Extraordinary preservation in a new vertebrate assemblage from the Late Cretaceous of Mongolia

WE report here a new locality, Ukhaa Tolgod ('brown hills'), from the Upper Cretaceous of the Gobi Desert of Mongolia, which shows an unmatched abundance of well preserved vertebrate fossils, including the highest concentration of mammalian skulls and skeletons from any Mesozoic site. In the main collecting area (about 4 km²), recovered and uncollected articulated skeletons of thero-pod, ankylosaurian and protoceratopsian dinosaurs represent over 100 individuals. Specimens collected also include skulls (many with associated skeletons) of over 400 mammals and lizards, skeletons (including the first known skull) of the bird Mononykus, and nest sites that preserve the first known theropod dinosaur embryos¹. In contrast to other Mesozoic localities, the diversity and abundance of theropods, mammals and lizards are unusually high. The exceptional preservation of vertebrates from the red-bed facies of the Gobi Upper Cretaceous has been attributed to arid conditions2-4, possibly involving catastrophic death and burial during major sandstorms⁵. Although fossils are found in fluvial facies at Ukhaa Tolgod, high concentrations of excellent specimens in aeolian facies support the argument for rapid entombment in sand. This contrasts with conditions for the terrestrial Upper Cretaceous in North and South America, where accretionary preservation of fossils in fluvial deposits predominates.