An ostrich chick hatches after 42 days - in some bird species it takes only eleven days: The development of birds in the egg is fast - on average, it takes twice as long for reptiles. Since birds are the descendants of dinosaurs, it was previously assumed that their egg incubation times were already similarly short. But was that really the case - or were they more like those of today's reptiles? This is the first question explored by Gregory Erickson of the Florida State University in Tallahassee.
Dino embryos in focus
As part of their study, the researchers studied the fossil teeth of two dinosaur embryos preserved in the egg: one was a small protoceratops - a roughly pig-sized dinosaur species. The eggs of these animals were relatively small with only about 194 grams of weight. The other embryo studied, on the other hand, came from a representative from the other end of the size spectrum of dinosaurs: Hypacrosaurus was a large duckbill dinosaur whose eggs weighed over four kilograms.
First, the researchers examined the embryonic jaws of the two dinosaurs using computed tomography (CT). They then analyzed the fine structures of the teeth using microscopy. Thus, fine growth lines were visible, which also have our teeth. "These are structures that arise when animal teeth develop, " explains Erickson. "They are like tree rings - but they are formed daily. So we literally counted them to see how long the dinosaurs have evolved in the egg. "
Surprisingly long in the egg
In this way, the researchers found that the Protoceratops embryo was about three months old when it came to an end and the process of fossilization began. The Hypacrosaurus embryo had even grown up in the egg for about six months. This incubation period is more like that of the reptile cousins of dinosaurs and not of birds, say the scientists. display
According to them, this result suggests that the faster incubation period of bird eggs did not develop until the branch of evolution of the ancestors of today's birds split off from the pedigree of the other dinosaurs. More information could be provided by embryos of species that are evolutionary closer to birds, such as velociraptors. Unfortunately, according to the researchers, hardly any corresponding fossils are available for research.
An extinction factor?
Nevertheless, interesting information is already in the current results, emphasize Erickson and his colleagues. They may shed light on the question of why the dinosaurs disappeared in the great mass extinction around 65 million years ago, while the ancestors of today's birds survived. Perhaps their shorter egg phase was an advantage in the turmoil at the end of the Cretaceous. Dino eggs, on the other hand, may have been more affected by environmental risk factors, the researchers say.
Original work of the researchers:
- PNAS, DOI: 10.1073 / pnas.1613716114