New York, Jan 3 (IANS) The eggs of non-avian dinosaurs took nearly between three to six months to hatch, similar to crocodiles and lizards, which explains the reason behind the extinction of the dinosaur species, scientists have found.

It was long assumed that the duration of dinosaur incubation was similar to birds, whose eggs hatch within 11 to 85 days, but they are more like of reptiles whose eggs take weeks to months to hatch.

"Some of the greatest riddles about dinosaurs pertain to their embryology -- virtually nothing is known. Did their eggs incubate slowly like their reptilian cousins? Or rapidly like living dinosaurs -- the birds," asked lead author Gregory Erickson, Professor at Florida State University. 

For the study, the team examined the fossilised teeth of two extremely well-preserved ornithischian dinosaur embryos: Protoceratops -- a pig-sized dinosaur, whose eggs weighed 194 grams -- and Hypacrosaurus -- a very large duck-billed dinosaur, with eggs weighing more than 4-kg.

Analysing the pattern of "von Ebner" lines -- the growth lines that are present in the teeth of all animals, as well as humans --, the researchers found that the Protoceratops embryos were about three months old when they died and the Hypacrosaurus embryos were about six months old. 

"These are the lines that are laid down when any animal's teeth develops. They're kind of like tree rings, but they are put down daily. And so we could literally count them to see how long each dinosaur had been developing," Erickson added.

In addition, the study found that the prolonged incubation may have affected dinosaurs' ability to compete with more rapidly generating populations of birds, reptiles, and mammals following the mass extinction event that occurred 65 million years ago. 

The long incubation period also exposed the non-avian dinosaur eggs and attending parents to predators, starvation, and environmental disruptions such as flooding, the researchers stated, in the paper published in the journal PNAS.