A Cretaceous nursery
The Erlian Basin in Inner Mongolia, China, is a location for Late Cretaceous fossils. The region, sometimes called "Dinosaur City," has yielded thousands of dinosaur eggs, showing the reproductive habits of these animals. The discoveries are concentrated in formations of Cretaceous sandstone, where nests containing multiple eggs have been found buried. One nest discovered in 1989 held 13 well-preserved eggs arranged in a distinct pattern, with the large ends pointing inward and the small ends facing out.
The geological conditions of the Erlian Basin during the Early Cretaceous were shaped by crustal extension, forming a series of faulted lake basins. This environment, filled with sandy conglomerates and volcanic rock, proved ideal for the rapid burial and fossilization of dinosaur nests. The eggs found here are often oval-shaped, with some measuring approximately 142 millimeters long and 138 millimeters wide. These fossils date back to a period of intense geological activity that preserved them for millions of years.
Peeking inside the eggs
The scientific value of the Erlian Basin eggs is their quantity and arrangement. Some of these fossils contain embryonic remains, showing dinosaur development. While not specific to Erlian, advanced imaging techniques like synchrotron X-ray scanning have been used on similar dinosaur embryos to create detailed 3D reconstructions of their skeletons, down to the level of individual bone cells. This non-destructive method allows paleontologists to study the fragile structures without damaging the fossil.
These scans reveal the developmental stages of dinosaurs before they hatched. Studies on other dinosaur embryos, such as Massospondylus, have shown two distinct types of teeth present in the jaw. One set of simple, triangular teeth would have been resorbed or shed before hatching, a trait seen in modern geckos and crocodiles. The second set represented the teeth the dinosaur would have used after emerging from the egg. This level of detail provides a direct comparison between the embryonic development of dinosaurs and their living relatives, like birds and crocodiles. The local Erenhot Dinosaur Museum displays many of the fossils found in the area, including skeletons of Gigantoraptor erlianensis and numerous dinosaur eggs.