Birds that lived at the time of the dinosaurs might have been too heavy to sit on clutches of eggs without breaking them, according to an analysis of primitive avian fossils. The findings suggest that incubation might be a defining feature of modern birds, evolving only in the past 100 million years.
Some palaeontologists have criticized the study, in part because the idea flies in the face of evidence that some non-avian dinosaurs closely related to birds sat atop nests on the ground. The accepted view by many researchers was that some dinosaurs were already brooding nests to incubate their eggs, which would suggest that the behaviour evolved long before the proliferation of modern groups of birds, following a mass-extinction event 66 million years ago.
Many fossils of early birds have been discovered in the past three decades — particularly in China — but direct evidence of their reproductive behaviour has been elusive, says palaeontologist Charles Deeming at the University of Lincoln, UK, who led the analysis.
More at: https://www.nature.com/articles/d41586-018-03447-3