April 9, 2002
Mammals: Placental Orders and Dentition
(Be sure to be familiar with the major placental orders mentioned in class and lab, as well as dental specializations.)
Mammalian teeth are differentiated into four basic types: incisors, canines, premolars, and molars. In non-mammalian cynodont therapsids, molars and premolars are undifferentiated (and referred to simply as "cheek teeth"). Molar teeth, unlike the other tooth types, are not represented in the first set of teeth (termed milk teeth or "baby teeth") and only erupt in the permanent second set. All teeth in the lower jaw arise from the dentary; in the upper jaw, incisors erupt from the premaxilla, and canines, premolars, and molars erupt from the maxilla.
Skulls and teeth are the most commonly fossilized elements of mammalian skeletons. Tooth arrangement and structure are strongly correlated with diet in mammals, and tooth reduction (loss of teeth) has been a common theme (especially in herbivores). Therefore, mammalian paleontologists make extensive use of dental formulas, a numerical listing of tooth types, starting from the center of the mouth and working back along either side. Barring trauma, adults have a symmetric arrangement in the left and right half of each jaw, but the upper jaw need not agree with the lower jaw. For example, most dogs have a dental formula of 3-1-4-2 / 3-1-4-3.
Most of the mammals alive today are placentals, characterized by long periods of gestation and by a primitive dental formula of 3-1-4-3 / 3-1-4-3. Lost teeth typically never are regained within a lineage, so placentals do not have more than three incisors or more than three molars per quadrant (half-jaw). (A notable exception are toothed whales, which have evolved a multitude of sharp, needle-like teeth like those seen in crocodiles and other piscivores [fish-eaters]).
Marsupials (with short gestation and a long period of lactation) dominate the modern fauna of Australia (kangaroos, koalas, wombats, etc.) and are common in South America (opossums) as well. The primitive marsupial dental formula is 5-1-3-4 / 4-1-3-4. The living monotremes (egg-laying mammals, the platypus and echidna) are toothless as adults.
Carnivores vs. Herbivores
Just behind the maxilla is the zygomatic arch (common to mammals and their cynodont forbears) composed of outward projections of the jugal and squamosal bones. In addition to allowing extra room for the passage of muscles to work the lower jaw, the arch also allows for differentiation of separate muscle groups to be involved in biting and chewing. Masseters attach from the dentary (specifically, the masseteric fossa labelled here) to the zygomatic arch and onto the maxilla in front of the arch, providing crushing force. The temporalis attaches from the dentary (specifically, the coronoid process labelled here) to the side of the braincase, providing torque about the axis of jaw articulation.
In comparing the skulls of carnivores and herbivores, it can be seen that the shearing force of the temporalis is somewhat more important to carnivores, which have more room on the braincase (this is not unrelated to carnivoran intelligence) and commonly develop a sagittal crest (running from posterior to anterior on the skull) providing yet additional room for temporalis attachment. However, a few large herbivores, such as gorillas, have sagittal crests as well. The jaw joint in carnivores tends to lie within the plane of tooth occlusion; an arrangement that further emphasizes shearing (as in a pair of scissors).
In herbivores, the crushing force of the masseters is relatively more important than is shearing. The jaw joint is generally well above the plane of tooth occlusion, allowing extra room for masseteric attachment on the dentary and causing the rotation of the lower jaw to be translated into straight-ahead crushing force between the teeth of the upper and lower jaws.
Major Placental Orders and Dental Specializations
Order Carnivora includes carnivores, omnivores, and even a few primarily herbivorous species, such as the giant panda, Important teeth for carnivorans are the large, slightly recurved canine, used to dispatch prey, and the carnassial complex (last upper premolar and first lower molar), used to rend meat from bone and slice it into digestible pieces. Incisors are retained by carnivorans; the third incisor is commonly large, sharp, and caniniform (canine-like). Dogs have molar teeth behind the carnassial for crushing bones, but cats (including this tiger) have only a greatly reduced, functionless molar behind the carnassial in the upper jaw. Cats will strip bones clean but will not crush them to get the marrow inside. Omnivores, such as bears and raccoons, have developed blunt, molar-like carnassials (third tooth from the back of the mouth in each case). Carnassials are a key adaptation for terrestrial vertebrate predation; all other placental orders are primarily herbivores, insectivores, or aquatic.
Order Artiodactyla (even-toed ungulates) includes most living large herbivores. The order contains pigs, hippopotami, and camels, but the most numerous artiodactyls are the ruminants: bovids (cattle, sheep, goats, antelopes), deer, giraffe, and the pronghorn (American "antelope"). In addition to their stomach, ruminants have three large stomach-like pouches within the esophagus. The first of these is the rumen, a large "fermentation vat" where bacteria break down the cellulose in plant matter. This matter is then returned to the mouth in the form of a cud for further chewing, later swallowing, and more complete digestion in the other esophagal pouches and the true stomach. Ruminants lack upper incisors and generally upper canines as well. The lower incisors and a small incisorized lower canine crop vegetation by striking a hard keratin pad in the upper jaw. Molars and the molarized premolars have vertical ridges of enamel that allow for grazing (eating coarse, abrasive grass). The ridges run front-to-back; ruminants chew in a side-to-side manner. The enamel ridges ensure that this most resistant part of the tooth (and not softer dentine) is brought into contact with the grass. In typical mammalian molars, the enamel is restricted to a thin layer on the outside of the tooth. Herbivores without enamel ridges are typically browsers (feeding on softer vegetation: fruits, leaves, nuts, tubers). Pigs are examples of nonruminant artiodactyls, in which the canines are retained but splayed to the side for defense. Pig molars, like those of other omnivores such as bears and humans, contain broad, blunt cusps and basins, with enamel restricted to the outer surface.
Order Perissodactyla (odd-toed ungulates) include many extinct lineages and the living horses, rhinoceroses, and tapirs, which are all large herbivores. In horses, which are grazers, all incisors are retained, and a canine is retained in males. The molars and molarized premolars have vertical ridges of enamel that are even more complexly folded than in ruminant artiodactyls.
Order Rodentia contains the greatest number of known species among placental orders, including rats, mice, squirrels, beavers, and porcupines. In all rodents, a single large pair of incisors is present in both upper and lower jaws, with left and right incisors pressed together to form a sharp chisel. The chisel remains sharp not only due to the continued growth of the incisors throughout the life of the rodent, but also through the presence of enamel only on the anterior (front) side. Softer dentine behind the enamel is worn away more rapidly, leaving a thin, sharp, leading edge of enamel. Canines are absent in rodents, and a lenghty diastema (gap in the tooth row) exists between the incisor and cheeck teeth in both jaws. Three molars are retained, commonly with well developed, side-to-side enamel ridges (rodent chewing is back-and-forth). In some cases, including squirrels, a molarized premolar or two is present as well.
Order Lagomorpha, mostly rabbits, are small herbivores with slightly less specialized dentition as compared to rodents. The fenestrated ("windowed") nature of the maxilla in lagomorphs is unique and diagnostic. Two pair of upper incisors are present, the tiny second pair directly behind the larger first pair. Canines are absent, and a large diastema is present. Molars and premolars are not as well adapted as those of many rodents to a diet of coarse vegetation. As a result, rabbits will commonly consume their fecal pellets to obtain the maximum nutritive benefit from their food (which is not adequately digested in the first cycle of consumption).
Shrews (order Insectivora, and most bats, order Chiroptera) exhibit simple, pointed teeth in the front of the mouth for catching their insect prey. Their molars have sharp cusps for piercing the exoskeleton. Note: many larger insectivorous feeders (such as anteaters, the pangolin, and the aardvark, all in separate orders) lack incisors and catch insects with their sticky tongues.
Stance is variable among mammals, the primitive condition being plantigrade, with the whole of the foot, including tarsals, in contact with the ground surface. Plantigrade stance offers the slowest terrestrial locomotion, but the greatest flexibility. It is commonly retained in burrowers (such as moles) and arboreal (tree-dwelling) mammals (such as primates) as well as graviportal (large, heavy) mammals. Greater running speed is obtainable through digitigrade stance, in which only the phalanges of the digits (toes and fingers) make contact with the ground. Even greater running speed can be obtained through unguligrade stance, in which one or a few ungual (last) phalanges, fused to form a hoof, make contact with the ground. Despite a need for speed, carnivores use their claws to dispatch their prey and thus employ digitigrade stance (there are no living hoofed carnivores).