It’s a world first: a fossilised specimen of a non-avian dinosaur crouching over her nest of eggs with the embryos still encased inside the shells.
The fossil – found in Ganzhou City in southern China’s Jiangxi Province – consists of an incomplete skeleton of a large adult oviraptorid “crouched in a bird-like brooding posture over a clutch of at least 24 eggs”, say the researchers from Yunnan University, and at least seven of the eggs preserve “bones or partial skeletons of unhatched embryos inside”.
The evidence suggests she was a “caring parent”, having nurtured the brood to a late stage of development before disaster struck.
This late stage of development, and the proximity of the adult to the eggs, strongly suggests the latter died in the act of incubating its nest.
This behaviour is similar to that of its modern bird cousins, rather than the crocodile style of laying its eggs or simply guarding its nest.
The fossil dates to the third and final period of the Mesozoic era (commonly known as the Age of Dinosaurs) that extended from 145 to 66 million years ago.
More specifically, the new specimen was recovered from uppermost Cretaceous-aged rocks, some 70 million years old.
“Dinosaurs preserved on their nests are rare, and so are fossil embryos. This is the first time a non-avian dinosaur has been found sitting on a nest of eggs that preserve embryos in a single spectacular specimen,” explains lead author Dr Shundong Bi.
“This kind of discovery, in essence fossilised behaviour, is the rarest of the rare in dinosaurs,” explains co-author Dr Matthew Lamanna from the Carnegie Museum of Natural History.
“Though a few adult oviraptorids have been found on nests of their eggs before, no embryos have ever been found inside those eggs. In the new specimen, the babies were almost ready to hatch, which tells us beyond a doubt that this oviraptorid had tended its nest for quite a long time. This dinosaur was a caring parent that ultimately gave its life while nurturing its young.”
The team also conducted oxygen isotope analyses that indicate that the eggs were incubated at high, bird-like temperatures, adding further support to the hypothesis that the adult perished in the act of brooding its nest.
Moreover, although all embryos were well-developed, some appear to have been more mature than others, which in turn suggests oviraptorid eggs in the same clutch might have hatched at slightly different times.
This characteristic, known as asynchronous hatching, appears to have evolved independently in oviraptorids and some modern birds.
Another fascinating find is that the adult has a cluster of pebbles in her abdominal region.
These, say the researchers, are almost certainly gastroliths, or “stomach stones” – rocks that would have been deliberately swallowed to aid the dinosaur in digesting its food. This is the first time that undoubted gastroliths have been found in an oviraptorid, and as such, these stones may provide new insights into the diets of these animals.
Says Dr Xing Xu from the Chinese Academy of Sciences: “It’s extraordinary to think how much biological information is captured in just this single fossil. We’re going to be learning from this specimen for many years to come.”