Changes in tooth shape have played a major role in vertebrate evolution with modification of dentition allowing an organism to adapt to new feeding strategies. The current view is that molar teeth evolved from simple conical teeth, similar to canines, by progressive addition of extra "cones" to form progressively complex multicuspid crowns. Mammalian incisors, however, are neither conical nor multicuspid, and their evolution is unclear. We show that hypomorphic mutation of a cell surface receptor, Lrp4, which modulates multiple signaling pathways, produces incisors with grooved enamel surfaces that exhibit the same molecular characteristics as the tips of molar cusps. Mice with a null mutation of Lrp4 develop extra cusps on molars and have incisors that exhibit clear molar-like cusp and root morphologies. Molecular analysis identifies misregulation of Shh and Bmp signaling in the mutant incisors and suggests an uncoupling of the processes of tooth shape determination and morphogenesis. Incisors thus possess a developmentally suppressed, cuspid crown-like morphogenesis program similar to that in molars that is revealed by loss of Lrp4 activity. Several mammalian species naturally possess multicuspid incisors, suggesting that mammals have the capacity to form multicuspid teeth regardless of location in the oral jaw. Localized loss of enamel may thus have been an intermediary step in the evolution of cusps, both of which use Lrp4-mediated signaling.V ertebrates exhibit remarkable diversity in their dentitions, which is a feature of the importance of tooth shape in adaptation to new feeding strategies in evolution. Even quite closely related species of mammals can have different shapes of teeth and thus tooth development provides an excellent model for molecularly based evolutionary developmental biological studies (evo/devo). These tooth evolutional changes took place by the activation or inactivation of gene function, and thus evolutionary lost structures or gene activation/inactivation during evolution are occasionally retained as vestigial structures or latent gene activation/inactivation at embryonic stages.The current view is that all mammalian teeth evolved from simple ancestral teeth with a conical shape not dissimilar to mammalian canines (1). Mammalian heterodont dentitions contain a variety of tooth shapes and most evo/devo studies have focused solely on the molar dentition, with cuspal morphology being used as the main comparative feature between specimens (1). A cusp is a pointed or rounded projection of the tooth that is composed of both enamel and dentin, and the general consensus is that multicuspid teeth (molariform) evolved from conical teeth by progressive addition of extra "cones" (1). Incisors however are a uniquely mammalian tooth type that are neither conical nor multicuspid and their evolutional process is not understood.Among mammalian teeth, murine dentition has been used as a powerful tool for evo/devo studies because of the relative ease of gene manipulation. A major defining feature of R...