Mammal Teeth: Origin, Evolution, and Diversity

Conclusions

Conclusions What is life? Question ever asked—never answered—mystery from all time, as far as ever from solution. Yet without mouth and teeth, this problem would arrive at a negative solution since life would become extinct and in this way the mystery becomes negatively solved. — mosely, 1862 WHILE EVEN THE DEEPEST understanding of mammalian dentitions will not unravel all the mysteries of the universe, without teeth we would not be here to ponder them. Teeth are fundamental to the mammalian way of life and to the adaptive diversity of our biological class. Moreover , since teeth are the most common elements preserved in most fossil mammalian assemblages, they offer us the best prospect for comparing variation of form in the past and present and for studying adaptation over time. ADAPTIVE RADIATION OF THE MAMMALS Mammalia is an extraordinarily diverse class of animals, from the bumblebee bat, at about 1.7 g, to the blue whale, at up to more than 170 million g! Mammals swim, fly, climb, burrow, walk, and run though an incredible variety of habitats ranging from the Arctic to Antarctic pack ice, from the ocean’s depths to high mountain peaks, and from open desert to dense rainforest. Many lineages have undergone rapid morphological and ecological diversification over the course of evolutionary history (Schluter 2000). Life finds a way to spread out when circumstances permit. Environmental changes resulting from bolide impacts, tectonics, and regional or local factors have all led to new or vacated niches into which mammalian lineages have evolved and radiated. Evolutionary novelties have led to new adaptive zones, the extent of which depended on how distinctive the new adaptive type was and the variety of opportunities it provided (G. G. Simpson 1953). Important novelties included the first mineralized teeth, the earliest mammalian masticatory system, and the development of the tribosphenic molar. A true appreciation for the adaptive radiation of mammalian tooth form requires integration of the many different aspects of mammalian odontology presented in this book (Fig. C.1). Endothermy can be viewed as the trigger, opening new niches with new food opportunities but at the same time generating intense selective pressure for increased dietary efficiency. Abiotic and biotic environmental changes throughout the Mesozoic and especially the Cenozoic changed food options, and competition almost certainly led to niche differentiation. The resulting dietary diversification in part explains the radiation of mammal teeth, because digestive efficiency depends on the match between tooth form and food properties. On the other side of the equation, the drive to increase dietary efficiency in early endotherms 222 m a m m a l t e e t h Mammals have many options to choose from as they “belly up” to the sneeze guard at the biospheric buffet (see chapter 3). Carbohydrates, fats, and proteins can all yield energy, and mammals have developed innovative ways to wring calories and other nutrients from a very broad range of foods. Some mammals are herbivores and prefer graze or browse. Others are faunivores, with prey ranging from insects to other mammals. Some are dietary specialists, whereas others are opportunistic omnivores. The diversity of sizes, shapes, and underlying structures of mammal teeth and other feeding adaptations make this all possible. HOW DO TEETH WORK? While mammals have many food choices, living things typically do not want to be eaten. Organisms can mount substantial defenses, both chemical and mechanical, to prevent consumption and assimilation. Mechanical defenses against fracture include both stress-limited ones, which harden tissues against the initiation of cracks, and displacementlimited ones, which toughen them against the propagation of cracks (see chapter 3). These can make it difficult for a mammal to ingest and digest, and teeth evolve to meet these challenges to energy access (see chapter 4). Anterior teeth are typically used in food acquisition. Mammalian incisors and canines come in many sizes and shapes because foods differ in their external properties and other defenses against ingestion. The incisors and canines of faunivores vary with prey sizes and killing techniques, whereas those of herbivores relate to such things as food selectivity, size, and casing. Form-function relationships are further complicated by the roles that front teeth play in other activities, such as combat, digging, and grooming. Cheek teeth, on the other hand, have traditionally been viewed as guides for chewing, as opposing premolars and molars with steeper surfaces typically have steeper approaches to each other during mastication. And in many cases, crown shape likely limits motion as the teeth move into and out...