All crocodilians share a similar basic body form. They share characters such as a secondary palate, thick armoured bodies, a powerful tail (used for swimming), and an additional membrane to protect the eye while submerged. Their heads are heavily built, with large muscles capable of delivering a huge downward force, although the opposite muscles are so weak it is possible to hold their jaws shut with only two fingers.
Crocodilians have large, powerful skulls. Experiments with the American alligator showed that the jaw-closing muscles can generate 2,125 pounds (964 kilograms) of force. Even more impressive was the extinct giant crocodile Sarcosuchus, whose bite force has been estimated at 18,000 pounds (8,165 kilograms). The opposing muscles (reponsible for opening the jaws) are, however, so weak that a person can prevent a crocodile from opening its mouth using only two fingers.
All living crocodilians have similar numbers of teeth, five premaxillary, and 13-16 maxillary teeth. Most crocodiles have only 15 mandibular teeth, while alligators have 17-22. This gives total numbers of teeth of 60-72 in crocodiles and 72-82 in alligators. The only living gharial, Gavialis gangeticus, has a long slender snout and specialises as a piscivore, so it has the largest number of teeth: five premaxillary, 23-24 maxillary, and 25-26 mandibulars (making 106-110 in total). Crocodile teeth have a good nervous system, and when triggered by an object entering the mouth, the jaws snap shut with frightening speed.
Living crocodilians have a nictitating membrane, a second eyelid, that can be used both to clean the eye and protect it when the crocodile submerges.
(photograph by the author)
The palate is the roof of the mouth. Normally, reptiles have a direct connection between the external and internal nostrils, so they breathe straight into their mouths. In crocodilians, the external nostrils are right at the tip of the jaws, and a secondary. bony palate has grown back in the roof of the mouth. This pushed the internal nostrils right back to lie above the back of the throat. This allows the animal to open its mouth underwater without drowning.
At the back of the palate is the palatal valve, a flap of tissue that can be opened and shut. The palatal valve is sealed when the crocodile is submerged, but it can open if the nostrils are above the water. So, crocodilians can breathe underwater when only the tip of the snout is above the surface.
With the exception of some marine genera of Mesosuchia, all crocodilians have 24 presacral vertebrae, 2 sacrals, and a tail comprising of 30-40 vertebrae. In modern crocodiles the first nine vertebrae are cervicals (neck vertebrae) and the ribs associated with the atlas and axis are simple rods with slightly expanded heads.
All modern crocodilians are quadrupedal and on land have a wide sprawling stance. They have three modes of terrestrial locomotion: crawling on their belly, walking with their body raised off the ground, and galloping. A large crocodile can reach speeds of ~7mph and ~10mph when crawling and galloping respectively.
Crocodilians have a long powerful tail which is used for swimming (the limbs are not used at all). Although cumbersome on land, crocodilians are very capible swimmers and can move with great speed when required. Such is the power of the tail that crocodiles can lunge and jump out of the water to catch prey (see picture left). In one extinct genus (Metriorhynchus) the tail supports a large caudal fin.
With the exception of some early crocodylomorphs and some fully aquatic forms, all crocodilians have a tough covering of scales that act as a dermal armour. Although a fully grown crocodilian has no natural predators, they do fight for territory and it is not uncommon for whole limbs to be ripped off during combat.
Crocodilians generally live in muddy, bacteria-infested waters. In these conditions even the slightest graze would become infected, and yet crocodilians can sustain savage wounds with no adverse effects. Recently, a unique antibody has been identified in their blood, aptly named "crocodillin", which gives crocodilians one of the most effective immune systems in the world. The only time crocodiles suffer from an infection is when they become stressed and their health declines.
Crocodilians have a four-chambered heart, as in birds and mammals. This is not surprising as birds and crocodilians share a common ancestor. The four-chambered heart provides a number of advantages over a two-chambered (lizard) heart as it allows oxygenated blood from the lungs to be kept separate from deo-oxygenated blood from the body.
Crocodilians are unique as they have evolved a means of bypassing the pulmonary system altoghether. They have a valve called the "foramen of panizza," which divides the blood flows from the lungs and body. This valve shuts off the pulmonary (lung) system during diving, when the pulmonary system is redundant until the animal resurfaces and takes a breath. Blood is therefore transfered from the pulmonary system to the body, providing greater aerobic capacity with submerged.