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Crocodilia

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Crocodilia
Temporal range: Triassic - Recent
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Sauropsida
Order:
Crocodilia

Owen, 1842
Suborders

Eusuchia
Protosuchia †
Mesosuchia †
Sebecosuchia †
Thalattosuchia †

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


Spelling

The group is often spelled "Crocodylia" for consistency with the genus Crocodylus Laurenti, 1768. However, Richard Owen used the -i- spelling when he published the name in 1842, so by the usual rules of scientific classification his name has priority. In any case, the -i- spelling is a more accurate Latinization of the Greek κροκόδειλος (krokodeilos, literally "pebble-worm", referring to the shape and texture of the animal).


Description

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 properties.

All crocodilians have, like humans, thecodont dentition (teeth set in bony sockets) but unlike mammals, they replace their 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 an almost "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 special part of the tongue called the "palatal valve" 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.

They lack a vomeronasal organ (yet it is detectable in the embryo) and a urinary bladder.

They have alveoli in their lungs and a unique muscular attachment to the liver and viscera that acts as a piston to breathing, separating the thoracic and abdominal cavities (similar to the diaphragm of mammals). Although tegu lizards have a primitive proto-diaphragm, separating the pulmonary cavity from the visceral cavity and allowing greater lung inflation, this has a different evolutionary history.

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 is a very successful one. Their form is well-adapted to their semi-aquatic niche, which is why modern forms look similar today as they did when the dinosaurs were still around (though only to the semi-aquatic forms). Mammals too have adapted to this body plan at least once in history. One ancestral whales family, Ambulocetidae, was an aquatic predator living in rivers and lakes. Ambulocetids 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 Crocodylia). 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, it is in fact specialised to their semi-aquatic lifestyle. Their distant ancestors were fast-moving terrestrial predators, like Junggarsuchus sloani. An extinct terrestrial species, Pristichampsus rollinatii, even had hoof-like toes.

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.

The gender of the juvenile 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.

Their spool-shaped vertebrae in their ancestors went from being biconcave to having a concave front and a convex back in the modern forms. This made the vertebral column more flexible and strong, a useful adaptation if you are hunting in water.

They possess ribs of dermal origin restricted to the sides of the ventral body wall. The collar bone (clavicle) is absent.

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.

West African dwarf crocodile from the forests of West and West Central Africa

The upper and lower jaws are covered with sensory pits, seen as small, black speckles on the skin, the crocodile version of the lateral organ we see in fish and many amphibians. But they have a completely different origin. These pigmented nodules encase bundles of nerve fibers that respond to the slightest disturbance in surface water, detecting vibrations and small pressure changes in water, making it possible for them to detect prey, danger and intruders even in total darkness. These sense organs are known as DPRs (Dermal Pressure Receptors). While alligators and caimans only have them on their jaws, crocodiles have similar organs on almost every scale on their body. The function of the DPRs on the jaws are clear, but it is still not quite clear what the organs on the rest of the body in crocodiles actually do. They are probably doing the same as the organs on their jaws, but it seems like they can do more than that, like assisting in chemical reception or even salinity detection.

Evolution

When their extinct species are examined, crocodiles prove to have been a very diverse and adaptive group of reptiles. Crocodiles are an ancient group of animals. The earliest species, the Sphenosuchians, evolved during the Late Triassic. They were highly gracile, terrestrial forms built like greyhounds. Several terrestrial species during the Cretaceous evolved herbivory (Simosuchus clarki and Chimaerasuchus paradoxus). During the Jurassic and the Cretaceous marine forms in the family Metriorhynchidae such as Metriorhynchus evolved forelimbs that were paddle-like and had a tail similar to modern fish. Dakosaurus andiniensis a closely related species to Metriorhynchus had a skull that was adapted to eat large sea reptiles.

The evolution of extant forms, Eusuchia (which includes the crown group Crocodylia), first appear in the Lower Cretaceous of Europe. The recently-described fossil, Isisfordia duncani, lived approximately 95 to 98 million years old (during the Cenomanian epoch of the Upper Cretaceous) is the second oldest known eusuchian, and the earliest crocodilian yet found in Australia. The clade Eusuchia underwent a mass radiation during the Late Cretaceous and the Paleogene, in which they evolved into numerous forms, such as semi-aquatic dinosaur eating species (Deinosuchus); hooved, terrestrial carnivores (Pristichamps) and 'hatchet'-shaped skulled forms (Baru).

Extinct Species


Modern Species

See individual family pages for a full list of species.

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