It’s hard to imagine a mighty T. rex kneeling delicately above a clutch of eggs, but new research surrounding a fossilized oviraptor suggests that this behavior may have indeed been practiced by some dinosaurs. The first non-avialan dinosaur (species outside of the clade of dinosaurs related to living birds) fossil to feature an adult dinosaur sat on top of a clutch of eggs that contain embryonic remains has been detailed in Science Bulletin. What's more, the embryos were at different stages, suggesting the eggs would hatch at different times, something that is usually determined by when the parent starts incubating.
“This isn’t the first time an oviraptorid has been found in such a way, nor are these the first-ever oviraptorid embryos,” study author Shundong Bi, a professor at the Indiana University of Pennsylvania, told IFLScience. “But it is the first time that an adult has been found atop embryo-bearing eggs. It’s also the first nesting oviraptorid to be discovered outside the Gobi Desert.”
Brooding, seen in chickens that sit on their eggs to incubate them during development, was thought to be an unlikely behavior in non-avialan dinosaurs whose heavy bodies would surely squish their progeny. However, this new fossil found near Ganzhou, China, is the first discovered having preserved a non-avialan dinosaur atop an egg clutch that still contains embryonic remains. The researchers believe the presence of an adult on eggs containing embryos at advanced growth stages provides strong support for the brooding hypothesis in some non-avialan dinosaurs.
Interestingly, the embryos inside the eggs are at different developmental stages, which points to the possibility that had they survived the eggs would’ve hatched at different times. “The asynchronous hatching was not widespread among dinosaurs,” said Bi. “This phenomenon, known as asynchronous hatching, is pretty peculiar and uncommon even in modern birds, the living descendants of dinosaurs.”
The researchers say their findings demonstrate that the evolution of reproductive biology along bird-line archosaurs (a large group of vertebrates that includes dinosaurs and pterosaurs and is represented today by birds) was complex and not the linear, incremental process it’s previously assumed to have been. They theorize that some aspects of non-avialan theropod reproduction may have been unique to these dinosaurs and not passed to the avialan ancestors that eventually gave rise to modern birds.
Recent research detailed how the avialan feature of flight likely evolved twice in dinosaurs before the clade containing modern birds’ ancestors came into the picture. This new insight presents a further trait of avialan dinosaurs and animals that may have been shared by some of their distant cousins.