Data from: Enlargement of sternum traits facilitated the evolution of powered flight in birds

An enlarged sternum with a prominent keel is a central feature of the
flight apparatus of modern birds. However, sterna of near-bird dinosaurs
(Pennaraptora) and early avialans are either significantly different from
those of living birds or absent altogether, raising questions on how
specialized sternal structures evolved in birds and how they are related
to function. This remains poorly understood due to the fragmentary nature
of the fossil record, and the challenges in inferring form and function
from crushed fossils. We employ ancestral character estimations to trace
sternal trait acquisition through the bird stem group, and multivariate
phylogenetic regressions to analyze relationships between sternum
morphology, body mass, and flight capabilities. We find that sternum
evolution was episodic: basal members of Pennaraptora had proportionally
small sterna, which became larger and more craniocaudally elongated in
Avialae. This enlargement precedes the appearance of a midline ridge, a
possible precursor of the sternal keel, in Pygostylia. Sternum size
increased again in crownward Ornithuromorpha, alongside a fully formed
sternal keel and enlarged caudal projections, both critical areas of
flight muscle attachment. Sternal experimentation in relation to flight
characteristics occurs multiple times throughout Pennaraptora, including
within Paraves and Enantiornithes, indicating that powered flight may have
evolved multiple times before proliferating in crown-group birds.