A Flawless Set of Teeth
The shark's jaws exhibit characteristics unique in the animal world, no terrestrial predator has jaws that come anywhere near matching them in their perfection.
The ancestors of modern sharks were equipped with an upper jaw fixed to the skull (as in man) and with a mouth located at the extremity of the head. Since the age of these fossil sharks, which had to content themselves with relatively small prey, the mouth has moved underneath the head, and the upper jaw, disengaged from the skull, has become mobile. The shark is therefore equipped with two mobile and independent jaws, enabling it to swallow much larger prey and to tear off much larger pieces of flesh.
The spectacular distance these jaws can open would be nothing were they not subtended by muscles of exceptional power. The shutting force of a shark's jaws was measured a few years ago using an apparatus tested by J. N. Snodgrass which he called the "gnathodynamometer". With the help of Dr. Perry Gilbert he measured the dentition strength of the Tiger Shark, the Lemon Shark and the Dusky Shark. The maximum force recorded for for a single tooth of a Dusky Shark was 600 kilos per 2mm squared, or 3 tons per square cm. Even then it should be noted that their specimens were not more than three meters long, so doubtless this enormous force must be doubled for animals of 5 or 6 meters in length. As an indication, the strength exerted by a human jaw is 220 kilos per square cm for a man weighing 70 kilos (11 stone). It must also be remembered that, in addition to cutting, these jaws are capable of crushing bones.
The Great White Shark as seen by Bloch in 1785 in his Histoire naturelle du poisson. This encyclopedia entry served as a model for a very long time and even its errors were taken up (in particular here the inversion of the upper and lower teeth).
(Click the thumbnail for a larger image)
Besides its extensibility and its Herculean strength, the jaw of the shark possesses a third formidable feature: a set of teeth whose anatomy and manner of replacement are also unique in the animal kingdom. The shark's teeth can be renewed indefinitely as fast as they are lost either at the time of an attack or when they fall out spontaneously. Several sets of reserve teeth exist behind the set that is functional at the time, i.e. the one that sits on the ridge of the jawbone. There are thus at least five sets, covered to varying degrees with buccal mucous membrane depending on whether they are number five, four, three or two in line. If one or more teeth in the functional row are exposed at the root, broken or torn out, the corresponding tooth or teeth of the next row will move up and become functional.
Furthermore, the teeth are held fast on a very strong fibrous tissue, allowing them to be erected when the mouth is opened. The opening movement makes them turn forwards and outwards, enabling the shark to bite firmly and to hold on to what it bites. Taking into account the fact that each one of these teeth itself bears smaller teeth, making it a veritable saw, and that its concavity at the rear gives it qualities of a hook, one can understand why the wounds left by such a formidable jaw are always dramatic, if not fatal.
Even when dead, the animal remains a potential danger to guard against. A jaw that has been cut away from its surrounding muscles, and placed on the deck of a boat to dry in the sun has a good chance of suddenly snapping shut again owing to retraction of the fibrous articulations. Woe betide the hand or arm that lingers too long; accidents have occurred on a number of occasions.
For the same reasons, the best way of removing the hook from the shark's mouth is to wait until the following day. Apparent death in this animal is often only an illusion, even several hours after it has been out of the water.
Our information on the rate of tooth replacement in the sharks stems in particular from the studies done by the American Samford Moss on species of scavenging shark. The Lemon Sharks, for example, renew their teeth every 8.2 days for the lower jaw and every 7.8 days for the upper jaw. This is doubtless an extreme case, and it seems that the largest species have a longer cycle, of the order of six months to a year. It has been possible to demonstrate this in a crude fashion through teeth gathered on the bottom of certain large aquaria.
The shape of the teeth is highly characteristic according to the order to which the animal belongs, and thus not only allows for the easy identification of a shark once captured, but also aids the identification of the species of shark responsible for an attack, when all or part of a tooth is found in the victim's wounds.
Dental Formula - This helps in the identification of a shark. 13/13, for example, means 13 teeth in the upper half-jaw and the same number in the lower half-jaw. In certain species (particularly in the requiem sharks), a few very small teeth can be seen in the center of each jaw, causing the formula to vary.
