The fossilized dinosaur egg from the Hekou Formation, Ganzhou, Jiangxi province, southern China, is elongate ovoid in shape with dimensions of 16.7 cm long by 7.6 cm wide, and has characteristics typical of the egg family Elongatoolithidae. Dubbed ‘Baby Yingliang,’ the embryo belongs to an oviraptorosaur, a toothless theropod dinosaur closely related to birds. Among the most complete dinosaur embryos ever found, the fossil suggests that oviraptorosaurs took on a distinctive tucking posture before they hatched, a behavior that had been considered unique to birds. It also raises the possibility that tucking behavior may have evolved first among non-avian theropods during the Cretaceous period.
An artist’s reconstruction of the oviraptorosaur egg and embryo from the Hekou Formation, Ganzhou, Jiangxi province, southern China. Image credit: Julius Csotonyi.
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.
‘Baby Yingliang’ was identified as an oviraptorosaur based on its deep, toothless skull.
“Dinosaur embryos are some of the rarest fossils and most of them are incomplete with skeletons disarticulated,” said Dr. Waisum Ma, a paleontologist at the University of Birmingham.
“We were surprised to see this embryo beautifully preserved inside a dinosaur egg, lying in a bird-like posture.”
Xing et al. found an exceptionally preserved, articulated oviraptorid embryo inside an elongatoolithid egg, from the Late Cretaceous Hekou Formation of southern China. The oviraptorid skeleton is 23.5 cm in total length, measured from the anterior tip of the skull to the last preserved caudal vertebra, and occupies nearly the entire width of the egg and most of the length, with the exception of a∼1.9 cm space between the dorsal vertebrae and the blunt pole of the egg. Image credit: Xing et al., doi: 10.1016/j.isci.2021.103516.
In the study, Dr. Ma and colleagues found that Baby Yingliang’s head lies ventral to the body, with the feet on either side, and the back curled along the blunt pole of the egg, in a posture previously unrecognized in a non-avian dinosaur.
That’s especially notable because it’s reminiscent of a late-stage modern bird embryo.
Comparison of the specimen to other late-stage oviraptorosaur embryos suggests that before hatching, oviraptorosaurs developed avian-like postures late in their incubation.
In modern birds, such coordinated embryonic movements are associated with tucking, a behavior that’s controlled by the central nervous system and is critical for hatching success.
The notion that such pre-hatching behavior may have originated among non-avian theropods can now be further investigated through more studies of other fossil embryos.
But first, the paleontologists will continue studying this rare specimen in even more depth, using various imaging techniques to image its internal anatomy, such as skull bones, and other body parts that are still covered in rocks.
“This dinosaur embryo inside its egg is one of the most beautiful fossils I have ever seen,” said Professor Steve Brusatte, a paleontologist at the University of Edinburgh.
“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 findings were published in the journal iScience.
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Lida Xing et al. An exquisitely preserved in-ovo theropod dinosaur embryo sheds light on avian-like prehatching postures. iScience, published online December 22, 2021; doi: 10.1016/j.isci.2021.103516
