Crocodylia:
Crocodylia is the largest living reptile and has 25 extant
species in 3 families. Some controversy between the molecular and morphological
phlylogeny exists in this group, resulting in different placements of Gavialis and Tomistoma. However, both molecular and morphological data has Crocodylus and Osteolaemus as sister groups. The three families, Gavialoidea, Alligatoroidea, and Crocodylidae are distinguished by their
diagnostic features, highlighted below.
Crocodylia is semiaquatic/aquatic with a robust skull, long
snout, strongly toothed jaws, short neck, cylindrical trunk, thick laterally
compressed tail, osteoderms (bony plates) in the skin, and short but strongly
developed limbs.
The skull is characterized by having extensive ornamentation
with pits and ridges so that the skin is tightly attached to the skull. It also
has extensive pneumatization of the skull with respect to the Eustachian tubes
and nasal cavities. The foramen magnum is formed by the basioccipital and exoccipital. An ear-flap is present on the skull table, which protects the ear
drum from underwater pressure. Look at the picture below for the specific bones of
the skull. Additionally, this group exhibits thecodont
teeth, whereby the tooth is deeply set in a socket. Furthermore, a
diagnostic characteristic between families is width of the mandibular
symphysis, where the two halves of the lower jaw meet. Alligatoridae have a narrow symphysis and Crocodylidae have a wide symphysis.
The postcranial skeleton of Crocodylians can be divided into
four regions: the cervical vertebrae, dorsal vertebrae, sacral vertebrae which
articulates with the illium, and the caudal, or tail vertebrae. The first
cervical vertebrae is the atlas, which directly articulates with the occipital
condyle of the skull and the second cervical vertebrae is the axis which
articulates with the atlas. The ribs are considered bicipital because they have
two articulation points with the ribs. Moving posteriorly, the position of the
capitulum and the articulation with the centrum and traverse process changes.
Note the changes from the cervical and dorsal vertebrae on the picture below
(PIC). The pectoral girdle is composed of
CN ankle |
Pelvic girdle (left), vertebral sections (top right), atlas-axis complex and bicipital ribs (bottom right) |
Reproduction
Crocodylians have internal fertilization via a penis and are
oviparous and form nests out of vegetation, which acts as a compost pile to
encourage rapid development. The temperature of the nest is extremely
important because it determines the sex of the babies, a phenomenon called
temperature-
Cloaca and possibly a penis |
Diet
Crocodylians are opportunistic carnivores and eat a variety
of prey. Potential prey items will vary depending on the size of the
individual. The morphology of the snout and skull can indicate what they may
eat. A broad, short and stout snout is typically modified for greater crushing
force whereas a long narrow snot is specialized for snapping and eating fish.
Families of
Crocodylians
Family Gavialoidae
o
Gavialis
gangeticus
Gavialis gangeticus |
§
This family is found in parts of India and
Southeast Asia and is extinct in many areas. It can grow to lengths of 6.5 m
and have their teeth outside their jaw. They have a characteristic boss on the
end of their long and narrow snout These are one of the most endangered
Crocodylian and one of the most endangered reptile.
o
Tomistoma
schlegelii
§
These are found in parts of India and South Asia
and can grow up to 4m. They have a long and narrow snout with the teeth inside.
This species prefers to eat tetrapods, especially monkeys.
Family Alligatoroidea
·
Subfamily Alligatoridae
o
Alligator
mississippiensis and Alligator
sinensis
Alligator |
o
This subfamily is found throughout Eastern North
America, Central and South America, and China and are the only Crocodylian that
live in areas where seasonal temperatures drop below freezing. This subfamily
is found in freshwater and has paired nasal foramina. They reach sexual
maturity at 2 meters but males usually wait to mate until they are larger so
they are better competitors
·
Subfamily Caimaniae
o
3 genera: Caiman,
Melanosuchus, Paleosuchus
o
This subfamily is found in slow moving
freshwater rivers and are found throughout Mexico and South America. They can
range from 1.7-5 m and build vegetation nests adjacent to termite mounds to
obtain additional heat for the nest.
Family Crocodyloidea
· 3 genera: Crocodylus
(15 spp, pantropical), Mecistops (1
spp, Wester, West-central Africa) and Osteolaemus
(1 spp, Wester, west-central Africa)
·
This family can range in length from 2.5-7m and
have a very long snout. The 4th mandibular tooth on the lower jaw sits
outside the mouth when it is closed and the 1st mandibular tooth
fits into a groove on the premaxilla.
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