An international team of scientists has announced the discovery of an extraordinary fossilized nest in China, which preserves at least eight separate dinosaurs from 70 million years ago.
The ancient egg clutch belongs to a medium-sized adult oviraptor, and we know this because the father is actually part of the fossil. The skeleton of this ostrich-like theropod is found crouched over two dozen eggs, at least seven of which were on the verge of hatching and still contain embryos inside.
The ancient scene is unprecedented and provides the first compelling evidence that dinosaurs were raising their parents, laying their eggs, and incubating them for quite some time.
“This type of discovery, essentially fossilized behavior, is the rarest of the rare in dinosaurs,” says paleontologist Matt Lamanna of the Carnegie Museum of Natural History (CMNH).
“Although some adult oviraptorids have been found in their egg nests before, embryos have never been found within those eggs.”
Since the 1980s, paleontologists have unearthed numerous dinosaur nests that contain fossilized eggs. Some rare ones have even been found with the skeleton of the parents on top. Other oviraptor eggs suggest that they may have been a blue-green color.
However, inferring the behavior of these fossils has proven problematic. While it appears that the oviraptor parents are incubating in their nests, it is also possible that these dinosaurs died while laying or caring for their eggs, not necessarily incubating them. This is more similar to how crocodiles manage their nests, not modern birds.
The new specimen was recovered from the Ganzhou Nanxiong Formation in southern China, a region famous for the world’s largest collection of fossilized dinosaur eggs, but it is unlike anything scientists have found before.
The relationship between the dinosaur’s father and the embryo has never been as close as this. The body of the adult oviraptor is kept “very close to the eggs”, with little or no sediment in between.
In at least seven of the eggs, exposed embryonic material was found, including ossified bones in identifiable shapes.
One of the eggs may actually contain a complete skeleton, with its vertebrae, dorsal ribs, a humerus, both ilia and femur, and a tibia placed in a curved position.
By analyzing the oxygen isotopes of these embryos, the researchers found that the estimated incubation temperature was consistent with the father’s body temperature, between 30 and 38 degrees Celsius (86 to 100 degrees Fahrenheit).
“In the new specimen, the babies were almost ready to be born, which tells us without a doubt that this oviraptorid had cared for its nest for quite some time,” explains Lamanna.
“This dinosaur was a loving father who finally gave his life while feeding his young.”
Interestingly, however, not all embryos were at the same stages of development. This suggests that ultimately the clutch may have hatched at different times, a feature that was thought to appear much later, only in some types of birds.
While oviraptors are often considered an intermediate stage in this evolutionary process, it appears that they may have independently drifted away from simultaneous hatching, and this suggests that the evolution of bird reproduction was not a simple linear process.
Most modern birds will wait until they lay all their eggs before incubating them, sometimes with the help of the mother and father, and this leads to synchronous hatching.
While the oviraptors may also have waited to hatch until all the eggs were laid, the authors suggest that the upper eggs could have been closer to the incubating adult and therefore could have developed more rapidly. However, this is just an idea. We will need more data to find out why some eggs hatched earlier than others.
However, in other ways, the oviraptor shares similar traits to modern birds. The sex of the fossilized father, for example, may have been male, suggesting that the father may have also participated in the rearing, similar to ostriches mothers and fathers, who take turns hatching their young.
The sex of the adult oviraptor is still up for debate (it could be a male or a female depending on the available data), but the idea is consistent with other analyzes of theropod nests, which suggest some level of parental care.
As if all that reproductive information wasn’t enough, this remarkable fossil has also given us insight into the oviraptor’s potential diet. For the first time, scientists have found small stones in the stomach of this type of dinosaur, which would likely have been swallowed to aid digestion.
“It is extraordinary to think how much biological information is captured in this single fossil,” says paleontologist Xing Xu of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing.
“We will learn from this specimen for many years.”
The study was published in the Science bulletin.