10.2. Homodont Animals

Tooth types

Most toothed vertebrates exhibit repetitions of a single type of tooth (homodont) that varies in size across the dentition. Diet and feeding habit can cause natural selection to gradually mold tooth shape producing remarkably specialized teeth. Carnivory in fishes favors the evolution of flat teeth with a sharp edges for cutting meat. They tend to be triangular with a pointy tip to facilitate penetration. Piranhas are South American freshwater fish with carnivorous habit. They are famous for having very sharp teeth and quickly aggregating and dismantling large animals at the first sign of blood in the water.

Other morphological adaptations observed in carnivore fish include serrated teeth, commonly found in sharks. A serrated cutting edge has many small points of contact with the material being cut. By having less contact area than a smooth blade or other edge, the applied pressure at each point of contact is greater and the points of contact are at a sharper angle to the material being cut. The tooth produces many small splits in the surface of the material being cut and merges them as it moves.

Some herbivore fish, on the other hand, specialize in eating seeds and have broad and flat teeth designed for crushing hard items. The pacu (order Characiformes) is a relative of the piranha with very different teeth that grossly resemble human molars. Pacus can reach large body sizes (> 80 cm) and they mostly feed on nuts that fall from the trees into the rivers that they inhabit.

Living amphibians typically have small teeth, or none at all, since they commonly feed only on soft foods. These pedicellate teeth are flexible due to connective tissue and uncalcified dentine that separates the crown from the base of the tooth. In addition to the tiny denticles found along the jaws, some frogs have vomerine teeth, found on the roof of the mouth. These tend to be sharp and inclined posteriorly to help prevent the loss of food items in the mouth.

Reptile teeth are generally simple and conical in shape. Most lizards and crocodilians have large numbers of conical teeth designed to perforate and retain live food items. They mostly ingest their prey whole as opposed to biting pieces off such as carnivore fishes with blade-like teeth do. An interesting variation can be seen in snakes, in which some of the teeth are specialized into venom-injecting fangs.

The position and size of the fangs and the associated venom glands is highly variable in snakes. Their teeth also vary in the presence and position of groves on the teeth, which serve the role of transporting venom from the base to the tip of the tooth. Most snakes can be placed into one of four groups, based on their teeth, and this grouping correlates strongly with lineage and venom type.

Aglyphous (lacking groves) snakes have no specialized teeth. All teeth are similar in shape and often size. Most aglyphous snakes, including boas and pythons are non-venomous. Some are considered mildly venomous, but they are generally not harmful to humans.

Opisthoglyphous (rearward grooves) snakes use a pair of enlarged teeth at the back of the maxillae to inject venom. These bones normally angle backward and are grooved to channel venom into the puncture. Since these fangs are not located at the front of the mouth the snake must move the prey into the rear of the mouth and then penetrate it with its fangs. Opisthoglyphous snakes such as garter snakes are found in family Colubridae.

Proteroglyphous (forward grooved) snakes have shortened maxillae bearing few teeth except for a substantially enlarged fang pointing downwards and completely folded around the venom channel, forming a hollow needle. Because the fangs are only a fraction of an inch long in even the largest species these snakes must hang on, at least momentarily, as they inject their venom. Some spitting cobras have modified fang tips that allow them to spray venom at an attacker's eyes. Proteroglyphous dentition is unique to elapids such as coral snakes and cobras.

Solenoglyphous (pipe grooved) snakes have the most derived venom delivery method of any snake. Each maxilla is reduced to a nub supporting a single hollow fang and it can rotate over its articulation with the skull. The fangs, which can be as long as half the length of the head, are folded against the roof of the mouth, pointing posteriorly. The fangs rotate erecting anteriorly when the jaws open. Solenoglyphous snakes open their mouths almost 180 degrees allowing their fangs to penetrate deep into the prey. Solenoglyphus dentition is unique to vipers.

Most dinosaurs had conical teeth like those of lizards and crocodilians. As notable exceptions, several carnivore species had flattened triangular teeth, some of them with serrated edges. Large herbivore species faced accelerated wearing of the teeth through abrasion produced by the silicon crystals contained in the grass that they consumed. Hadrosaurs exhibited ‘tooth batteries’ with rows of teeth densely packed side by side on the jaw and also over each other forming vertical layers.

Summary

Homodont animals have all teeth of a same type. A great diversity of tooth structures is found among species, however. Diet and feeding habit are the main driving forces shaping the evolution of teeth. Specializations are found for cutting meet, seizing prey, crushing seeds, dealing with abrasion by grass, and injecting venom.

Keywords

Carnivory, herbivory, homodont, cusp, serrated teeth, pacu, flat teeth, vomerine teeth, pedicellate teeth, conical teeth, fangs, viper, venom injecting, aglyphous, opisthoglyphous, proteroglyphous, solenoglyphous, tooth battery.

Figure credits

Figure 1 by Peter van der Sluijs - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=19049352

Figure 2 by User:Chris huh - File:Tiger shark teeth.jpg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=17350720

Figure 3 by Nisamanee wanmoon - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=35460603

Figure 4 by Everglades NPS from Homestead, Florida, United States - Alligator TEETH, NPSPhoto, R. Cammauf, Public Domain, https://commons.wikimedia.org/w/index.php?curid=44378671

Figure 5 by Brimac The 2nd - https://commons.wikimedia.org/wiki/File:Bitis_gabonica_fangs.jpg, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=42558205

Figure 6 by Mokele at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10890158

Figure 7 by Mokele at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10890199

Figure 8 by Mokele at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10890297

Figure 9 by Mokele at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10890403

Figure 10 by Paleopod - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=55294934