Dentition

Probably more than any other part of the skeleton, the dentition of fossil mammals plays a critical role in taxonomy, assessment of phylogenetic position, and interpretation of behavior (primarily diet, but also such activities as grooming, gnawing, or even digging). In part, this reflects the durability of teeth (enamel, the hard outer layer of most mammal teeth, is the hardest substance in the body), which accounts for why they are generally more common than other skeletal remains. But it is also because the dentition usually exhibits species-specific differences, not so readily distinguished in other parts of the skeleton, that can often be detected even in individual teeth. Especially useful general accounts of the dentition in vertebrates generally, and mammals in particular, include Gregory (1922), Peyer (1968), and Hillson (1986).

One of the characteristics of mammals (inherited from their nonmammalian cynodont ancestors) is the regional differentiation of the dentition into incisors (I), canines (C), premolars (P), and molars (M), known as heterodonty (Fig. 2.1). The postcanine teeth are collectively called cheek teeth. Incisors are typically involved in procuring and ingesting food. Canines usually function for stabbing or holding prey, for aggression, or for display. Premolars hold or prepare food for the molars, which shear, crush, and grind the food. In most mammals, the antemolar teeth are replaced once during life, a diagnostic mammalian condition called diphyodonty. The first set of teeth, the deciduous or milk teeth (indicated by "d," such as dP4), erupts more or less in sequence from front to back, followed by the molars, which are actually part of the first generation of teeth. Most of the antemolar teeth are sequentially replaced by permanent teeth after some or all of the molars are in place.

The number of teeth present in each part of the dentition varies among mammals and is an important taxonomic characteristic. It is expressed in shorthand by the dental formula, I.C.P.M/I.C.P.M, which specifies the number of teeth in each quadrant, that is, on each side, above and below. Thus the dental formula of primitive extant placentals is 3.1.4.3/3.1.4.3 X 2 = 44; this translates to three incisors, one canine, four premolars, and three molars in each upper and each lower quadrant, for a total of 44 teeth. These teeth are conventionally identified as I1-3C1P1-4M1-3/I13C1P1_4M1_3. The dental formula of primitive marsupials (e.g., the opossum Didelphis) is 5.1.3.4/4.1.3.4 X 2 = 50. Generalized marsupials typically differ from primitive living placentals, then, in having more incisors (and more of them in the upper jaw than in the lower), one more molar, and one less premolar. The postcanine teeth are conventionally identified as P1-3, M1-4 (in both upper and lower jaws), although some accounts use a different numbering system. Obviously the number of teeth has varied considerably among mammals. Some primitive Mesozoic types had more premolars and/ or molars than do most modern species, whereas some living mammals have many homodont (similar) teeth (e.g., porpoises) and others have no teeth at all (e.g., anteaters). As a general rule, however, no mammal has more than one canine, and living marsupials and placentals rarely increase the number of premolars and molars beyond the primitive state.

Fossil evidence suggests, however, that the primitive eutherian dental formula was 5.1.5.3/4.1.5.3. To achieve the dental formula common in the most generalized living placentals, it is probable that incisors were lost from the back of the series and a premolar was lost from the middle (P3), very early in the history of placentals (McKenna, 1975a; Novacek, 1986b). This hypothesis suggests that the four premolars present in most primitive extant placentals could be dP1.P2.P4.P5, although the last two are conventionally identified as P3 and P4. This convention has been adopted because there is little direct evidence of how the reduction to four premolars took place, and whether it represents a single event in eutherians or occurred multiple times. Whatever position was lost, there is general agreement that the remaining teeth are probably homologous, and they continue to be almost universally identified as (d)P1.P2.P3.P4. This practice is also followed here, acknowledging that it is an assumption. Nearly all Cenozoic placentals have no more than three incisors and four premolars, hence a dental formula of 3.1.4.3/3.1.4.3 may be considered the primitive condition among Paleocene and Eocene mammals.

Although the dental formula is an important characteristic of mammals, equally or more important are the homologies of the teeth. For example, an enlarged central incisor evolved in many clades of mammals, but the tooth involved is not always homologous. In some cases it is I1, whereas in others it is I2 or a retained deciduous I2. When all the incisors are present, homologies are easily determined, but deciphering true homologies when the number of incisors is reduced to one or two requires developmental or evolutionary evidence. Unusually specialized premolars have also arisen independently in various lineages, as demonstrated by their occurrence at different tooth loci in different clades.

Several positional and other descriptive terms are commonly used when describing teeth. Buccal refers to the external or lateral surface, which faces the cheek (=labial, facing the lips, especially near the front of the jaw); lingual denotes the internal or medial surface, toward the tongue. The anterior end of the toothrow is also called mesial, the posterior end distal. Tooth length is measured mesiodistally whereas width is measured transversely (buccolingually). Teeth are implanted in the alveoli (sockets) of the jaw by the root(s); the neck is approximately at the gum line, and most of what is exposed is the crown, usually covered by enamel. Enamel is an extremely hard, largely inorganic substance composed of hydroxyapatite crystallites. The underlying dentine is an avascular tissue consisting of hydroxyapatite, collagen, and water, and is softer than the enamel. Cemen-tum is a bonelike tissue usually found covering the roots of teeth, but it is also found in the crowns of the teeth of many herbivores. Teeth with relatively low crowns are characterized as brachydont, whereas those with high crowns (higher than the roots, or higher than the length or width; Simpson, 1970c) are hypsodont. Teeth that grow continuously throughout life and never form roots are called hypselo-dont (e.g., Simpson, 1970c) or euhypsodont (Mones, 1982); these are essentially equivalent terms. The most obvious examples of hypselodont teeth are the incisors of rodents, but the condition has evolved independently in multiple lineages, and at different tooth loci.

Incisors may be small to very large and ever-growing, and the crowns vary from pointed to broad and spatulate, chisel-like, bilobed, or multicuspate; upper incisors tend to be larger than lowers. Canines are usually relatively large, conical teeth, but in some forms they are reduced or lost, whereas in others they are huge and saberlike or gliriform (like rodent incisors) and may be ever-growing. Both incisors and canines are almost always single-rooted. Premolars may be simple with one main cusp, or more complex, sometimes closely resembling molars. They usually increase in size and complexity posteriorly. In some mammals the posterior premolars are greatly enlarged, and in this case they may be swollen or bladelike. Despite these interesting variations in antemolar teeth, which are sometimes diagnostic of particular taxa, the crown morphology of molars is particularly distinctive and almost always carries substantial weight for taxonomy, phylogenetic assessment, and dietary inference.

Extant mammals, as well as most of the fossil groups dealt with in this book, have molars derived from the basic tribosphenic condition that evolved in the Mesozoic ancestors

Marsupial Molar Nomenclature
Fig. 2.2. Structure of tribosphenic molars (anterior to the left): (A) left upper; (B) left lower. (From Bown and Kraus, 1979.)

of marsupials and placentals. Some more primitive groups, discussed in the chapter on Mesozoic radiations (Chapter 4), were not yet tribosphenic. Here I focus on the structure of tribosphenic molars (Fig. 2.2) and defer a discussion of how tribosphenic molars evolved until Chapter 4. In general, tri-bosphenic molars have divided roots, two for each lower molar, located below the trigonid and talonid, and three for each upper, under each of the three main cusps. Generalized tribosphenic upper molars are transversely wider than they are long, and the three main cusps form a triangle with two cusps arranged buccally and one lingually. On the buccal side, the paracone is anterior, the metacone posterior; the lingual cusp is the protocone. Between the para-cone-metacone and the buccal margin of the tooth is the stylar shelf, on which smaller cusps may be present, such as the parastyle, stylocone, mesostyle, and metastyle. Conules (paraconule and metaconule) are commonly present between the paracone or metacone and the protocone. A hypocone is frequently developed posterolingually, especially in herbivores, and may result in a quadrate upper molar. It is generally assumed that these cusps on adjacent teeth of an individual are serially homologous.

Mammalian cusp nomenclature is largely topographic: although it is probable that the three main cusps are almost always homologous across species, this is not true for the hypocone, mesostyle, and various other smaller cusps, which have demonstrably arisen multiple times independently (Van Valen, 1994a; Hunter and Jernvall, 1995). Indeed, developmental evidence has shown that relatively small changes during tooth formation can result in substantial changes in the size or number of small cusps (Jernvall, 2000). Although this instability helps to explain the frequent appearance of new cusps in different clades, it also means that variations in small cusps may have little phylogenetic significance, which should be remembered when using minor variations in cusp pattern as evidence for or against relationship.

Cusps are often joined by crests, and in some teeth crests predominate. The centrocrista is the crest between the paracone and metacone in generalized molars. When this crest is better developed, or when it links the centrocrista to the parastyle, metastyle, or mesostyle, it is called the ectoloph. Other crests are usually named with respect to the cusps they join. For instance, the preprotocrista and postproto-crista run anteriorly and posteriorly to the protocone, from the paracone or paraconule, and metacone or metaconule, respectively. Parallel transverse crests joining the paracone to the protocone and the metacone to the hypocone are the protoloph and metaloph, respectively. They are particularly well developed in herbivorous forms. A low (basal) shelf on any margin of the tooth is a cingulum.

Tribosphenic lower molars are longer than wide and consist of a trigonid anteriorly and a talonid posteriorly. As its name implies, the trigonid consists of three cusps, but in tribosphenic molars these cusps are arranged in a triangle that is inverted compared to that of the upper molars. Lower molar features end in the suffix -id; the two lingual cusps of the trigonid are the paraconid and metaconid, and the buccal cusp is the protoconid. The trigonid is almost always taller than the talonid. When it first evolved, the talonid was little more than a short "heel" with a single cusp, but in tribosphenic molars it usually has two or three cusps, the en-toconid lingually hypoconid buccally, and the hypoconulid in between. As in the upper molars, crests commonly join various cusps: the paracristid (or paralophid) between para-conid and protoconid, the protocristid (sometimes called the metacristid or metalophid in certain mammals) between protoconid and metaconid, and the postcristid (=hypolo-phid) between hypoconid, hypoconulid, and entoconid. The cristid obliqua is a crest that runs from the hypoconid anteriorly to the back of the trigonid, often oriented obliquely toward the metaconid. An entocristid may be present mesial to the entoconid. The crests of the talonid usually encircle a depression of variable size, forming a talonid basin that occludes against the protocone. Additional talonid cusps are sometimes present, including a metastylid behind the metaconid (really an accessory trigonid cusp), an entoconulid anterior to the entoconid, or a mesoconid on the cristid obliqua. A basal cingulum (cingulid) is often present on the buccal side, sometimes extending to the mesial or distal ends but almost never lingually.

Molars with sharp or bladelike cusps or crests are described as secodont or sectorial; specialized sectorial teeth called carnassials are characteristic of carnivorous mammals. Teeth with low, rounded cusps are bunodont. An occlusal pattern dominated by crescentic crests with a mesio-distal long axis is selenodont, whereas a pattern characterized by transverse ridges is lophodont. These and other

Bunodont Molar

Fig. 2.3. Comparison of various dentitions and molar types (not to scale): (A) brachydont and bunodont (Ellipsodon); (B) hypsodont (Notostylops); (C) sectorial (Batodonoides); (D) lophodont (Triplopus); (E) selenodont (Poabromylus); (F) bunoselenodont and dilambdodont (Eotitanops). (A from Matthew, 1937; B from Simpson, 1948; C from Bloch et al., 1998; D from Radinsky, 1967a; E from Wilson, 1974; F from Osborn, 1929.)

Fig. 2.3. Comparison of various dentitions and molar types (not to scale): (A) brachydont and bunodont (Ellipsodon); (B) hypsodont (Notostylops); (C) sectorial (Batodonoides); (D) lophodont (Triplopus); (E) selenodont (Poabromylus); (F) bunoselenodont and dilambdodont (Eotitanops). (A from Matthew, 1937; B from Simpson, 1948; C from Bloch et al., 1998; D from Radinsky, 1967a; E from Wilson, 1974; F from Osborn, 1929.)

modifications of the primitive tribosphenic pattern have enabled mammals to adapt for diverse diets (Fig. 2.3) and are one of the keys to their success.

The microscopic structure of the enamel also provides information relevant to phylogeny and function (e.g., Koe-nigswald and Clemens, 1992; Koenigswald, 1997a,b). Enamel is composed of long, needlelike crystallites of carbonate hy-droxyapatite. In the most primitive Mesozoic mammals, the crystallites are parallel and radiate from the enamel-dentine junction to the surface. This relatively simple type of enamel is called aprismatic or nonprismatic enamel. In most mam mals, however, the crystallites combine into bundles called prisms, each of which is surrounded by a prism sheath, also composed of crystallites. Although there is considerable variation in the morphology of the prisms and their sheaths, the significance of this variation is unknown. Groups of prisms are often arranged in the same orientation. In some cases all the prisms are oriented similarly and are either arranged radially from the enamel-dentine junction (radial enamel) or bend together (tangential enamel). In most eutherian mammals that weigh more than a few kilograms, the enamel consists of decussating groups of prisms that change orientation together, known as Hunter-Schreger bands (HSB; see Fig. 15.14). This specialized arrangement of prisms is thought to help strengthen the enamel, but the functional significance of different types of HSB is poorly understood. Although some patterns of HSB appear to be phylogenetically significant, the extent of homoplasy can make it difficult to distinguish them from functionally related patterns. Enamel microstructure is particularly important in rodents and is further discussed in Chapter 15.

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Responses

  • Cosma
    Why is dentition important in mammal taxonomoy?
    8 years ago
  • joel ritchie
    Why dental formula differ among mammal?
    2 years ago
  • santina
    Which animal has dentition formular of 44?
    2 years ago
  • RUBY NOAKES
    Why dental formula differs among mammals?
    2 years ago

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