The Developing Bird Pelvis Passes Through Ancestral Archosaurian and Dinosaurian Conditions.

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 hypothesized 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. If this is the case, then developing bird embryos should possess the same character states as taxa along the avian stem lineage (i.e., non-avian dinosaurs and their close relatives). We used embryological imaging techniques (modified CLARITY protocol) to examine the early morphogenesis of avian pelvic tissues (Japanese Quail, Common Parakeet, Chilean Tinamou), allowing direct comparison with the fossil record. We found that many ancestral dinosaurian features (e.g., forward-facing pubis, short ilium, pubic 'boot') are transiently present in the early morphogenesis ('pre-cartilage') of birds and arrive at their typical 'avian' form after transitioning through a prenatal developmental sequence that mirrors phylogeny. Using 3D geometric morphometrics, we quantitatively demonstrate that avian pelvic ontogeny closely parallels the dinosaur-to-bird transition and provide evidence for an evolutionary module within the pelvis that is conserved across Archosauria. The same pattern was observed in a paleognath, a galloanseriform, and a neoavian, strongly suggesting that this is the ancestral avian condition. These indicate that the avian pelvis evolved via terminal addition, and that the presence of ancestral states in avian embryos may stem from these conserved modular relationships. This modularity suggests a previously unrecognized mechanism for the retention of ancestral states in development, hinting that terminal addition may be common across evolutionary transitions.