New evidence suggests dinosaurs took three to six months to develop in the egg, more like modern reptiles than birds. The researchers speculate that slow developers were at a disadvantage in the changed environment at the end of the Cretaceous period, and this may have contributed to their extinction.

Birds and reptiles approach egg laying differently. Birds generally produce fewer eggs in a clutch, often just one, but make them larger relative to the size of the parents. Incubation periods last from 11 to 85 days. Reptiles can lay far more eggs at once – think of hatching sea turtles – and make them smaller. Less obviously, they also tend to take longer to hatch.

A team led by Dr Gregory Erickson of Florida State University set out to see how dinosaurs compared, using the fact that teeth contain age lines like tree rings.

Humans develop their teeth through a process of layering with daily mineralization, which can be counted to show how many days went into their development. It might be expected that day-night cycles don't matter in the egg, but embryonic teeth from crocodiles and their relatives show the same pattern, known as incremental lines of von Ebner. Erickson and his colleagues used the incremental lines to measure the incubation periods for two dinosaur species, whose large nesting sites have provided us with suitable fossils.

It took Protoceratops andrewsi an average of 48 days to produce an embryonic tooth, Erickson reports in Proceedings of the National Academy of Sciences. Based on the ratios found in related species, this led the authors to estimate an incubation time of least 83 days. For Hypacrosaurus stebingeri, the figures are 99 and 171 days.

H. stebingeri was a late Cretaceous hadrosaur that grew much larger than any bird – even on hatching it was 1.7 meters (5.6 feet) long – so it lacks obvious avian comparisons for its long incubation time. P. andrewsi is a different matter. Also a late Cretaceous species, this horned dinosaur was more on the scale of an ostrich.

P andrewsi's incubation period was at least twice as long as that for similar-sized avian eggs. When the researchers modeled the time it would take to hatch a bird's eggs of similar size to those of H. stebinger, they got figures half as long, with crocodiles even shorter.

These results surprised the authors, since dinosaurs are more closely related to birds than crocodiles and had been thought to resemble them in this way. The authors propose rapid incubation evolved around the time toothless dinosaurs appeared. Long incubation times would have been hazardous when food became scarce as a result of the drastic events that ended the Cretaceous, forcing parents to spend too long nesting.