The clutch of ancient eggs belongs to a medium-sized adult oviraptor, and we know that because the parent is actually part of the fossil. The skeleton of this ostrich-like theropod is positioned in a crouch over two dozen eggs, at least seven of which were on the brink of hatching and still contain ᴇᴍʙʀʏᴏs inside.
These eggs now join the ranks of another famous oviraptorid ᴇᴍʙʀʏᴏnic egg, known as Baby Louie, whose 90 million-year-old remains were also found in China.
The ancient scene is unprecedented, and provides the first hard evidence that dinosaurs were brooding parents, laying their eggs and incubating them for quite a long time.
“This kind of discovery – in essence, fossilized behavior – is the rarest of the rare in dinosaurs,” says paleontologist Matt Lamanna from the Carnegie Museum of Natural History (CMNH).
“Though a few adult oviraptorids have been found on nests of their eggs before, no ᴇᴍʙʀʏᴏs have ever been found inside those eggs.”
Oviraptorids, which are theropods — a group of mostly meat-eating bipedal dinosaurs that includes Tyrannosaurus rex and Velociraptor — flourished during the Cretaceous period (145.5 million to 65.5 million years ago). This brooding oviraptorid was discovered in rocks dating to the last age of the Cretaceous period, known as the Maastrichtian age (72 million to 65.5 million years ago), alongside the Ganzhou railway station in Jiangxi province.
The fossil isn’t complete, as the adult’s sᴋᴜʟʟ and a few of its ʙᴏɴᴇs, including parts of its vertebrae, are missing; but its nest is remarkably well-preserved with the remains of at least 24 oval-shaped eggs. At least seven of those eggs, each about 8.5 inches (21.5 centimeters) long and just over 3 inches (8.5 cm) across, contain ʙᴏɴᴇs or partial skeletons of ᴇᴍʙʀʏᴏnic dinosaurs.
The adult oviraptorid was sitting directly over the clutch, with its forelimbs (or arms) covering the edges of the nest, the researchers wrote in the study. Many of the ᴇᴍʙʀʏᴏs were about to hatch. This indicates that the adult was likely incubating its eggs, a behavior also seen in modern birds, the descendants of dinosaurs, rather than guarding its nest like a crocodile (crocodiles are archosaurs, meaning they are distant cousins of dinosaurs), the researchers said.
Analyzing the oxygen isotopes of these ᴇᴍʙʀʏᴏs, researchers found the estimated incubation temperature was consistent with the body temperature of the parent, sitting somewhere between 30 to 38 degrees Celsius (86 to 100 degrees Fahrenheit).
The team figured this out by comparing the oxygen isotopes in the eggshells, which would have matched the mother’s oxygen isotopic makeup because she laid the eggs, with the chemistry of the ᴇᴍʙʀʏᴏnic ʙᴏɴᴇs, which would have changed over time because of the incubation heat applied to them. This finding adds another layer of evidence that the adult oviraptorid was sitting on the nest to keep the eggs warm. In contrast, reptiles tend to keep their eggs at cooler temperatures of about 79 to 90 F (26 to 32 C), the researchers said.
Interestingly enough, however, not all the ᴇᴍʙʀʏᴏs were at the same stages of development. This suggests the clutch may ultimately have hatched at different times – a feature that was thought to show up much later, in only some types of birds.
The adult oviraptorid revealed one more secret; the researchers found a cluster of pebbles near its abdominal region. These pebbles were likely gastroliths (“stomach stones” in Latin) that the dinosaur likely swallowed to help it grind and digest its food. This is the first instance of stones that are clearly gastroliths to be found in an oviraptorid specimen, the researchers said.