Reptiles Database

Subclass Archosauria
Order Crocodylia
Suborder Eusuchia

Family Crocodylidae (Crocodiles and relatives)

Appearance: Large, heavily armored, lizard-like reptiles with elongate snouts and powerfull tails. Osteoderms (heavy plates of bone) underneath the dorsal scales form the armor and are present ventrally in many species. Teeth are thecodont (set into sockets, from Greek thece = container and odus = tooth). All crocodiles are adapted to aquatic life by having webbed feet, nostrils on top of their snout that can be closed by valves, and eyes with a transparent membrane that is drawn across the eye under water. Other features are unique among reptiles, e.g. a muscular partition separating th epectoral and abdominal cavities (similar to the diaphragm of mammals) and alveoli in the lungs. Crocodiles also have a heart with four chambers which mostly prevents mixing of oxygenated and deoxygenated blood.

Size: Alligatorids are up to 6 meters in total length (e.g. Melanosuchus niger), but small species are less than 1.7 m (Paleosuchus). Crocodylus porosus grows to more than 7 meters and is probably the largest living crocodile. Osteolaemus tetraspis reaches only 2 meters. Gavialis reaches 6.5 metes and Tomistoma about 4 meters.

Distribution: Alligatorinae: North-, Central-, and South America. Alligator sinensis in China. Crocodylinae: in Asia from Iran to Indonesia and New Guinea, Australia, Africa, Southern Mexico to northern South America, Caribbean, southern Florida, Madagascar. Gavialinae: Pakistan east to Indochina and south to Indonesia.

Habitat: All crocodiles are more or less aquatic. They are usually excellent swimmers, but some can even gallop on land (Crocodylus johnsoni). Alligatorids inhabit rivers, lakes, swamps, and lagoons (except for Paleosuchus which inhabits small rivers and pools of forested areas of the Amazon area). Alligator mississippiensis even enters coastal marine waters.

Food: Fishes, turtles, birds, mammals and other animals. Small crocodiles eat insects, frogs, and other small animals living in or near water. Gavialis and Tomistoma east mostly fish.

Behavior: Crocodiles show a more complex social behavior than most other reptiles. They are territorial during the breeding season and aggressively warn intruders, often with vocalizations. Sounds produced by crocodiles are quite diverse including roars, grunts, coughs, and purs. Crocodiles hear well and their senses of smell and ehesight are also well developed. Predation is usualy nocturnal and in or near water. Drowining is often used to kill prey. Crocodiles don't chew and swallow their food as a whole or as big chunks.

Reproduction: Oviparous. Eggs are laid in mounted nests constructed from plant matter and soil or directly in soil on beaches or nearby areas (Gavialis and a few species of Crocodylus). All species examined so far show temperature-dependent sex determination. Crocodiles guard their nests and some species might even help the hatchlings to leave the nest buy breaking it open and carrying the babies to the water in their mouths. Paleosuchus trigonatus lays eggs in mound nests next to termite nests in the forest.

Relationships: Crocodiles are more closely related to birds than to any other (living) reptile although recent molecular studies suggested a close relationship to turtles (Hedges & Poling 1999). The three subfamilies of crocodiles are often considered as separate families, Alligatorinae, Crocodylinae, and Gavialinae. Morphological data seem to place Tomistoma within the Crocodylinae (Frey et al. 1989, Norell 1989, Tarsitano et al. 1989) whereas molecular evidence separates that genus together with Gavialis from the other crocodiles (Densmore and White 1991, Hass et al. 1992, Gatesy and Amato 1992). Although combined data sets seem to support a Gavialis-Tomistoma clade the relationship of Tomistoma are still not unambigously solved (Poe 1996). Poe (1996) also synonymized Melanosuchus with Caiman.

Diagnosis: In alligatorids, the teeth of the lower jaw fits into pits in the upper jaw and cannot ber seen when the mouth is closed. In crocodylids, the fourth tooth in the lower jaw is accommodated in a notch in the upper jaw and is visible when the mouth is closed.

List of genera:

Subfamily Alligatorinae (Alligatoridae)

Subfamily Crocodylinae (Crocodylidae)

Subfamily Gavialinae (Gavialidae)

Phylogeny

Tree of crocodiles

Consensus Bayesian trees illustrating the relationships of crocodylians using sequences from the DMP1 and C-mos genes (modified after Willis et al. 2007). Another recent study based on mtDNA sequences (Roos et al. 2007) suggested to include Alligator, Caiman, and Paleosuchus in the family Aligatoridae and the remaining genera in the family Crocodylidae.

Selected References:

Densmore, L.D., III.; White, P.S. (1991)
The systematics and evolution of the Crocodilia as suggested by restriction endonuclease analysis of mitochondrial and nuclear ribosomal DNA.
Copeia 1991 (3): 602-615

Gatesy, John; Amato, George D (1992)
Sequence similarity of 12S ribosomal segment of mitochondrial DNAs of gharial and false gharial.
Copeia 1992 (1): 241-243

Hass, C. A.; Hoffman, M. A.; Densmore, L. D.; Maxson, L. R. (1992)
Crocodilian evolution: insights from immunological data.
Mol Phylogenet Evol 1 (3): 193-201

HEDGES S.B. AND L.L. POLING. 1999
A molecular phylogeny of reptiles.
Science 283 (5404): 998-1001

McAliley, L. Rex; Ray E. Willis, David A. Ray, P. Scott White, Christopher A. Brochu and Llewellyn D. Densmore III (2006)
Are crocodiles really monophyletic? &emdash; Evidence for subdivisions from sequence and morphological data.
Molecular Phylogenetics and Evolution 39 (1): 16-32

Norell, M. A. (1989)
The higher level relationships of the extant crocodylia.
Journal of Herpetology 23 (4): 325-335

Poe, Steven (1996)
Data set incongruence and the phylogeny of Crocodilians.
Systematic Biology 45 (4): 393-414

Roos, Jonas; Ramesh K. Aggarwal, Axel Janke (2007)
Extended mitogenomic phylogenetic analyses yield new insight into crocodylian evolution and their survival of the Cretaceous–Tertiary boundary.
Molecular Phylogenetics and Evolution 45: 663–673

Schmitz, Andreas; Patrick Mausfeld; Evon Hekkala; Tara Shine; Hemmo Nickel; George Amato; Wolfgang Böhme (2003)
Molecular evidence for species level divergence in African Nile Crocodiles Crocodylus niloticus (Laurenti, 1786).
C. R. Palevol 2: 703&endash;712

Tarsitano, Samuel F., Eberhard Frey, and Jürgen Riess. 1989.
The evolution of the Crocodilia: A conflict between morphological and biochemical data.
Am. Zool. 29: 843-856.

Willis, Ray E.; L. Rex McAliley, Erika D. Neeley and Llewellyn D. Densmore III 2007
Evidence for placing the false gharial (Tomistoma schlegelii) into the family Gavialidae: Inferences from nuclear gene sequences.
Molecular Phylogenetics and Evolution 43 (3): 787-794

Wu, Xiao-Bing; Wang, Yi-Quan; Zhou, Kai-Ya; tong, Zong-Zhong; Nie, Ji-Shan; Wang, Chao-Lin and Xie, Wan-Shu (2001)
Systematic relationships of 8 crocodilian species suggested by 12S rRNA gene sequence [in Chinese].
Acta Zool. Sinica 47 (5): 522-527

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