WASHINGTON • Research on the teeth of fossilised dinosaur embryos indicated that the eggs of some dinosaurs took between three and six months to hatch, researchers said.

Because birds are living dinosaurs, scientists have long assumed that the duration of dinosaur incubation was similar to birds', whose eggs hatch within 11 to 85 days. Comparable-sized reptilian eggs typically take twice as long - weeks to many months.

The new study, published in the US journal Proceedings Of The National Academy Of Science, showed that contrary to previous assumptions, dinosaur incubation is more similar to that of typical reptiles than of birds.

"Some of the greatest riddles about dinosaurs pertain to their embryology - virtually nothing is known," lead author Gregory Erickson, a professor at Florida State University, said in a statement.

In the study, the research team looked at the fossilised teeth of two well-preserved dinosaur embryos on each end of the size spectrum.

One is Protoceratops, a pig-sized dinosaur found in Asia whose eggs were quite small, while the other is Hypacrosaurus, a large duck-billed dinosaur found in Canada, with eggs weighing more than 4kg.

The results showed that the Protoceratops embryos were about three months old when they died and the Hypacrosaurus embryos were about six months old.

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