Paleontologists in China have discovered what they say is the first non-avialan dinosaur fossil known to preserve an adult skeleton atop an egg clutch that contains embryonic remains.

Paleontologists in China have discovered what they say is the first non-avialan dinosaur fossil known to preserve an adult skeleton atop an egg clutch that contains embryonic remains.
An attentive oviraptorid theropod dinosaur broods its nest of blue-green eggs while its mate looks on in what is now Jiangxi Province of southern China some 70 million years ago. Image credit: Zhao Chuang.
Dinosaurs preserved on their nests are rare, and so are fossil embryos, said Dr. Shundong Bi, a paleontologist in the Centre for Vertebrate Evolutionary Biology at the Yunnan Universitys Institute of Palaeontology and the Department of Biology at the Indiana University of Pennsylvania.
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.
The 70-million-year-old fossil was recovered from the Upper Cretaceous Nanxiong Formation near the Ganzhou railway station of Ganzhou city, Jiangxi Province, China.
The specimen is a partial skeleton of a medium-sized oviraptorid theropod preserved atop an undisturbed clutch of at least 24 eggs, some of which are broken, exposing embryonic bones.
The eggs are 21.5 cm in length and 8.5 cm in width across their equatorial regions. The outer surface of all eggs exhibits ornamentation made up of fine, densely packed ridges approximately 2-3 mm in width.
Embryonic material is exposed in seven eggs, but ossified bones with identifiable morphologies are observed only in four eggs.
The late stage of development of the embryos and the close proximity of the adult to the eggs strongly suggests that the latter died in the act of incubating its nest, like its modern bird cousins, rather than laying its eggs or simply guarding its nest crocodile-style.
An oviraptorid specimen consisting of an adult skeleton preserved atop an embryo-bearing egg clutch: (a) photograph of the specimen; (b) interpretive drawing with bones and gastroliths in white and eggs color-coded by ring (A, red; B, green; C, blue); (c) restoration (white indicates bones preserved in the adult skeleton). Abbreviations: I – digit I; II – digit II; III – digit III; A# – egg in lowermost ring (A); as – astragalus; B# – egg in middle ring (B); C# – egg in uppermost ring (C); cav – caudal vertebra; ch – chevron; cv – cervical vertebra; di – manual digit; dr – dorsal ribs; dv – dorsal vertebra; em – egg known to preserve embryo; fe – femur; fi – fibula; ga – gastralium; gl – gastroliths; h – humerus; il – ilium; is – ischium; mt – metatarsal; O2 – egg sampled for oxygen isotope analysis; pb – pubis; pp – pedal phalanges; ra – radius; sl – semilunate carpal; ti – tibia; ul – ulna. Note that C11 and C12 are not paired eggs; the eggs that would have been paired with C11 and C12 are probably not preserved, as is the case for some other eggs and skeletal elements. Image credit: Bi et al., doi: 10.1016/j.scib.2020.12.018.
This kind of discovery, in essence fossilized behavior, is the rarest of the rare in dinosaurs, said Dr. Matthew Lamanna, a researcher in the Section of Vertebrate Paleontology at 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 researchers 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. This suggests that oviraptorid eggs in the same clutch might have hatched at slightly different times.
Known as asynchronous hatching, this characteristic appears to have evolved independently in oviraptorids and some modern birds.
Another interesting aspect of the specimen is that the adult preserves a cluster of gastroliths (stomach stones) in its abdominal region.
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.
Its extraordinary to think how much biological information is captured in just this single fossil, said Dr. Xing Xu, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology and the Center for Excellence in Life and Paleoenvironment at the Chinese Academy of Sciences.
Were going to be learning from this specimen for many years to come.
The findings appear in the journal Science Bulletin.
Shundong Bi et al. An oviraptorid preserved atop an embryo-bearing egg clutch sheds light on the reproductive biology of non-avialan theropod dinosaurs. Science Bulletin, published online December 16, 2020; doi: 10.1016/j.scib.2020.12.018