Living birds (Aves) have bodies dramatically modified from the ancestral reptilian condition. The avian pelvis in particular experienced dramatic changes during the transition from early archosaurs to living birds. This stepwise transformation is well documented by an excellent fossil record; however, the ontogenetic alterations that underly it are less well-understood. We used embryological imaging techniques to examine the morphogenesis of avian pelvic tissues in three dimensions, allowing direct comparison with the fossil record. Many ancestral dinosaurian features (e.g., forward-facing pubis, short ilium, pubic ‘boot’) are transiently present in the early morphogenesis of birds and arrive at their typical ‘avian’ form after transitioning through a prenatal developmental sequence that mirrors phylogeny. We quantitatively demonstrate that avian pelvic ontogeny closely parallels the dinosaur-to-avian transition and provide evidence for an evolutionary module within the pelvis that is conserved across Archosauria. The presence of ancestral states in avian embryos may stem from this conserved modular relationship. These are further indications that the avian pelvis evolved via terminal addition—a mechanism whereby ancestral states shift to derived states during late development, resulting in retention of ancestral character states. This phenotypic modularity suggests a previously unrecognized mechanism for the promotion of terminal addition, hinting that the retention of ancestral states in development may be common across evolutionary transitions.