The "Baby Yingliang" dinosaur egg fossil at Yingliang Stone Nature History Museum in Fuzhou, Fujian province. [Photo provided to chinadaily.com.cn]

By comparing "Baby Yingliang" with the embryos of other dinosaurs and birds, the research team proposed that such prehatching behavior, which was previously thought unique to birds, evolved in theropod dinosaurs tens or hundreds of millions of years ago.

Fion Waisum Ma, joint first author and PhD researcher at the University of Birmingham, said the dinosaur embryo study could help bolster people's understanding of 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 behaviors," Ma added.

Steve Brusatte from the University of Edinburgh, who is part of the research team, said this little prenatal dinosaur looks just like a baby bird curled up in its egg, providing yet "more evidence that many features of today's birds first evolved in their dinosaur ancestors".

Dinosaur embryos are some of the rarest fossils. To further test the hypothesis, more embryo fossil discoveries are needed, said coauthor Xing Lida, professor at the China University of Geosciences in Beijing.