ISSN 0031-0301, Paleontological Journal, 2013, Vol. 47, No. 11, pp. 1252–1269. © Pleiades Publishing, Ltd., 2013. 1252 1 INTRODUCTION The Late Cretaceous continental sediments of the Gobi Desert of Mongolia are famous for the spectacu- lar dinosaur and mammal fossils, but in recent times small but interesting bird fossils—both enantiorni- thines and ornithuromorphs—have been discovered from three stratigraphic levels: Barun Goyot and Dja- dokhta formations of Campanian age and Nemegt Formation of Maastrichtian age (Elzanowski, 1981; Kurochkin, 1996; Clarke and Norell, 2002; Chiappe et al., 2001). These formations have been considered to represent at least two distinct Late Cretaceous envi- ronments and vertebrate faunal horizons. The spec- tacular orange-red sediments of the Djadokhta/Barun Goyot were deposited in arid environment and have yielded protoceratopsids, armoured ankylosaurs, liz- ards, and mammals, but the overlying Nemegt beds 1 The article is published in the original. were laid down under more humid conditions and have produced massive dinosaurs such as hadrosaurs, sauropods, and tyrannosaurs (Novacek, 1996). Simi- lar to dinosaur distributions, these two horizons tend to show two distinct avialan assemblages; the lower two formations, Djadokhta and Barun Goyot are dominated by enantiornithines, whereas ornithuro- morphs birds are more common in the overlying Nem- egt Formation, represented by various taxa of Hesper- ornithes, a derived bird Teviornis, and several unnamed taxa (Clarke and Norell, 2004). Fragmen- tary and isolated remains represent many of the avi- alan taxa that make it difficult to ascertain their affin- ities. So far, Apsaravis is the only known ornithuro- morph bird from the Djadokhta Formation represented by excellent postcranial material (Clark and Norell, 2002). Fossils of Mesozoic birds are extremely rare from the Gobi Desert, and embryonic skeletal remains are even more so. The fossil record of avian embryos is An Embryonic Enantiornithine Bird and Associated Eggs from the Cretaceous of Mongolia 1 E. N. Kurochkin a , S. Chatterjee b , and K. E. Mikhailov a a Borissiak Paleontological Institute, Russian Academy of Sciences, 123 Profsojuznaya Street, Moscow GSP-7, 117868 Russia b Museum of Texas Tech University, Lubbock, Texas 79409-3191, USA e-mail: sankar.chatterjee@ttu.edu Received May 27, 2012 Abstract—Enantiornithes is the most speciose clade of Cretaceous birds, but many taxa are known from iso- lated postcranial skeletons. Two embryonic enantiornithine bird skeletons of Gobipipus reshetovi gen. et sp. nov. from the Upper Cretaceous (Campanian) Barun Goyot Formation of the Gobi Desert in Mongolia provide new insights into the anatomy, radiation, and mode of development of early avialans. In recent times, both enantiornithine and ornithuromorph birds are known from the Barun Goyot Formation as well as from the Djadokhta and Nemegt Formations. The 80-million-year-old Gobipipus skeletons encased within eggshells shows several features characteristic of enantiornithine birds. The wing skeleton and shoulder girdle show morphological features indicating that Gobipipus achieved sophisticated powered flight. Gobipipus reshetovi gen. et sp. nov. is quite distinct from the sympatric enantiornithine species Gobipteryx minuta from the same strata in many anatomical features. Phylogenetic analysis of 26 avialan ingroup taxa based on distribution of 202 characters indicate that Gobipipus is a basal member of enantiornithine birds along with Confuciusornis and shares more characters with ornithuromorphs than previously recognized. The embryonic nature of Gobipipus specimens sheds new light on the developmental history of enantiornithine birds. The well-ossified bones of the fore- and hind limbs, and fusion of many skeletal elements indicate a precocial mode of devel- opment in Gobipipus. Apparently Gobipipus hatchlings could walk away from the ground nests as soon as they emerged from their eggs. The asymmetry of egg poles are unique features of Gobipipus eggs (oogenus Gobioolithus) among Cretaceous avialans. The microstructure of the shell in Gobioolithus eggs with the embryos of Gobipipus is typical avian (of ornithoid basic type) and less ratite-like in morphology of the spongy layer than is that in the other possible egg-remains of enantiornitine birds (oofamily Laevisoolithidae). Keywords: embryos and eggs, enantiornithine bird, Gobipipus, Gobioolithus, Upper Cretaceous, Barun Goyot Formation, Gobi Desert DOI: 10.1134/S0031030113110087