Shara Murun Formation

The deposits of the Shara Murun Formation are situated in the Erlian Basin, a structural basin located in central Inner Mongolia in northern China,^[1][3][4] close to the border between China and Mongolia.^[3][4] The first study on the Erlian Basin was published by the Russian geologist Vladimir Obruchev in 1892.^[4] The basin was later extensively studied by the Central Asiatic Expeditions of the American Museum of Natural History,^[1][4] over the course of five field seasons^[4] in the 1920s.^[1] The researchers of the Central Asiatic Expeditions were responsible for naming most of the formations in the basin, including the Shara Murun Formation.^[4] Later studies and field expeditions in the Erlian Basin were conducted by the Sino-Soviet Paleontological Expedition in 1959–1960, the Regional Geological Survey of the Nei Mongol Autonomous Region in the 1960s and 1970s, and the Institute of Vertebrate Paleontology and Paleoanthropology, American Museum of Natural History, and Carnegie Museum of Natural History since the 1980s.^[4]

The Erlian Basin preserves near-continuous sedimentary layers from the late Paleocene to the early Oligocene.^[1][4] The Shara Murun Formation preserves strata of Middle Eocene age.^[1] It overlies the Tukhum Formation and underlies the Ulan Gochu Formation.^[4]

Because the Eocene sediments in the Erlian Basin are near-continuous, there has been much debate historically over where certain stratigraphic units end and others begin.^[3] The Asian land mammal ages (ALMA) of the Eocene are mainly based on the different fossil faunas found in the Erlian Basin;^[1][4] the fauna of the Shara Murun Formation is the type fauna of the Sharamurunian ALMA.^[1] Debate on the stratigraphy have complicated reconstructions of the Asian Eocene, since definitions and interpretations of the stratigraphic units impact the definitions of the land mammal ages.^[1][3] In some cases, different researchers have used entirely different frameworks and stratigraphic unit names.^[3] Much work has been done in the 21st century to establish a functional framework, though some stratigraphic issues still remain.^[1]

The Erlian Basin experienced a subtropical and humid climate during the Eocene. The vegetation was highly diverse and dominated by deciduous trees and shrubs.^[5]