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Crocodilia is an order of large reptiles that appeared about 220 million years ago. They are the closest living relatives of birds.

Like mammals and unlike most other reptiles, crocodiles have a four-chambered heart (although, monitor lizards have a four-chambered heart, as well); however, unlike mammals, oxygenated and deoxygenated blood can be mixed when the foramen of Panazzi is open, which bridges both ventricles in the heart. This opening is typically only open during diving, in order to shunt blood away from the lungs. Their blood has shown to have strong antibacterial powers.

All crocodilians have "thecodont" dentitions (teeth set in bony sockets) but unlike mammals, they replace these teeth throughout life. Juvenile crocodilians replace teeth with larger ones at a rate as high as 1 new tooth per socket every month. After reaching adult size in a few years, however, tooth replacement rates can slow to two years and even longer. Very old members of some species have been seen in the "edentulous" (toothless) state, after teeth have been broken and replacement has apparently ceased. The result of this is that a single crocodile can go through at least 3,000 teeth in its lifetime. Each tooth is hollow, and the new one is growing inside the old. In this way, a new tooth is ready once the old is lost.

From the top: Head of an American alligator (Alligator mississippiensis), a Nile crocodile (Crocodylus niloticus), and an Indian gharial (Gavialis gangeticus).Crocodilians have a secondary bony palate that enables them to breathe when partially submerged, even if the mouth is full of water. Their internal nostrils open in the back of their throat, where a flap of skin closes off their respiratory system when they are underwater. This way they can open their mouths underwater without choking. Most reptiles lack a secondary palate, but some skinks (family Scincidae) have evolved a bony secondary palate too, to varying degrees.

The tongue is attached to the floor of its mouth, making it hard to move at all. Like all reptiles they have a relatively small brain, but it is more advanced than in other reptiles. Among other things it has a true cerebral cortex.

Crocodiles are often seen laying with their mouths open, called gaping. One of its functions is probably to cool them down, but since they are also doing this at night and/or when it is raining, it is possible that gaping has a social function too.

The crocodile basic body plan has turned out to be very successful, as it appears to be a good solution which works very well. This is the reason why modern forms look very much the same way today as they did when the dinosaurs were still around. Even mammals have more or less adapted their body plan at at least one point in history. One primitive ancestor of the whales, Ambulocetidae, was an aquatic predator living in rivers and lakes. Ambulocetids seems to have filled an ecological niche similar to the modern crocodiles.

They have a semi-erect, semi-sprawled posture, holding their legs more directly underneath them than most other reptiles (the chameleons are probably the only reptiles with a more erect posture than Crocodilia). This makes it possible for some species to even gallop on land if necessary; an Australian species can reach a speed of over 16 km/h while galloping on an irregular forest floor. But their ancestors actually had a fully erect posture; their sprawling and semi-erect posture are secondary and evolved after they adapted to a life in water. Especially the ankle bones (tarsals) are highly modified. In other words, their locomotion is not primitive, instead it turns out it is very specialized and quite unique. It seems their distant ancestors most likely were fast-moving terrestrial predators, like Junggarsuchus sloani. An extinct and very early terrestrial crocodile, pristichampsus rollinatii, even had hoof-like toes. The front feet have five toes and the hind feet have four webbed toes. The three inner toes on the front feet and the three inner toes on the hind feet have claws.

As in many other aquatic or amphibian tetrapods, the eyes, ears, and nostrils are all located on the same plane. They see well at day and may even have color vision, plus the eyes have a vertical, cat-like pupil which also gives them excellent night vision. The iris is silvery, a light reflecting layer of tapetum behind the retina greatly increases their ability to see in weak light, this also makes their eyes glow in the dark. A third transperant eyelid, the nictitating membrane, protects their eyes underwater. But they can't focus under water, which means other senses are more important when they are under the surface. While birds and most reptiles have a ring of bones around each eye which supports the eyeball, a ring called the sclerotic ring, the crocodiles lack these bones (just like mammals and snakes). The eardrums are located behind the eyes and are covered by a movable flap of skin. This flap closes, along with the nostrils and eyes, when they dive, preventing water from entering their external head openings. The middle ear cavity has a complex of bony air-filled passages and a branching eustachian tube. There is also a small muscle (which is also seen in gecko) next to or upon the stapes, the stapedius, which probably functions in the same way as the mammalian stapedius muscle does, dampening strong vibrations.

What sex the juvenile will be is determined by the incubation temperature.

The skin is covered with non-overlapping scales composed of the protein keratin (the same protein that forms hoofs, skin, horns, feathers, hair, claws and nails in other tetrapods), which are shed individually. On the head the skin is actually fused to the bones of the skull. There are small plates of bone, called osteoderms or scutes, under the scales. Just like a tree, crocodile osteoderms have annual growth rings, and by counting them it is possible to tell their age. Osteoderms are found especially on the back, and in some species also on the belly. The overlapping rows of scutes cover the crocodile's body from head to tail, forming a tough protective armor. Beneath the scales and osteoderms is another layer of armor, both strong and flexible and built of rows of bony overlapping shingles called osteoscutes, which are embedded in the animal's back tissue. The blood-rich bumpy scales seen on their backs acts as solar panels.

Crocodiles and gharials have modified salivary glands on their tongue, salt glands, used for excreting excess salt from their body. Alligators and caimans have them too, but here they are non functioning. This tells us that at some point the common origin of the Crocodilia were adapted to saline water and marine environments. This also explains their wide distribution across the continents. Species like the saltwater crocodiles (C. porosus) can still survive long periods in the sea. True crocodiles are probably the most original forms, while alligators and caimans have evolved from the crocodiles.

They are known to swallow stones, gastroliths ("stomach-stones"), which act as a ballast as well as help to crush up the bones of their prey. The crocodile stomach is divided into two chambers, the first one is described as being powerful and muscular, like a bird gizzard. This is where the gastroliths are found. The other stomach has the most acidic digestive system of any animal, and it can digest mostly everything from their prey; bones, feathers and horns.

Crocodiles are an old group of animals, but they have evolved much since their body plan first formed many million years ago. After dinosaurs became extinct, some crocodiles became more terrestrial, filling niches earlier occupied by meat-eating dinosaurs. Some extinct forms were probably herbivorous (Simosuchus clarki and Chimaerasuchus paradoxus). Primitive and long-extinct species such as Hesperosuchus and Gracilisuchus were facultatively bipedal. Others were much more adapted to a life in water than any of the species living today. The marine crocodile Metriorhynchus had modified its legs into flippers, and Dakosaurus andiniensis had a skull that was adapted to eat large sea reptiles. If extinct forms are included, the crocodiles are a very diverse and adaptive group of reptiles.

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