Birds are dinosaurs, but dinosaurs weren't birds

For decades now, the drumbeat of dinosaur news has been their similarity to birds. They were warm-blooded! They had feathers! And they are still around, because birds are actually dinosaurs. All true, but those that were non-avian dinosaurs, as they are now called, were not all beak and tweet. They were closely related to other living reptiles like crocodiles, and new findings about how long their eggs took to hatch bring that point home.

Scientists reported recently that by using a new technique on exceedingly rare fossils of unhatched dinosaur embryos, they determined that those embryos took twice as long to hatch as bird eggs of a similar size. The embryo of a large duck-billed dinosaur took at least six months to hatch, and the eggs of larger dinosaurs may have taken even longer.

The long incubation times complicate thinking about dinosaur behaviour. While some kinds of dinosaurs may have tended their eggs and young, for others the difficulty of hanging around for most of a year to watch buried eggs would have been too much. And long incubation times mean slow reproduction, a definite disadvantage when a comet or asteroid slams into the planet, as happened 65 million years ago.

But not birds. Gregory M Erickson of Florida State University, USA, the lead author of the study, in Proceedings of the National Academy of Sciences, said the study was undertaken because fossil dinosaurs in the egg are so rare that “virtually nothing is known about their embryology.” David J Varricchio at Montana State University, USA, who has studied fossilised dinosaur eggs and was one of the scientists who reviewed the paper for the journal, said the research took a new line of evidence — embryonic tooth age — and the technique could prove valuable in future studies.

Scientific opinions have varied on incubation times. David and other scientists had studied how porous fossilised eggshells were, which led them to conclude that the vast majority of dinosaur eggs were buried. And that behaviour, similar to living reptiles, suggested long incubation times, he said.

But, Gregory said, most researchers thought that since dinosaurs were closely related to modern birds their incubation rates were birdlike. Gregory used teeth from rare fossil embryos found in fossilised eggs that were about to hatch. He and his colleagues counted daily growth markers in the teeth, calculating that tooth growth accounted for about 40% of incubation time.

He worked with Mark A Norell, the head of paleontology at the American Museum of Natural History, USA, a co-author of the paper, to study a sample of teeth from 71- to 75-million-year-old embryos of Protoceratops, a sheep-sized dinosaur found in Mongolia. They came up with an incubation time of at least 83 days. Gregory said the information from embryo teeth was the first direct evidence of how many days non-avian dinosaurs took to hatch.

Such long incubation periods raise all sorts of questions, Gregory said. How could dinosaurs have migrated, as some have suggested, if they spent most of a year with their eggs? The very long incubation times would have meant dinosaur parents staying in one place for a whole year defending eggs and young. Long incubation periods also meant that the dinosaurs had to pick nesting sites that would be protected for many months from floods, drought and predation.

They were also not reproducing as fast as other animals at the time of the mass extinction 65 million years ago, which may have contributed to their disappearance. Birds, for example, had already appeared, and their incubation times were probably shorter. The incubation times would have been only one strike against the dinosaurs surviving a planetwide catastrophe, however.

Gregory said they had other disadvantages. “These animals were profligate wasters of energy,” he said. They were big and warm-blooded and “even the smallest dinosaurs took over a year to mature,” including time after hatching. “The dinosaurs found themselves holding some bad cards,” he said. “They had a dead man’s hand.”