Monophyodonty, the presence of a single tooth generation, also characterizes cetaceans. Homodonty and polydonty are acquired in cetacean evolution gradually, and even in mysticete evolution polydonty precedes their complete loss of teeth in modern forms.

Cetaceans use their teeth to grab and hold, but not to chew their food. It has been proposed that these profound changes in dentition are related to the absence of mastication in cetaceans ( Werth, 2000). The absence of precise occlusion in cetaceans is associated with the evolution of both homodonty and polydonty ( Fig. 1). The only large radiation of mammals to lose precise dental occlusion is the order Cetacea. Its teeth are homodont, unicuspid and did not occlude precisely. The giant armadillo ( Priodontes maximus) exhibits polydonty, or a significantly increased tooth number (approximately 20 per jaw quadrant) beyond the basal condition for placental mammals (3 incisors, one canine, 4 premolars, and 3 molars per quadrant, 3.1.4.3). Interestingly, up to 10% of these seals also have supernumerary teeth a much higher rate of anomalies than most mammals ( Cruwys & Friday, 2006). The morphology of their post-canine teeth reflect this shift away from precise occlusion and rhythmic mastication by showing a reduced number of mesial-distally arranged tooth cusps lacking tightly integrated occlusal surfaces ( Miller et al., 2007 Jernvall, 2000). For example, in grey seals ( Halichoerus grypus) the postcanine teeth act primarily in holding and puncturing prey ( Adam & Berta, 2002). While most mammals maintain some characteristics of this pleisomorphic dentition, a few species, such as phocid seals, armadillos, and cetaceans, have lost several characteristic mammalian dental traits along with precise dental occlusion. These changes in dental patterns contributed significantly to the success of mammals and have been maintained in most species since the origin of mammals 220 million years ago ( Luo, 2007). The evolution of multicuspid teeth and stable tooth number in the ancestors of mammals allowed for precise occlusion and hence the maintenance of a high metabolic rate through efficient food processing during mastication. In contrast, mammals have diverged from this state: their pleisomorphic dental pattern consists of four morphologically-distinct tooth classes (incisors, canine, premolars and molars) per jaw quadrant and a limited number of teeth erupting during a lifetime ( Gregory, 1922). Most non-mammalian vertebrates have dentitions with simple tooth crowns, a single morphological tooth class (homodonty), and varying numbers of teeth per jaw ( Gregory, 1922). Such major developmental changes drive morphological evolution and are correlated with major shifts in diet and food processing during cetacean evolution. We hypothesize that the functional constraints underlying mammalian occlusion have been released in cetaceans, facilitating changes in the genetic control of early dental development.

By contrast, dolphins have lost these regional differences in dental morphology and the Bmp4 domain is extended into the caudal region of the developing jaw.

Here we show that pigs, a cetacean relative with regionalized tooth morphology and complex tooth crowns, retain the typical mammalian gene expression patterns that control early tooth differentiation, expressing Bmp4 in the rostral (mesial, anterior) domain of the jaw, and Fgf8 caudally (distal, posterior). The developmental controls of tooth differentiation and tooth number have been studied in a few mammalian clades, but nothing is known about how these controls differ between cetaceans and mammals that retain functional occlusion. The fossil record documents the critical aspects of occlusal evolution of cetaceans, and allows us to pinpoint the evolutionary timing of the macroevolutionary events leading to their unusual dental morphology among mammals. Evolution toward these specializations began immediately after the time cetaceans transitioned from terrestrial-to-marine environments. Most toothed whales have an increased number of simple crowned teeth that are similar along the tooth row. Cetaceans are one of the few groups of mammals in which precise occlusion has been secondarily lost. These major evolutionary innovations characterize most members of the Class Mammalia. A high metabolic rate is sustainable thanks to efficient food processing and that in turn is facilitated by precise occlusion of the teeth and the acquisition of rhythmic mastication. The evolutionary success of mammals is rooted in their high metabolic rate.

Dolphin teeth