SITTING ON EGGSHELLS
The earliest birds were too heavy to sit on their eggs without breaking them, according to a new study, raising doubts about the idea that dinosaurs incubated their clutches.
Birds emerged from a lineage of dinosaurs called therapods. Fossilised dinosaur nests containing adult bones suggest that therapods such as Oviraptors incubated their eggs and, therefore, that early birds did, too. But estimates of the egg size of 21 early bird species, including Archaeopteryx – made by measuring the diameter of the pelvic canal through which their eggs had to pass – suggest that the eggs were too fragile to bear the parents’ weight.
So is there still life in the idea of egg-brooding therapod dinosaurs? “From my perspective, no there isn’t,” says Charles Deeming of the University of Lincoln, who led the study. “Oviraptors sat in a ring of eggs that were multilayered and angled inwards. This minimises the contact between egg and the body, which would lead to poor heat transfer even with eggs at the top.” He thinks it more likely that, like crocodiles, they simply guarded their eggs rather than incubated them.