Jaws and teeth provide specialized food capture and mechanical breakdown of food.

Jaws and teeth were an important evolutionary innovation for active predators. Some of the first vertebrates, the jawed fishes, became dominant in their aquatic environment through the ability to swim and bite forcefully to obtain their food. Fig. 40.10 shows a phylogenetic tree of vertebrates indicating the appearance of jaws. Jawed fish evolved from jawless ancestors, and jaws can be found in present-day fish, amphibians, reptiles, and mammals. As you can see from the large number of jawed groups, jaws are key to the evolutionary success of vertebrates. Jaws evolved from cartilage that supported the gills, providing a spectacular example of an organ adapted for one function (gill support) changing over time to become adapted for an entirely different function (predation).

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FIG. 40.10 A phylogeny of vertebrates, showing the appearance of jaws. Jaws, which evolved more than 400 million years ago from a group of jawless ancestors, contributed to the success of many groups of animals.

Among vertebrates, mammals evolved a specialized jaw joint, the temporomandibular joint, as well as a great diversity of specialized forms of teeth. These teeth have cutting and crushing surfaces, enabling mammals to break down a variety of foods mechanically before swallowing. Fig. 40.11 illustrates the arrangement of specialized teeth in mammals with different diets. Teeth in the front of the mouth, the incisors, are specialized for biting. Others, such as the canines of dogs, cats, and other carnivores, are specialized for piercing the body of their prey. Saber-toothed cats, which lived as recently as 10,000 years ago, had remarkably long canines (up to 16 inches or 40 cm!) specialized for killing large prey. Premolars and molars are teeth in the back of the mouth that are well adapted for crushing and shredding tougher foods, such as meat and fibrous plant material.

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FIG. 40.11 Jaws and teeth of carnivorous, herbivorous, and omnivorous mammals. Mammals evolved a diverse arrangement of teeth for diverse diets.

Herbivorous animals, such as cattle, sheep, and horses, have specialized premolars and molars with prominent surface ridges. These teeth can shred tough plant material before it is swallowed and digested. Mammalian herbivores use their front incisors and canines to bite grasses and leaves, and then move the food to the back of the mouth, where it is ground and crushed between their ridged premolars and molars.

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