SOME FUNCTIONAL ASPECTS OF MOLAR EVOLUTION

Theories of molar evolution have long been dominated by the cusp concept. In the 19th century the Concrescence Theory compared each cusp with a simple "reptilian cone" (see the review by Neuville, 1932). Cope (1873) ascribed the origin of the multicuspid molars to "anteroposterior repetitive acceleration" and "lateral repetition," by which he seems to imply that the process of development of a simple tooth is repeated in each cusp of the complex tooth. The idea that the molar is made up of cusps is deeply embedded in the Tritubercular Theory, even though the cingulum was recognised as an additional element. What sort of individuality does a cusp have? Strictly, a cusp can be recognised only at its tip, which stands on all sides higher than the adjacent tooth surface. More basally, it merges with adjacent cusps, and though in some cases grooves can be used to delimit cuspal areas, in others the surfaces of neighbouring cusps are continuous and boundary lines must be arbitrary. Moreover, when teeth of related animals are compared cusps seem to appear from (or disappear into) the surfaces of other cusps, most often from ridges running down their slopes, or from cingula. The same is observed when comparison is made between adjacent teeth in a dentition. The close relationship between cusps and ridges is shown also in ontogeny. The cusp appears as a separate centre of calcification, but sooner or later it joins with its neighbours: the shallower the intervening valley, the sooner the junction is made (Butler, 1956). Ridges develop in the same way as cusps (Butler, 1967), the only difference being that, as there is no intervening valley, their calcification is joined to that of a cusp from the beginning. When it is realised that the shape of the tooth surface is a result of the folding of a sheet of epithelium (the inner dental epithelium of the enamel organ), probably under the influence of the adjacent mesenchyme (Kollar and Baird, 1968), the difficulty of defining a cusp becomes understandable. It is a remarkable fact that tooth patterns, though they show much individual variation in minor detail, are sufficiently constant in their main features to provide useful taxonomic data. The manner of folding of the dental epithelium is evidently under genetic, and ultimately under selective control. The relation between genotype and tooth pattern is very complex (e.g., Griineberg, 1951, 1965) and much remains to be discovered about the genetic control of tooth development. Nevertheless, it is reasonable to suppose that those features of the pattern that are constant within living species or have persisted over long periods of time in mammalian evolution have been maintained by stabilizing selection because they have survival value. In other words, details of the molar pattern have functional significance. That cusps have functions was recognised by the trituberculists. Ryder (1878) and Cope (1889) related the shape and position of cusps to the direction of chewing movements and Osborn (1888) made use of occlusal diagrams to explain the origin of the tritubercular molar. It was believed that in primitive mammals jaw movements were vertical (orthal) due to a hinge action of the mandibular joint, and that occlusal relations could be adequately interpreted by superimposing drawings of upper and lower teeth seen in crown view. Many occlusal diagrams of this type were produced by Gregory (e.g., 1920a, 1920b,