Data from: Pelvis morphology suggests that early Mesozoic birds were too heavy to contact incubate their eggs

Numerous new fossils have driven an interest in reproduction of early
birds but direct evidence remains elusive. No Mesozoic avian eggs can be
unambiguously assigned to a species, which hampers our understanding of
the evolution of contact incubation, which is a defining feature of extant
birds. Compared to living species eggs of Mesozoic birds are relatively
small, but whether the eggs of Mesozoic birds could actually have borne
the weight of a breeding adult has not yet been investigated. We estimated
maximal egg breadth for a range of Mesozoic avian taxa from the width of
the pelvic canal defined by the pubic symphysis. Known elongation ratios
of Mesozoic bird eggs allowed us to predict egg mass and hence the load
mass an egg could endure before cracking. These values were compared to
the predicted body masses of the adult birds based on skeletal remains.
Based on 21 fossil species, we show that for non-ornithothoracine birds
body mass was 130% of the load mass of the eggs. For Enantiornithes body
mass and egg load mass were comparable to extant birds, but some early
Cretaceous ornithuromorphs were 110% heavier than their eggs could
support. Our indirect approach provides the best evidence yet that early
birds could not have sat on their eggs without running the risk of causing
damage. We suggest that contact incubation evolved comparatively late in
birds.