An international team led by Prof. BI Shundong from Indiana University of Pennsylvania and Dr. XU Xing from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences, for the first time, discovered a dinosaur preserved sitting atop a nest of its own eggs with fossilized babies inside.
The related study was published in Science Bulletin on Dec. 16.
Co-interim director and lead dinosaur paleontologist Dr. Matt Lamanna from Carnegie Museum of Natural History also participated in the study.
"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," said Dr. BI.
The new specimen was recovered from uppermost Cretaceous-aged rocks, some 70 million years old, in Ganzhou City in southern China's Jiangxi Province. It belongs to oviraptorosaur, a group of bird-like theropod dinosaurs that thrived during the Cretaceous Period, the third and final time period of the Mesozoic Era (commonly known as the 'Age of Dinosaurs') that extended from 145 to 66 million years ago.
The fossil consists of an incomplete skeleton of a large, presumably adult oviraptorid crouched in a bird-like brooding posture over a clutch of at least 24 eggs. At least seven of these eggs preserved bones or partial skeletons of unhatched oviraptorid embryos inside.
The late stage of development of the embryos and the close proximity of the adult to the eggs suggested 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 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 by ZHAO Chuang)
"This kind of discovery - in essence, fossilized behavior - is the rarest of the rare in dinosaurs," said Dr. Lamanna. "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 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."
Oxygen isotope analyses indicated 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 appeared to be more mature than others, which in turn suggested that 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.
One interesting aspect of the new oviraptorid specimen is that the adult preserved a cluster of pebbles in its abdominal region. These are almost certainly gastroliths, or "stomach stones," rocks that had 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.
"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," said Dr. XU.
The ~70-million-year-old fossil in question: an adult oviraptorid theropod dinosaur sitting atop a nest of its eggs. Multiple eggs (including at least three that contain embryos) are clearly visible, as are the forearms, pelvis, hind limbs, and partial tail of the adult. (Image by BI Shundong, Indiana University of Pennsylvania)
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