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.