Slow incubation of dinosaur eggs: why only birds survived

Cast of Mossospondylus eggs and embryo
Photo from Royal Ontario Museum by Daderot (CC)

Most people have heard that birds are dinosaurs. But that raises an obvious question: why did birds survive the end-Cretaceous mass extinction event that did for their non-avian kin? A paper just published in PNAS suggests this might be explained by different reproductive strategies.

Reconstruction of Protoceratops andrewsi
AntoninJury (Wikimedia Commons) CC BY-SA 4.0

In brief, the authors made CT scans of teeth from fossilized dinosaur embryos. They then counted the von Ebner lines, which reflect the incremental pattern of dentine formation. Applying some quite reasonable assumptions to the data, they estimated the incubation time of the dinosaur eggs as minimum 2.8 months for Protoceratops andrewsi and 5.8 months for Hypachrosaurus stebingeri. The incubation times of modern birds tend to be much shorter though the upper end of the range (11-85 days) overlaps with P. andrewsi.

It is suggested that the relatively long generation times of non-avian dinosaurs put them at a disadvantage in competing with birds, reptiles and mammals during the Cretaceous-Palaeogene transition.