Since a fossilized egg is rock, both inside and out, it's not so easy to discover what's inside. Sometimes scientists are lucky enough to discover hatchlings fossilized beside eggs, so they can assume that the embryos inside are very similar. In other cases, natural conditions have caused fossilized eggs to break open and expose the material within. CAT scans can be used on whole eggs to discover what's in them. The whole eggs can also be soaked in mild acid that will, over the course of a year, eat away the eggshell and reveal whether there's an embryo inside.

By their very nature, the discovery of fossilized eggs is always cause for excitement among paleontologists. The eggs can reveal much about how the creatures who left the lived. In 2004, for example, a total of three fossilized eggs from pterosaurs were discovered in China (two eggs) and Argentina. The Argentine egg is 100 million years old, and its location among many other fossils of the same species -- ranging in age from young to adult -- told paleontologists that Pterodaustro guinazui lived in communities that nested in colonies and took care of their young after hatching. The creatures bore a resemblance to modern-day flamingoes. The site where the egg was found, Loma del Pterodaustro, contained thousands of fossils that had been retrieved, all of them, save a couple of fish fossils, being  Pterodaustro pterosaurs. This led scientists to conclude the site, once likely filled with salty lakes, was suitable only to that creature [source: Roach].

Both Chinese eggs were pterosaurs that lived about 121 million years ago. They were found in the fossil-rich Yixian formation, where 100 million years ago pterosaurs may have nested in the area replete with lakes. Paleontologists estimate the egg shells were similar to that of a crocodile or turtle [source: Roach].