaches to the carapace adjacent
to the first trunk vertebra. Ventrolaterally, the two bones form the glenoid fossa,
which articulates with the head of the humerus. An acromion process extends
medially from each scapula to articulate with the entoplastron via a plastroacromial
ligament. The fan-shaped procoracoid bones extend posteromedially. Each terminates in a crescent-shaped coracoid cartilage. A flat acromialcoracoid ligament
connects the acromion and the posterior part of the coracoid on each side.
1123_C02.fm Page 53 Friday, November 15, 2002 11:16 AM
53
attaches to the carapace (Figure 2.7) and is limited in its range of movement. The
vertebral arches of the successive cervical vertebrae have articulating processes with
sliding joints that allow limited dorsal–ventral bending of the neck, but little twisting.
Each dorsal (or trunk) vertebra is composed of a separate dorsal arch and
ventral vertebral body, which articulates bilaterally with a pair of ribs. Rib heads
are aligned with the junctions of adjacent vertebral bodies. In Dermochelys,
there is no hypertrophy of intermembranous bone between the ribs of the carapace or formation of peripheral bones. The bony carapace skeleton remains
composed solely of an expanded nuchal bone, ribs, and vertebrae. Cheloniid
vertebrae and ribs are connected via intermembranous bone to produce the
carapacial bones. Neural bones are incorporated into the vertebral arch elements;
the ribs and their intermembranous expansions form pleurals, and peripheral
bones form the margins of the carapace. In L. kempii, the peripheral bones also
widen with age and increasing body size. The spaces between the ribs and the
carapace, the fontanelles, are closed by a thick membranes of cartilage and
fibrous connective tissue underlying the scutes. The anterior-most bone is the
nuchal, and the posterior-most of peripheral bones are the pygals. Between the
last neural bone and the pygal are the suprapygals (one to three; usually two),
which lack any vertebral fusion (Figure 2.7). The lateral processes of the sacral
vertebrae, deep to the suprapygals, are not fused to the carapace. These lateral
processes are formed by rib-like processes that articulate with the ilium. The
caudal vertebrae of females are short and decrease in size distally; those of
mature males are large with robust lateral and dorsal processes (Figure 2.12).
The cheloniid plastron is composed of four pairs of bones: epiplastron, hyoplastron, hypoplastron, and xiphiplastron, and one unpaired bone, the entoplastron (Figure 2.8). The leatherback plastron is formed from a peripherally located ring of
reduced, paired plastron bones. No entoplastron is present (Deraniyagala, 1939;
Pritchard, 1997).
2.3.3 APPENDICULAR SKELETON
The appendicular skeleton of turtles differs from that of other reptiles in that the
shoulder and pelvic bones are located inside the ribs and ventral to the vertebrae.
This morphological difference results in rotation and repositioning of many bony
structures in the limbs and of the associated soft tissues (muscles, nerves, blood
vessels). The anterior appendicular skeleton is composed of flippers (Figure 2.13)
and triradiate pectoral girdles (Figure 2.14). The latter are formed of two bones, the
scapula, with its ventral acromion process, and the procoracoid (also called a coracoid). The scapula is aligned dorsoventrally and attaches to the carapace adjacent
to the first trunk vertebra. Ventrolaterally, the two bones form the glenoid fossa,
which articulates with the head of the humerus. An acromion process extends
medially from each scapula to articulate with the entoplastron via a plastroacromial
ligament. The fan-shaped procoracoid bones extend posteromedially. Each terminates in a crescent-shaped coracoid cartilage. A flat acromialcoracoid ligament
connects the acromion and the posterior part of the coracoid on each side.
1123_C02.fm Page 53 Friday, November 15, 2002 11:16 AM