The evolution of the mammalian dental complex was accompanied by a radical change in the role of dentition itself. Teeth became forward projections of the mammal’s digestive system, and were characterized by three distinctive features — occlusion, heterodonty, and the emergence of at most two functional generations of teeth. First, occlusion means that the teeth of the upper and lower jaws meet precisely together to permit mastication. Second, heterodonty implies that mammals have specialized teeth for performing different functions (incisors, canines, premolars and molars), and thirdly, teeth were no longer continuously replaceable as in the case of non-mammalian vertebrates, but limited to two sets — the milk or deciduous set replaced by the permanent set. Additionally, the dental needs of mammals gave rise to changes in hearing and jaw structure. Unlike non- mammalian vertebrates such as reptiles, mammals have one bone in each jaw instead of several, and a large false palate that permits simultaneous breathing and chewing instead of no false palate. Tempro- mandibular joints on each side of the jaw work in opposition to each other to make mastication possible.
This specialized dentition serves a major piece in the mammalian biological pattern because it enables mammals with their high metabolic rates to meet their energy needs by absorbing the optimal amount of nutrients from the ingested food. Processing food before swallowing it increases digestive efficiency. Mammalian dentition, well preserved in the fossil record, can also help us trace the adaptive radiations of mammals through evolution. The diversity of dentitions in mammals has enabled them to adopt diverse dietary patterns and consequently occupy a wide range of environmental niches in our world. Mammals inhabit all kinds of habitats — savannahs, deserts, tropical forests, temperate woodlands, polar regions and the aquatic world. These mammals might eat flesh (carnivorous), plant matter (herbivorous) or a combination of both (omnivorous).
The distinctiveness of the mammalian dental complex can be better understood by glancing at the dentition of the earliest vertebrates known to us through fossil evidence. The Agnathans or jawless fish (the ancestors of present day hagfish and lampreys) didn’t have teeth. Instead, they fed by means of a filter mechanism in which water was drawn through an open feeding tube and passed through a gill-like structure that strained off food particles which were then ingested. In contrast to the mammalian heterodont dentition, this made food choice and the adaptations available in turn, limited. The gill openings, supported by cartilage, were later adapted to a respiratory use through evolution, and the cartilage evolved into a jaw structure. Scales around the mouth became teeth as these early vertebrates made the transition from filter feeding to grasping food with jaws. The Placoderms which became extinct during the Paleozoic, possessed the movable jaw in which the dermal armor plates that covered the head end of the organism folded into the mouth forming a hard crushing surface made of dentin. The core of the teeth of all subsequent vertebrates continues to be made of dentin. At later stages of evolution — fish, amphibians and most reptiles, possessed several continuously replaceable teeth made of dentin that were covered by a layer of dense, hard tissue called enamel. These organisms didn’t chew, but grasped and tore food to swallow it.
Subsequently, the precursors to mammals, cynodonts, featured well differentiated teeth – incisors, canines, and multicusped cheek teeth (premolars and molars) that occluded precisely to allow for different foods to be processed differently. With the evolution of the cynodonts, the previously single jaw adductor muscle – separated into two muscles — the precursors of the masseteric and temporalis muscles of mammals. Gradually the mammalian dental complex evolved the distinctive features discussed earlier.