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
|Pelvic girdle (left), vertebral sections (top right), atlas-axis complex and bicipital ribs (bottom right)|
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|
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
o 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.
· Subfamily Alligatoridae
o Alligator mississippiensis and Alligator sinensis
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.
· 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.