Fossilized Egg Suggests Dinosaurs had Similar Pre-hatching Posture to Modern Birds

In one of the most well-preserved dinosaur embryos ever found, a baby dinosaur curled its back and tucked its head in a position that is similar to modern birds before they hatch, a discovery that scientists say could shed new light on how dinosaurs developed in their early stages.

The best dinosaur embryos

The embryo, dubbed ‘Baby Yingliang’, was discovered in the Late Cretaceous rocks of Ganzhou, southern China and belongs to a toothless theropod dinosaur, or oviraptorosaur.

“It is one of the best dinosaur embryos ever found in history,” University of Birmingham researcher Fion Waisum Ma, who co-authored a paper in the journal iScience, told AFP.

The oviraptorosaur embryo ‘Baby Yingliang’ provides a rare glimpse into the prehatching behaviour of non-avian dinosaurs. Xing et al., 2021, Author provided

Ma and colleagues found Baby Yingliang’s head lay below its body, with the feet on either side and back curled – a posture that was previously unseen in dinosaurs, but similar to modern birds.

In modern birds, such postures are related to ‘tucking’ — a behaviour controlled by the central nervous system and critical for hatching success. After studying egg and embryo, researchers believe that such pre-hatching behaviour, previously considered unique to birds, may have originated among non-avian theropods.

The posture of Baby Yingliang appears similar to a roughly 17-day-old “pre-tucking” chicken embryo. Ultimately a chicken embryo will achieve a tucking posture on the 20th day of development in preparation for hatching on day 21.

Interestingly, we also noticed similarity in the posture of a previously reported oviraptorosaur embryo and a roughly 18-day-old chicken embryo, which is in the first stage of tucking. These observations suggest that oviraptorosaurs may have exhibited similar pre-hatching behaviours to modern birds.

Fossil forgotten for years

Oviraptorosaurs, which means “egg thief lizards,”  were feathered dinosaurs that lived in what is now Asia and North America during the Late Cretaceous period. 

They had variable beak shapes and diets, and ranged in size from modern turkeys at the lower end to massive Gigantoraptors, that were eight meters (26 feet) long.

Baby Yingliang measures around 27 centimeters (10.6 inches) long from head to tail, and lies inside a 17 centimeter-long egg at the Yingliang Stone Nature History Museum.

Researchers believe the creature is between 72 and 66 million years old, and was probably preserved by a sudden mudslide that buried the egg, protecting it from scavengers for eons.

It would have grown two to three meters long if it had lived to be an adult, and would have likely fed on plants.

The specimen was one of several egg fossils that were forgotten in storage for decades.