FIG. 1 in Pennaraptoran Theropod Dinosaurs Past Progress And New Frontiers

FIG. 1. Macroevolution and microevolution of avian beak from toothed ancestor's snout. The avian beak is inferred to have evolved from the snout of toothed nonavian dinosaurian ancestors through alterations to the signaling pathways involving bone morphogenetic proteins (BMPs) and other molecules. BMP, ALX homeobox protein 1 (ALX1), calcium-modulated protein (CALM) and high-mobility group AT-hook 2 (HMGA2) are all important molecules that regulate the diversity of beak shapes. Not to scale.