Crocodile

A crocodile is a large amphibious reptile. It lives mostly in large tropical rivers, where it is an ambush predator. 

The Crocodylia is an order of mostly large, predatory, semiaquatic reptiles. Their ancestors, the clade Pseudosuchia, appeared about 250 million years ago in the early Triassic period. 

Crocodiles as we know them first appeared 94 million years ago in the Upper Cretaceous period. They are the closest living relatives of birds: the two groups are the only survivors of the Archosauria.

One species, the Australian saltie, travels in coastal salt water as well as rivers. In very dry climates, crocodiles may aestivate and sleep out the dry season. 

They are basically Archosaurs, a group which also includes the dinosaurs. There are various species of crocodiles including the American, slender-snouted and Orinoco crocodile.

The crocodile can quickly snap its jaw shut with much power. However, crocodiles have little strength when opening their jaws, such that a person can hold the jaw shut with their hands.

Present-day crocodiles range in size from African dwarf crocodiles that measure rarely over 5 feet (1.5 m) to saltwater crocodiles which can approach 23 feet (7 m).

Description 
Crocodiles live in rivers, lakes and dams in parts of America, Asia, Africa and Australia. Some of the crocodiles from Australia live in salt water. These saltwater crocodiles are normally bigger than the ones that live in fresh water. While crocodiles spend most of their time in water, they can come out and move around on the land. 

Crocodiles cannot breathe underwater: they breathe air. This is usual for land animals which take to the water later in their evolution. Whales, turtles, ichthyosaurs and plesiosaurs are good examples of this.

When they are not active, crocodiles can hold their breath for about two hours underwater before drowning. Normally, when underwater, they are active and can hold their breath for a maximum of 20 minutes to one hour.

Behavior

Feeding 
Crocodiles in East Africa take their chances when herds migrate in search of food. To make their annual journey up the East coast, herbivores have to cross rivers.

The crocodile looks like it is a log, and hides beneath the surface of the water. When an animal comes down to the water to drink, the crocodile lunges out of the water, grabs the animal in its jaws, and pulls it underwater, where it drowns. The crocodile grab a piece of meat in its jaws and tears it off with a spin-around. 

Adult crocodiles can go long periods without eating. When crocodiles are very young, they mainly eat insects, crustaceans, snails, small fish, frogs and tadpoles.

Reproduction 
Reproduction is different in several ways from other reptiles. 
First of all, their sex at birth depends on their temperature, not upon sex chromosomes (which they do not have). Eggs are laid in mounds near the water in sand or mud. They are guarded by the female, though she does have to go into the river to eat. The females are unusual for reptiles in that they continue to guard and protect the babies once they are born and in the river. They have a very distinctive behaviour when in the river: if the mother opens her mouth they swim back into it. When danger passes, she opens her mouth, and they swim out. Protection of the babies is one possible explanation of how the crocodiles have been able to survive. Much of this applies to alligators as well.

What they look like 

Their colors range from brown to grey or greenish-brown. Crocodile tongues are not free. They are held in place by a membrane that can not move. Crocodiles are unable to stick out their tongues.

Alligators and crocodiles 
Although there is not much difference in their life-style, biologists put alligators in a separate family. Gharials are also in a separate family, and Caimans are a sub-family of alligators.

An obvious difference between an alligator and a crocodile is that one can not see the fourth tooth in the lower jaw of an alligator when the alligator's mouth is closed. The fourth tooth in the lower jaw of a crocodile sticks out when its mouth is closed.

Ancient crocodiles 
Some ancient crocodiles walked on two legs like dinosaurs.  The scientists found preserved footprints in South Korea.  They found hind foot prints but no front foot prints or marks from a tail dragging behind.

They were able to tell that these ancient crocodiles walked on their hind legs with their front legs and tails off the ground.  They used the whole foot, the way a human or bear does, not just the toes, the way a dinosaur, bird or dog does.

Crocodylomorpha 
Crocodylomorpha is a group of pseudosuchian archosaurs which includes the crocodilians and their extinct relatives. They were the only members of Pseudosuchia to survive the end-Triassic extinction.

During Mesozoic and early Cenozoic times, crocodylomorphs were far more diverse than they are now. Triassic forms were small, lightly built, active terrestrial animals.  These were a mixed group of small-bodied forms with long limbs that walked upright. This was the ancestral form of the Crocodylomorphs. These forms persisted until the end of the Jurassic.

Largest crocodile 
There are several examples of present-day crocodiles reaching 6 m (20 ft) in length and weighing 1,200 kg (2,600 lb). 

The diagram left shows that the largest crocodyliforms were about 30 feet long.

Species
The order Crocodilia has 27 living species in nine genera. This does not include hybrid species or extinct prehistoric species. Modern molecular studies indicate that the nine genera can be grouped into three families.

Family Alligatoridae (Alligators and caimans)
Genus Alligator: two species
Genus Caiman: three species
Genus Melanosuchus: one species
Genus Paleosuchus: two species
Family Crocodylidae ("True" crocodiles)
Genus Crocodylus: fourteen species
Genus Mecistops: two species
Genus Osteolaemus: one species
Family Gavialidae (Gharial and false gharial)
Genus Gavialis: one species
Genus Tomistoma: one species

References

Other websites

 Crocodile (Wikiversity)
 Saltwater crocodiles in freedom: Authentic pictures and descriptions
 Supersize Crocs - an interactive guide to crocodiles from PBS 
 Man shares bungalow with crocodile BBC News.
 Crocodile -Citizendium

Crocodilia
Living fossils
Reptiles