How did dinosaurs take over

Dinosaurs weren't quick breeders

Tallahassee (USA) - It took much longer for a cub to hatch from a dinosaur egg than previously thought. This is what US and Canadian researchers conclude from studies of fossil eggs with almost fully developed embryos. The analysis of certain tooth structures showed that the dinosaurs took three to six months to hatch, depending on their size. Thus, the incubation time of the eggs corresponds more closely to that of crocodiles than that of birds, the scientists report in the journal “Proceedings of the National Academy of Sciences (PNAS)”. This suggests that the lizards only buried their eggs and did not keep them warm themselves. If the embryos had developed as quickly as bird chicks - taking the egg size into account - the dinosaurs would have hatched after half the time now calculated. The slow development of the young could have been a major disadvantage in adverse environmental conditions and contributed to the extinction of the dinosaurs.

"Our results could probably help to understand why the dinosaurs became extinct at the end of the Cretaceous period, while other reptiles as well as amphibians, birds and mammals survived to this day," says Gregory Erickson of Florida State University in Tallahassee, the head of the research team. Different bird species only incubate their clutches for 11 to 85 days, depending on the size of the eggs. In reptiles living today, converted to the same egg size, it takes twice as long for the embryo to develop. One reason for this is the warming during breeding in the bird's nest. Since at least some species of dinosaurs were warm-blooded, it was assumed that their young hatched as quickly as the chicks of the birds and that the birds, as descendants of the dinosaurs, would have adopted this trait. Now it turns out that this is not the case.

The researchers selected two types of dinosaurs from the Ornithischia group, of which fossil egg finds with embryos already developed were available. The egg of the nearly two meter long Protoceratops andrewsi weighed 194 grams. The approximately nine meter long Hypacrosaurus stebingeri from the hadrosaur group laid eggs the size of a volleyball, which, weighing more than four kilograms, are among the largest dinosaur eggs. With the help of computed tomography scans of the jaws, the scientists first determined the stage of development of the tooth formation. They then examined specimens of individual teeth with a special microscope. In the dentin, the bone-like core substance of all teeth, they identified growth zones that are reminiscent of the annual rings of a tree trunk - exactly one growth line per day was created during tooth formation. "We just had to count the lines to find out how long it took to develop," says Erickson.

The Protoceratops embryos were found to be 83 days old and the larger Hypacrosaurus embryos 171 days old when they died. These development times are more than twice as long as they would be for birds that - theoretically - would lay eggs of the same size. In contrast, the times determined agree well with those that would be expected with today's reptiles. Accordingly, the incubation period in birds was shortened in the course of evolution only after the evolutionary lines of birds and reptiles had already separated. It would now be interesting to study dinosaur eggs, which, like the feathered velociraptors, are more closely related to the later birds, the authors write.

The very long development time in the eggs could have turned out to be disadvantageous for the dinosaurs. As a result, the offspring were threatened by nest robbers for a long time and exposed to adverse environmental influences without protection. The significantly faster development in early birds and mammals may have given these competitors a survival advantage that helped them survive the great extinction of species 65 million years ago - which the dinosaurs failed.

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