Nicknamed “Baby Yingliang” paleontologists describe in detail an egg-preserved dinosaur embryo whose posture sheds new light on the relationship between modern birds and their ancestors. The offspring of a dinosaur species that walks on two legs take a special curved position that was previously thought to be unique to birds before hatching. This suggests that this egg positioning behavior appeared in the ancestors of bird dinosaurs, the researchers said.
They say the dinosaurs aren’t really extinct. Because if you want to admire one of them, it is usually enough to look out the window: birds are considered direct descendants of giant lizards from the Jurassic and Cretaceous periods. They evolved from theropod dinosaur species and, as numerous studies have shown, their ancestors possessed many supposedly avian traits. This includes aspects of reproductive biology: finds of fossil nests and eggs have provided numerous clues about bird traits, behavior and development processes. Fossilized eggs containing embryonic remains have also been found, but detailed specimens are very rare. As the natural state of babies during their lifetime is usually difficult to recognize due to the fossilization process: structures are often missing or small bones are displaced from their former positions.
Literally peeled from an egg
But that is not the case with a specimen currently reported by an international team of palaeontologists. A 17 cm long fossil egg was discovered along with others in late Cretaceous rock in southern China. “These fossils of dinosaur eggs were then dissected and our Baby Yingliang eventually appeared in one of them,” says co-author Lida Xing of the China University of Geosciences in Beijing. The fossil shows the finest details of the prenatal dinosaur and seems to be exactly as it was at the time of the tragedy. “This dinosaur embryo in an egg is one of the most beautiful fossils I’ve ever seen,” says co-author Steve Brusatte of the University of Edinburgh, explaining the significance of the find.
Baby Yingliang was assigned to representatives of Oviraptorosaurus based on the characteristics of the skull. It is a group of small and medium-sized theropods known from the Cretaceous period of Asia and North America. Fossil nests have already been discovered containing the eggs of these feathered bipeds – in some cases also with traces of unhatched pups. But Baby Yingliang now provides the clearest evidence of animal behavior in an egg. In particular, scientists have established a previously unknown attitude in dinosaur embryos. The baby oviraptorosaurus has its head on its belly, its feet are set to the sides, and its back is curved along the blunt end of the egg.
Like a chick in an egg
As the scientists explain, this position is characteristic of modern bird embryos at some stage before hatching. “Interestingly, this dinosaur embryo and the chicken embryo in the egg take a comparable posture, indicating similar behavior before hatching,” says lead author Fion Waisum Ma of the University of Birmingham. The researchers explain that chicks are known to make a series of typical changes of position inside the egg until they finally arch and tuck their heads under the wings. Avian embryos that fail to adopt these positions are more likely to be trapped in the egg.
It is different with reptiles: for example, today’s crocodile embryos assume a kind of sitting position in the egg. As previously established, this seemed to be the case with dinosaurs as well. But as it turns out, that was clearly not the case, at least with Oviraptorosaurus. Hence, paleontologists view the findings as an indication that the behavior of an egg previously thought to be unique to birds developed earlier in theropods. “This little prenatal dinosaur looks exactly like a chick wrapped in an egg, further proof that many of the characteristics of modern birds first evolved in their ancestors, the dinosaurs,” says Brusatte.
He and his colleagues now hope for more embryonic fossil finds, the deeper ones Provide insight into the presumably bird-like behavior of some species of dinosaur in the egg.
Source: University of Birmingham, Article: iScience, doi: 10.1016 / j.isci.2021.103516