Fig. 2 in Variation in premaxillary tooth count and a developmental abnormality in a tyrannosaurid dinosaur

Fig. 2. Diagrams that represent developmental models to explain regulation of tooth count. A. The inhibitory cascade model (Kavanagh et al. 2007) in which a developing tooth successively inhibits subsequent ones in a hypothetical mammal. B. The zone of inhibition model (Osborn 1971, 1978, 1998; Kulesa et al. 1996) in which a tooth develops outside the sphere of inhibition around an already developing tooth in a hypothetical reptile. Roman numerals show the sequence of tooth site formation, whereas Arabic numerals indicate the spatial order of tooth positions. The inhibitory cascade model explains regulation of tooth size, whereas the zone of inhibition model deals with spatial regulation of tooth positions. The pattern of tooth formation is spatially and temporarily sequential in the inhibitory cascade model (applicable to mammals), but does not have to be spatially sequential in the zone of inhibition model (applicable to mammals and non−mammals alike).