The External, Musculoskeletal and Neuro-Anatomy of Sea Turtles
47
The tails of sea turtles are dimorphic in juveniles approaching adulthood and in
adults, but not in hatchling and immature turtles. The tails of mature or nearly mature
males extend beyond the carapace margin, and the cloacal openings are found near
the tail’s tip. The tails of immature or female cheloniids normally do not extend beyond
the carapace’s posteriormost margin, and the cloacal opening is closer to the plastron
than to the tail’s tip (Wyneken, 2001). In D. coriacea , the tails of females sometimes
extend beyond the caudal peduncle of the carapace (especially if the terminal end of
the carapace has been damaged). However, the cloacal openings of adult female and
immature leatherbacks are closer to the plastron than those of immature males. The
position of the cloaca is a reliable indicator of sex in mature leatherbacks.
2.2.3 A PPENDAGES : F LIPPERS AND H IND L IMBS
The forelimbs of all species are elongated as winglike flippers. The hind limbs are
less elongated and more paddle-like in form. Leatherbacks have proportionately
longer flippers than cheloniids of all size classes (Pritchard, 1979; Wyneken, 2001),
lacking claws and scales. Cheloniids have claws on fore and hind limbs, and flippers
are covered with scales. All cheloniids have three or more large scales along the
trailing edge of each flipper near the axilla. These distinctive scales are often used
as a site for flipper tag application. Hind limbs have few or no large scales.
The number of claws on the forelimbs is the same as on the hind limbs. Chelonia
and adult Natator have one claw (on the first digit) on each foot. The remaining
cheloniids ( Caretta , Eretmochelys , Lepidochelys , and some immature Natator ) have
two claws (Pritchard, 1979; Zangerl et al., 1988). Claw I is usually larger than claw II
(on digit II), and in adult males, the first claw curves strongly toward the ventral surface.
2.3 SKELETAL ANATOMY
The skeleton is composed of bones and cartilages, and is partitioned into three parts:
the skull, axial skeleton, and appendicular skeleton. In sea turtles, each of these bony
groups is a composite of several structures. The skull includes the braincase, jaws,
and hyoid apparatus. The axial skeleton is composed of the carapace ( nuchal , dorsal ,
neural , pleural , suprapygal , pygal , and peripheral bones, Figure 2.7) and plastron
( epiplastron , entoplastron , hyoplastron, hypoplastron , and xiphiplastron bones, Figure 2.8), which includes ventral derivatives of the vertebrate axial skeleton as well
as unique bones (Zangerl, 1939; Gilbert et al., 2001). The appendicular skeleton
includes the flippers, hind limbs, and their supporting structures (the pectoral and
pelvic girdles). The skeleton at the hatchling stage, like that of many vertebrates, is
not completely ossified.
In chelonid hatchlings and all size classes of D. coriacea , the carapace is
composed of ribs, a nuchal bone, and vertebrae (illustrated by Deraniyagala, 1939).
In cheloniids, the shell becomes increasingly ossified with increase in size. Intermembranous bone hypertrophies between the ribs and grows laterally, further ossifying the carapace. The ribs grow laterally to meet the peripheral bones in cheloniids
(Pritchard, 1979; Gilbert et al., 2001; Wyneken, 2001). Hatchlings have distinct
flattened ribs, with little intermembranous bone between adjacent ribs. Individual
1123_C02.fm Page 47 Friday, November 15, 2002 11:16 AM
47
The tails of sea turtles are dimorphic in juveniles approaching adulthood and in
adults, but not in hatchling and immature turtles. The tails of mature or nearly mature
males extend beyond the carapace margin, and the cloacal openings are found near
the tail’s tip. The tails of immature or female cheloniids normally do not extend beyond
the carapace’s posteriormost margin, and the cloacal opening is closer to the plastron
than to the tail’s tip (Wyneken, 2001). In D. coriacea , the tails of females sometimes
extend beyond the caudal peduncle of the carapace (especially if the terminal end of
the carapace has been damaged). However, the cloacal openings of adult female and
immature leatherbacks are closer to the plastron than those of immature males. The
position of the cloaca is a reliable indicator of sex in mature leatherbacks.
2.2.3 A PPENDAGES : F LIPPERS AND H IND L IMBS
The forelimbs of all species are elongated as winglike flippers. The hind limbs are
less elongated and more paddle-like in form. Leatherbacks have proportionately
longer flippers than cheloniids of all size classes (Pritchard, 1979; Wyneken, 2001),
lacking claws and scales. Cheloniids have claws on fore and hind limbs, and flippers
are covered with scales. All cheloniids have three or more large scales along the
trailing edge of each flipper near the axilla. These distinctive scales are often used
as a site for flipper tag application. Hind limbs have few or no large scales.
The number of claws on the forelimbs is the same as on the hind limbs. Chelonia
and adult Natator have one claw (on the first digit) on each foot. The remaining
cheloniids ( Caretta , Eretmochelys , Lepidochelys , and some immature Natator ) have
two claws (Pritchard, 1979; Zangerl et al., 1988). Claw I is usually larger than claw II
(on digit II), and in adult males, the first claw curves strongly toward the ventral surface.
2.3 SKELETAL ANATOMY
The skeleton is composed of bones and cartilages, and is partitioned into three parts:
the skull, axial skeleton, and appendicular skeleton. In sea turtles, each of these bony
groups is a composite of several structures. The skull includes the braincase, jaws,
and hyoid apparatus. The axial skeleton is composed of the carapace ( nuchal , dorsal ,
neural , pleural , suprapygal , pygal , and peripheral bones, Figure 2.7) and plastron
( epiplastron , entoplastron , hyoplastron, hypoplastron , and xiphiplastron bones, Figure 2.8), which includes ventral derivatives of the vertebrate axial skeleton as well
as unique bones (Zangerl, 1939; Gilbert et al., 2001). The appendicular skeleton
includes the flippers, hind limbs, and their supporting structures (the pectoral and
pelvic girdles). The skeleton at the hatchling stage, like that of many vertebrates, is
not completely ossified.
In chelonid hatchlings and all size classes of D. coriacea , the carapace is
composed of ribs, a nuchal bone, and vertebrae (illustrated by Deraniyagala, 1939).
In cheloniids, the shell becomes increasingly ossified with increase in size. Intermembranous bone hypertrophies between the ribs and grows laterally, further ossifying the carapace. The ribs grow laterally to meet the peripheral bones in cheloniids
(Pritchard, 1979; Gilbert et al., 2001; Wyneken, 2001). Hatchlings have distinct
flattened ribs, with little intermembranous bone between adjacent ribs. Individual
1123_C02.fm Page 47 Friday, November 15, 2002 11:16 AM