Fossilised dinosaur egg shows embryo preparing to hatch like a bird

Researchers have announced the exceedingly rare discovery of a perfectly preserved fossilised dinosaur embryo.

The embryo – named “Baby Yingliang” – was preparing to hatch from its egg in a distinctive “tucking” posture, previously considered to be unique to birds and never seen before in dinosaurs. The research has been published in Cell.

The ancient animal is an oviraptorosaur – a group of feathered theropod dinosaurs closely related to birds that lived during the Late Cretaceous period 100–66 million years ago. Theropods were a sup-group of obligate bipedal dinosaur with hollow bones and three-toed limbs, which also included Tyrannosaurus rex.

This discovery suggests that the modern birds’ pre-hatching tucking behaviour could have evolved in dinosaur ancestors prior to the origin of birds.

Baby Yingliang is one of the most complete non-avian dinosaur embryos ever discovered, as – according to corresponding author Waisum Ma, of the University of Birmingham, UK – most fossilised non-avian dinosaur embryos are incomplete and have disarticulated skeletons.

“We were surprised to see this embryo beautifully preserved inside a dinosaur egg, lying in a bird-like posture,” says Ma. “This posture had not been recognised in non-avian dinosaurs before.”

A modern bird embryo in the tucking position has its head placed under the right wing to stabilise it while using the beak to break out of the shell. The process is controlled by the central nervous system and is critical for hatching success.

Baby Yingliang’s head is tucked between its arms and legs and its body is curled into a C-shape inside its egg. This has never been observed in articulated non-avian dinosaur embryos due to their scarcity.

The fossil was found in 2000 in Ganzhou, southern China, but ended up in storage and was largely forgotten until about 10 years later, when it was unearthed by museum staff during the construction of Yingliang Stone Nature History Museum.

“Museum staff identified them as dinosaur eggs and saw some bones on the broken cross section of one of the eggs,” says Dr Lida Xing, of China University of Geosciences.

The fossil was initially studied by scraping off part of the eggshell to expose the embryo within, however researchers say that they will continue to study it in more depth using various imaging techniques to gain insight into its internal anatomy, such as skull bones, and other body parts still covered in rock.