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A remarkably well-preserved egg reveals what birds inherited from dinosaurs

Oviraptorosaurs are a group of dinosaurs that looked like birds and were part of the ancestral line of dinosaurs from which birds descended. Oviraptorosaurs walked on two legs, had a powerful toothless beak, and were covered in feathers.

One of the earliest known species, Oviraptor philoceratops, was discovered in the 1920s following the unearthing of a skeleton next to a nest that contained eggs in Cretaceous rocks in the Gobi Desert, Mongolia. Paleontologists at the time assumed that the animal died while trying to rob the nest of another dinosaur. Its name means “egg thief”.

It was only in the mid-1990s, thanks to another fossil, that we understood thatoviraptor had actually died while tending to his own eggs. Since then, several important discoveries of oviraptorosaur eggs and nests have helped paleontologists piece together the breeding and nesting habits of these distinctive bird-like dinosaurs.

In 2021, the latest such discovery revealed the incredibly well-preserved skeleton of an oviraptorosaur curled up in its egg. Our group of scientists from Canada, China and the UK led the study of this 70 million year old Chinese fossil known as “Baby Yingliang”.

Baby Yingliang

Baby Yingliang is the first skeleton of a baby dinosaur that can accurately see the position of the embryo in its egg. Although the remains of other embryos had already been found inside their egg, their position was unclear, as they were disarticulated or had many missing bones.

As part of our study of the oviraptorosaurus fossil, we noticed a feature long thought to be unique to birds and has to do with how the embryo is placed in the egg before hatching. In Baby Yingliang, its back is curved into the rounded end of the egg, and its head is against the abdomen, with the tail curled against the pointed end. The legs are so folded that they are on either side of the head and upper body. The embryo looks like a chick a few days before hatching.

Bird embryos, however, fold more than what we saw in Baby Yingliang. Just before hatching, the head slips under the wing and rests on the shoulder, a position that aids in pipping and exiting the egg. Perhaps Baby Yingliang would have been completely folded like a bird if he had lived a little longer – his position nevertheless suggests that such embryonic postures first appeared in dinosaurs before being passed on to birds.

Artistic reconstruction of Baby Yingliang.
(Lida Xing/Shoulin Animation), Provided by the author

Incubation behaviors

There are other egg-related characteristics that birds inherited from oviraptorosaurs. These include the structure of the layers of the shell, the shape of the egg (one end is more pointed), the pigments that give the eggs their color, and the style of open nest. Even the incubation behavior called brooding, where the parent sits on its eggs, long thought to be reserved for birds, existed in these dinosaurs.

The discovery some 25 years ago of an astonishing fossil, and a few others of the same type made since, made it possible to observe the skeleton of an oviraptorosaur relative crouching on its eggs in the manner of a bird.

What is interesting is that oviraptorosaurs arranged their eggs in the nest in a particular way, different from that of birds. The eggs, which were often more than 30 in a nest, were placed in two or three superimposed rings, and oriented like the spokes of a bicycle wheel. If these dinosaurs used their body heat to incubate their eggs like today’s birds, this arrangement may have been crucial to exposing all of the eggs to the parent’s body in the central space of the ring.

The few brooding oviraptorosaur fossils that have been discovered so far are all from species weighing 100 kilos or less, which were not expected to exceed the size of an ostrich. The species to which Baby Yingliang belonged must also have been of this size: the 17 centimeter egg must have weighed half a kilogram and was part of an arrangement of eggs forming a little more than half a meter in diameter. Giant oviraptorosaurs, which are quite rare, were much larger, as were their eggs and nests.

photograph of a circle of fossilized eggs
A fossilized giant oviraptorosaur nest in China.
(Kohei Tanaka), Provided by the author

Giant eggs, huge nests

In 2017, I was part of a team that studied another embryo skeleton, known as “Baby Louie”. The skeleton was discovered with a group of eggs belonging to a new species of giant oviraptorosaur, which we have named “Beibeilong”. These dinosaur eggs, the largest ever found, were over 45 centimeters long and weighed more than five kilograms each. According to their size, the female must have weighed more than 1,100 kilograms, and the diameter of the nest was about two meters.

Assuming that Beibeilong behaved like other oviraptorosaur species, an obvious question arises: how could this behemoth sit on the nest without crushing its eggs?

In 2018, with another research team, we examined the eggs and nests of oviraptorosaur species ranging in weight from 50 kilograms to Beibeilong. We found that the shell of Beibeilong’s eggs was probably not strong enough, with its two millimeters thick, to support the entire weight of the animal.

We noticed that while the eggs of almost all species of oviraptorosaurs were arranged in a ring with a free central space, the size of this space was relatively larger in giant species like Beibeilong. This leads to speculation that these giant species constructed their egg ring differently than smaller ones, so that there was plenty of room in the center to support body weight while probably reducing egg contact.

birds and dinosaurs

Birds inherited many of their seemingly unique characteristics from dinosaurs. Fossilized eggs and nests have revealed bird-like features associated with reproduction in oviraptorosaurs.

Several egg-laying traits, including incubation behaviors, are now known to have been passed down from dinosaurs, such as oviraptorosaurs, to birds. Baby Yingliang’s recent discovery provides a unique view of the embryonic position inside the egg, which remarkably resembles that of a bird embryo about to hatch.

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