An international team of scientists has announced the discovery of an extraordinary petrified nest in China that has preserved at least eight separate dinosaurs from 70 million years ago.
The coupling of ancient eggs belongs to a medium-sized oviraptor for adults, and we know this because the parent is actually part of the fossil. The skeleton of this ostrich-like theropod has been placed in a brooding more than two dozen eggs, at least seven of which are on the verge of hatching and still contain embryos inside.
The ancient scene is unprecedented and provides the first hard evidence that dinosaurs were parents that hatched, laid their eggs and bred them for a long time.
“This kind of discovery – essentially fossil behavior – is the rarest of the rare in dinosaurs,” says paleontologist Matt Lamanna of the Carnegie Museum of Natural History (CMNH).
“Although some adult oviraptorides have been found on their eggs before, no embryos have ever been found in the eggs.”
The 70 million year old fossil. (Shundong Bi / Indiana University of Pennsylvania)
Since the 1980s, paleontologists have excavated numerous dinosaur nests containing fossilized eggs. Some rare have even been found with the skeleton of the parent on top. Other oviraptor eggs suggest that they may have had a bluish-green color.
However, the derivation of behavior from these fossils was problematic. Although the oviraptor parents hatch on their nests, it is also possible that these dinosaurs perished while laying or protecting their eggs and did not necessarily hatch. This is more similar to how crocodiles handle their nests, not modern birds.
The new sample was obtained from Ganzhou’s Nanxiong Formation in southern China – a region known for the world’s largest collection of fossilized dinosaur eggs – but it is different from what scientists have previously found.
The relationship between the parent and the embryo of the dinosaur has never been closer. The body of the adult oviraptor is preserved in ‘extraordinary proximity of the eggs’, with little to no sediment in between.
In at least seven of the eggs, embryonic material was found to be exposed, including bones in recognizable shapes.
One of the eggs may contain a complete skeleton, with the vertebrae, ribs, a humerus, both ilia and femora, and a tibia in a curled position.
By analyzing the oxygen isotopes of these embryos, researchers found that the estimated incubation temperature corresponds to the body temperature of the parent and sits somewhere between 30 and 38 degrees Celsius (86 to 100 degrees Fahrenheit).
“In the new specimen, the babies were almost ready to hatch, which no doubt tells us that this oviraptorid has been caring for its nest for a long time,” Lamanna explains.
“This dinosaur was a caring parent who eventually gave his life while raising his young.”
Artwork of oviraptor dinosaur hatching on a nest of blue-green eggs. (Zhao Chuang / PNSO)
Interestingly, however, not all embryos were in the same stages of development. This suggests that the clutch eventually hatched at different times – a feature that would presumably appear much later, only in some species of birds.
Although oviraptors are often considered an intermediate stage in this evolutionary process, they appear to be moving away independently of simultaneous hatching, suggesting that the evolution of bird reproduction was not a simple linear process.
Most modern birds will wait until all their eggs are laid before incubating them – sometimes with the help of mother and father – and this leads to synchronous hatching.
Although oviraptors may also have waited to incubate until all the eggs have been laid, the authors suggest that the upper eggs might be closer to the hatching adult and that they could therefore develop more rapidly. However, this is just an idea. We need more information to find out why some eggs would hatch earlier than others.
In other ways, however, the oviraptor has similar characteristics to modern birds. The sex of the petrified parent, for example, may have been male, suggesting that the father might also have participated in breeding, similar to ostrich mothers and fathers, who in turn incubate their young.
The sex of the adult oviraptor is still under discussion (it could be a male or female based on available data), but the idea is consistent with other analyzes of teropodnests, suggesting some paternal care.
Artwork of the adult oviraptor skeleton; preserved bones displayed in white. (Andrew McAfee / Carnegie Museum of Natural History)
As if all this reproductive information is not enough, this remarkable fossil also gave us a glimpse into the potential diet of the oviraptor. For the first time, scientists have found small stones in the stomach of this type of dinosaur, which would probably be swallowed to promote digestion.
“It’s extraordinary to think how much biological information is captured in just this single fossil,” said paleontologist Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.
“We’ll be learning from this copy for many years to come.”
The study is in the Science Bulletin.