Dino embryo in bird-like attitude

Artist’s impression of “Baby Yingliang” in its egg. (Image: LIDA XING)

He is called “Baby Yingliang”: paleontologists report on a dinosaur embryo preserved in detail in the egg, the posture of which sheds new light on the connection between modern birds and their ancestors. The offspring of a two-legged walking dinosaur assumes a specially curved position that was previously considered unique for birds before hatching. This suggests that this positioning behavior in the egg already originated in the dinosaur ancestors of the birds, say the researchers.

Actually, the dinosaurs are not really extinct, they say. Because if you want to marvel at one, you usually only need to look out the window: Birds are the direct descendants of the giant lizards of the Jurassic and Cretaceous periods. They evolved from theropod dinosaur species and, as many studies show, their ancestors already possessed many supposedly bird-like characteristics. This also includes aspects of reproductive biology: Finds of fossil nests and eggs have provided numerous indications of bird-like characteristics, behavior and development processes. Fossilized eggs with embryo remains have also been discovered – but detailed specimens are very rare. Because the natural condition of the little ones during their lifetime is mostly barely recognizable due to the process of fossilization: Structures are often missing or the fine bones are shifted from their former positions.

Literally peeled from the egg

But this is not the case with the specimen that is now being reported by an international team of paleontologists. The 17 centimeter long fossilized egg was discovered along with others in rocks from the late Cretaceous period at a site in southern China. “These dinosaur egg fossils were then prepared and so our ‘Baby Yingliang’ came to light in one of them,” reports co-author Lida Xing from the Chinese University of Geosciences in Beijing. The fossil shows the finest details of the prenatal dinosaur and it appears to be positioned exactly as it was when fate overtook it. “This dinosaur embryo in its egg is one of the most beautiful fossils I’ve ever seen,” says co-author Steve Brusatte from the University of Edinburgh, explaining the importance of the find.

The astonishingly well-preserved fossil provides insights into behavior in the egg. (Image: Xing et al./iScience)

Baby Yingliang was assigned to representatives of the oviraptorosaurs due to its skull features. This is a group of small to medium-sized theropods known from the Cretaceous Period in Asia and North America. Fossil nests with eggs have already been discovered of these feathered bipeds – in some cases with traces of the unhatched young. But Baby Yingliang now provides the clearest evidence to date of the behavior of the animals in the egg. Specifically: the scientists determined a previously unknown posture in dinosaur embryos. The small oviraptorosaur has therefore placed its head on its stomach, the feet are positioned to the side and the back is curved along the blunt end of the egg.

Like a chicken chick in an egg

As the researchers explain, this position is characteristic of modern avian embryos in a certain phase before hatching. “It is interesting to see that this dinosaur embryo and a chicken embryo adopt a similar posture in the egg, which indicates similar behavior before hatching,” says first author Fion Waisum Ma of the University of Birmingham. As the researchers explain, chicks have been known to make a series of typical changes in position in the egg until they end up arching their bodies and bringing their heads under their wings. Avian embryos that do not adopt these postures are at greater risk of being trapped in the egg.

With reptiles, however, things are different: For example, today’s crocodile embryos assume a kind of sitting position in the egg. According to previous findings, this also appeared to have been the case with dinosaurs. But as it turns out, this was apparently not the case, at least for the oviraptorosaurs. The paleontologists therefore see the findings as an indication that the behavior in the egg, which was previously regarded as unique for birds, had already developed in theropod dinosaurs. “This little prenatal dinosaur looks exactly like a baby bird curled up in its egg, which is further evidence that many of the features that are characteristic of today’s birds first developed in their dinosaur ancestors,” says Brusatte.

He and his colleagues are now hoping for more embryo fossil finds, the deeper ones Provide insights into the presumably bird-like behavior of some dinosaur species in the egg.

Source: University of Birmingham, Article: iScience, doi: 10.1016 / j.isci.2021.103516

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