JERNVALL, J.; Univ. of Helsinki, Finland: Mammalian teeth: Grinding innovation
The mammalian fossil record, dominated by durable teeth, is relatively complete when compared to other vertebrate groups. Particularly cheek tooth, or molar morphology has played a central role in paleoecological reconstructions. These data have allowed appraisals of the frequency and environmental context in which dental novelties have evolved. A general theme emerging is the parallel acquisition of new morphologies in several mammalian lineages. During the Mesozoic, the therian pattern of triangual upper molars, or tribosphenic teeth, have been implicated to have originated at least twice. More recently, in the first half of the Cenozoic, quadritubercular teeth (the hypocone cusp added to the tribosphenic tooth) evolved over 20 times. During the Cenozoic also several mammalian lineages with quadritubercular teeth evolved multiple shearing crests. The evolution of cusps and crests resulted in increase of occlusal area devoted to crushing and shearing, respectively. Teeth with crushing morphologies indicate typically generalized diet that includes soft plant parts in addition to animals, while shearing crests are best developed in mammals specialized to eat fibrous plant parts. Both the evolution of cusps and crests is usually associated with increased species richness and commonness of individual fossil taxa. In itself the frequent parallel evolution of cusps and crests suggests that developmental changes required for the evolution of, for example, the hypocone have been fairly modest. Based on comparative developmental biology studies, the development of all the cusps involve iterative activation of the same set of genetic pathways. By linking a simple gene-network model with tissue growth, computer based simulations can mimic both the development and evolution of new cusps. Computer simulations suggest additionally that different changes in molecular signaling can result in the evolution of the same cusp, further increasing the likelihood for cusp evolution.
