Dinosaur Embryos Tucked Themselves in Just Like Birds

A stunning fossil egg has allowed paleontologists to find new clues about a dinosaur’s early development

Dinosaur Embryo In Egg
An artist’s reconstruction of a baby oviraptorid curled up inside its egg Julius Csotonyi

The little dinosaur was almost ready to hatch. Nestled inside its oblong egg, the beaked, almost parrot-like oviraptorosaur curled into a C-shape with its head tucked between its arms and legs. This wasn’t an accidental pose as the dinosaur neared a hatching day that would never come. Modern birds take up the same pose inside their eggs, a very ancient behavior that helps birds poke their way out of their eggs and goes back over 66 million years.

Figuring out how dinosaurs grew inside their eggs has been a challenging task for paleontologists. Well-preserved fossil eggs are rare, and fossil eggs with delicate, embryonic dinosaurs inside are even rarer still. But the discovery of an exquisite oviraptorosaur embryo labeled as YLSNHM01266 has offered paleontologists a delicate and detailed look at what some baby dinosaurs were doing inside their eggs as they were getting ready to break out, and this find may help experts better understand similar finds.

The intricate fossil, described by University of Birmingham paleontologist Waisum Ma and colleagues today in iScience, was discovered about 20 years ago in Ganzhou, China. At first, it wasn’t even clear that the fossil was an egg at all, but it was still purchased by Liang Lu of the stone company the Yingliang Group. The egg waited in storage for years until the construction of the Yingliang Stone Nature History Museum led museum staff to have another look. Not only was the fossil an egg, Ma says, but breaks in the shell showed tiny bones inside. When expert preparators scraped away the outer shell, they found one of the best-preserved dinosaur embryos ever seen.

“I was very surprised when I saw this dinosaur embryo,” Ma says. Often, dinosaur embryos are either incomplete or are preserved as a jumble of tiny bones inside the shell. Instead, YLSNHM01266 is complete from snout to tail, the hole cut into the egg by fossil preparators acting as a literal window into how the little dinosaur grew up.

Fossil Dinosaur Egg With Embryo Inside
The fossilized egg with the bones of the baby oviraptorosaur tucked inside Lida Xing et al 2021

The dinosaur inside is an oviraptorosaur. These feathery, omnivorous dinosaurs had beaked faces and, often, elaborate crests on their heads. Ironically, the first oviraptorosaur ever described—Oviraptor itself—was thought to have been caught in the act of a robbing a nest because it was found associated with eggs. Finds since then have revised the dinosaur’s image. Multiple oviraptorosaurs, and likely Oviraptor itself, have been found sitting on top of their nests and may have been among the most attentive of dinosaur parents. “We are learning so much about oviraptorosaur eggs that it’s hard not to get excited,” says University of Edinburgh paleontologist Gregory Funston, who was not involved in the new study. “We now know their nesting patterns, brooding habits, eggshell color and some fine details of how their embryos developed,” he adds, which offers a great deal of information to comparison to other dinosaurs—both our non-avian favorites of the prehistoric past and the birds we see around us today.

But it’s not just the preservation of the little oviraptorosaur that makes it remarkable. The dinosaur’s posture tells paleontologists something new about how these dinosaurs developed. The fossil is the ancient equivalent of a “tucking” posture seen in birds such as chickens. “Tucking posture is the head under the right wing with the body curled up,” Ma says, which happens in the days before hatching. It’s not happenstance, but a pose that helps stabilize the head when birds push and peck out of the eggshell, meaning that oviraptorosaurs likely did the same.

“I think the authors present a strong case for their interpretation,” says Funston. Whereas other oviraptorosaur embryos are incomplete, or sometimes even damaged by the way they were collected or prepared, the new fossil is exceptional and can act as a “Rosetta Stone” for other oviraptorosaur embryos.

Oviraptorid Dinosuar With Eggs and Young
A display shows an oviraptorid dinosaur watching over its eggs and young. Darla Zelenitsky

Not all dinosaurs hatched the same way. The long-necked, plant-eating sauropod dinosaurs—like Brachiosaurus and its relatives—usually had round eggs and were not so bird-like in their development. One recently-discovered sauropod embryo, for example, had a horn-like projection on its snout to help it pierce and push out of the egg in a way more like that of crocodiles and lizards. And even today, some flightless birds like emus kick their way out of their eggs. But the oviraptorosaur egg indicates that these dinosaurs, at least, shared some important traits in common with birds and that a variety of features that make modern avians distinct—such as feathers, a wishbone and brightly-colored eggs—were inherited from much more ancient dinosaurs. Finding these connections between the prehistoric and modern can help paleontologists better understand the lives of extinct dinosaurs that we’ll never get to see in the flesh.

“We currently know little about the prehatching behavior of theropod dinosaurs,” Ma says, but knowing that oviraptorosaurs developed like many modern birds allows experts to look at other embryos and estimate how long they had been developing. The little dinosaur in the egg, for example, corresponds to what domestic chickens do about day 17 in the egg. If the Cretaceous dinosaur grew at the same rate, then it was about three days from hatching when it was buried.

The task now is to find and identify more dinosaur embryos. If YLSNHM01266 was held in storage for years, perhaps there are others. “Ideally, we need to find more oviraptorid embryos to really figure out their developmental pattern,” says study co-author Darla Zelenitsky, “but we are definitely starting to put the pieces of the puzzle together.”

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