The vast family of oviraptorids flourished during the second half of the Cretaceous, between 90 and 66 million years ago. These dinosaurs were extremely similar to birds: they ran on two legs, were covered with feathers, had toothless beaks on their faces, and hatched offspring in nests. However, clutches of eggs are also left by crocodiles, which are also very close to extinct dinosaurs.
Their offspring hatch at the same time, whereas in modern birds the young usually hatch at different times, as maturation is complete. What about the oviraptorids? To find out, it is enough to look inside the egg. Unfortunately, dinosaur eggs have long been fossilized, so it's not easy to examine their contents - and even an X-ray won't work.
Paleontologists from Germany, Switzerland and China have used neutron sources operating at the MLZ accelerator center near Munich. They report their work in an article published in the journal Integrative Organismal Biology. The press release of the University of Bonn tells about it briefly.
Three eggs from the oviraptorid clutch / © Tzu-Ruei Yang
Thomas Engler and colleagues used specimens excavated in China's Jiangxi province in a single clutch, dated to about 67 million years old. Three eggs, roughly 18 centimeters long, were fossilized together, and biologists used neutron tomography to illuminate the slices and examine the extent of skeletal and ligamentous development in embryos.
Egg # 3, based on a series of photographs / © Jens Lallensack
A powerful stream of particles was able to break through the rock and allowed the visualization of minerals of different densities, including those that the bones of unborn oviraptorids have become over millions of years. The length and relative position of these bones showed that one of the eggs was laid earlier than a pair of others from the same clutch.
However, even in these two embryos were at different stages of development, which was confirmed by studying the microstructure of the samples. The fact is that as the embryos grow, the thickness of their shell gradually decreases, and the calcium from it is used to form bones.
This indicator also indicated differences in development in embryos in eggs from a common clutch, and all together, according to the authors of the work, unequivocally shows that oviraptorids were even closer to modern birds and even their offspring appeared, like theirs, at the same time.