Data For: 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 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.