A Cretaceous nursery
In the arid expanse of the Zaisan Basin, paleontologists have located the fossilized remains of vast dinosaur nesting colonies. These sites date to the Late Cretaceous period, likely the Maastrichtian age, just before the mass extinction event that ended the dinosaurs' reign. The fossils are preserved within the Tayzhuzgen Formation, a sequence of red-bed mudstone, siltstone, and sandstone up to 136 meters thick. During the Cretaceous, this area was not a desert but the shoreline of a massive inland body of water called paleo-Lake Zaysan.
The primary inhabitants of these nurseries were hadrosaurs, the large, plant-eating "duck-billed" dinosaurs. Like similar sites found in North America, the Zaisan grounds suggest colonial nesting behavior. Hadrosaurs likely returned to these same locations year after year to lay their eggs in relative safety. This strategy of nesting in large groups offered protection from predators through sheer numbers. Analysis of eggshell fragments reveals at least two distinct types: spheroolithid eggs, which are characteristic of ornithischian dinosaurs like hadrosaurs, and elongatoolithid eggs. The second type points to the presence of theropods, with some evidence suggesting the eggs of a large predator, possibly Tarbosaurus bataar, have also been found here.
An embryonic clock
The Zaisan nesting grounds contain clutches of eggs with rare, unhatched embryos still inside. These tiny skeletons show dinosaur development. Using high-resolution computed tomography (CT) scanners, scientists can digitally dissect the eggs without destroying them. The scans generate detailed 3D models of the embryonic bones, showing their precise stage of development at the time of death. This technology helps researchers reconstruct the growth sequence of the dinosaurs before they hatched.
This embryonic evidence helps for understanding dinosaur incubation. Early analysis of these and similar eggs suggested an incubation period of around 60 days, like that of modern crocodiles. More recent studies on other dinosaur species have refined this understanding by examining embryonic teeth. As teeth form, they create daily growth lines called von Ebner lines, which can be counted like tree rings. For one large hadrosaur, Hypacrosaurus, this method revealed an incubation period of nearly six months. A longer incubation, similar to that of reptiles and not the rapid hatching of birds, would have left nests vulnerable for extended periods and may have been a disadvantage for the dinosaurs when a cataclysm struck the planet 66 million years ago.