Gobioolithus is an oogenus of fossil bird egg native to Mongolia. They are small, smooth-shelled, and elongated eggs that were first discovered in the 1960s and early 70s during a series of fossil-hunting expeditions in the Gobi desert. Two oospecies have been described: Gobioolithus minor and G. major. The eggs were probably laid in colonial nesting sites on the banks of rivers and lakes. G. minor is unusual because it frequently is found with embryonic skeletons of the enantiornithine bird Gobipipus. These embryos have well-developed wings, which suggest they would be able to fly very soon after hatching, unlike most modern birds. Description Gobioolithus eggs are small and smooth-shelled. They are asymmetrically shaped, similar to many modern bird eggs, with one end pointier than the other. The two oospecies are distinguished mainly by their size: G. major ranges from 50 to 53.5 mm long and 25 to 32 mm across, with an eggshell thickness of 0.2-0.4 mm, whereas G. minor is only 30–46 mm by 20–24 mm and 0.1-0.2 mm thick. The microstructure of Gobioolithus''' eggshell has not been thoroughly studied, and heavy recrystallization of most specimens makes it difficult to examine the eggshell structure or pore system. The eggshell consists of two (or possibly three) structural layers. The inner layer, called the mammillary layer, is about half the thickness of the outer, or continuous, layer. On the outside, many specimens have a recrystallized outer layer. This could simply due to diagenesis or it could be a true external zone, which is a third layer present in most bird eggs but is rare in non-avian dinosaurs. However, a few specimens are unaffected by recrystallization. These do not have a third layer, but this does not rule out the possibility that the eggshell originally had a three layers since the external layer can easily separate from the rest of the eggshell. These specimens also reveal an angusticanaliculate pore system, which means that the pores have a low density, and are long, narrow, and straight.Styloolithus, another fossil enantiornithine egg from the Gobi, differs from Gobioolithus in that it is larger and has a thicker eggshell with a proportionately smaller mammillary layer. Laevisoolithids, which are also eggs of enantiornithines, are also larger than Gobioolithus, but they have a much thicker mammillary layer. Embryos Many Gobioolithus minor specimens contain embryonic remains of the enantiornithine genus Gobipipus. The embryos have well-ossified skeletons, implying that they