The 72 to 66-million-year-old oviraptosaur embryo, found in Ganzhou, Jiangxi Province, in southern China, reveals that the creatures took on a distinctive tucking posture before they hatched.
Analysis of the fossil, named “Baby Yingliang”, after the name of the Chinese company which bought the egg around 20 years ago, showed the embryo's head is lying below its body, with its feet on either side and its back curled along the blunt end of the egg.
The researchers said this behaviour had previously been considered unique to birds.
It raises the possibility that this tucking position before hatching may have evolved first among non-avian theropods during the Cretaceous period.
The research team from institutions in China, the UK and Canada said it was among the most complete dinosaur embryo ever found.
“Most known non-avian dinosaur embryos are incomplete with skeletons disarticulated,” said Waisum Ma of the University of Birmingham.
“We were surprised to see this embryo beautifully preserved inside a dinosaur egg, lying in a bird-like posture. This posture had not been recognised in non-avian dinosaurs before.”
“Dinosaur embryos are some of the rarest fossils and most of them are incomplete with the bones dislocated,“ she added.
“We are very excited about the discovery of Baby Yingliang – it is preserved in great condition and helps us answer a lot of questions about dinosaur growth and reproduction.”
“It is interesting to see this dinosaur embryo and a chicken embryo pose in a similar way inside the egg, which possibly indicates similar prehatching behaviours.”
The embryo was identified as an oviraptorosaur based on its deep, toothless skull.
Oviraptorosaurs are a group of feathered theropod dinosaurs, closely related to modern-day birds, known from the Cretaceous of Asia and North America.
Their variable beak shapes and body sizes are likely to have allowed them to adopt a wide range of diets, including herbivory, omnivory and carnivory, the research team said.
Modern birds are known to adopt a series of tucking postures, in which they bend their body and bring their head under their wing, soon before hatching. Embryos that fail to attain such postures have a higher chance of death due to unsuccessful hatching.
By comparing the specimen with the embryos of other theropods, long-necked sauropod dinosaurs and birds, the researchers now suggest that tucking behaviour may have first evolved in theropod dinosaurs many tens or hundreds of millions of years ago.
They said additional discoveries of embryo fossils would be invaluable to further test this hypothesis.
Professor Steve Brusatte from the University of Edinburgh, part of the research team, said: “This dinosaur embryo inside its egg is one of the most beautiful fossils I have ever seen.
“This little prenatal dinosaur looks just like a baby bird curled in its egg, which is yet more evidence that many features characteristic of today’s birds first evolved in their dinosaur ancestors.”
The research is published in the journal iScience.
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