The Bayan Shireh Formation (also known as Baynshiree/Baynshire, Baynshirenskaya Svita or Baysheen Shireh) is a geological formation in Mongolia, that dates to the Cretaceous period. It was first described and established by Vasiliev et al. 1959. Description The Bayan Shireh Formation is primarily composed by varicoloured claystones and sandstones with calcareous concretions and characterized by grey mudstones and yellowish-brown medium grained sandstones. Up to thick, the most complete sections are found in the eastern Gobi Desert, consisting of fine-grained, often cross-stratified gray sandstone interbedded with claystone and concretionary, intraformational conglomerates with relatively thick units of red to brown mudstone in the upper part. The Baynshire and Burkhant localities are mainly composed by mudstone, siltstone, sandstone, and conglomerates, with most of their sedimentation being fluvial. The environments that were present on the Bayan Shireh Formation consisted mainly on semi-arid climates with large water bodies, such as rivers or lakes. Although it is considered to be partially lacustrine, largescale cross-stratification in many of the sandstone layers at the Baynshire locality seem to indicate that a large meandering fluvial system was present. It has been implied that during the late Bayan Shirehnian times, large rivers with direct connections to the sea drained a prominent part of the eastern Gobi region. Stratigraphy According to Jerzykiewicz and Russell, the Bayan Shireh Formation can be divided into 2 informal units: upper and lower beds. Danilov and colleagues have suggested that the lower beds are Cenomanian to early Turonian, and the upper beds are late Turonian to Santonian in age. While the lower beds are composed by extensive conglomerate that indicates the ancient presence of very active rivers, the upper beds are mainly composed by mudstone and claystone that is interbedded by sandstone, indicating again, the presence of rivers but also lakes and other water bodies. Based on comparisons with other formations, Jerzykiewicz and Russell suggested that the Bayan Shireh paleofauna seems to correspond best with the Turonian through early Campanian stages of the Late Cretaceous, about 93 to 80 million years ago. However, examination of the magnetostratigraphy of the formation indicates that the entire Bayan Shireh lies within the Cretaceous Long Normal, which lasted only until the end of the Santonian stage, giving a possible Cenomanian through Late Santonian age, between 98 and 83 million years ago. In 2012, Averianov and Sues re-examined many formations from the Gobi Desert and using biostratigraphic occurrences and previous dating,