Ermaying Formation

The Ermaying Formation is a geological formation of Anisian (Middle Triassic) age in north-central China. It is found across much of the Ordos Basin, at outcrops within the provinces of Shaanxi, Shanxi, and Inner Mongolia. It is composed of up to 600 m thick sequence of mudstone and sandstone, overlying the Heshanggou Formation and underlying the Tongchuan Formation. In the southern part of the Ordos Basin, the Zhifang Formation is equivalent to the Ermaying Formation.

The Ermaying Formation is divided into two members, each with a distinctive assemblage of tetrapod fossils. A 2013 study used SHRIMP U-Pb radiometric dating to assign an imprecise age of 245.9 ± 3.2 Ma for the upper member. A 2018 study assigned a more precise age of around 243.53 Ma based on three ash samples near the base of the upper member. This would indicate that the Upper Ermaying Formation is no older than the late Anisian stage.

A few studies apply the name "Ermaying Formation" to a sedimentary unit in the Yanshan belt, a fold-thrust belt northeast of Beijing. In the Yanshan belt, reported exposures of the formation are dated to the Late Triassic, lying below the Early Jurassic Xingshikou Formation. The Yanshan belt exposures are also known as the Huzhangzi Formation, an alternative name proposed to reflect their chronological and geographic divergence from exposures in the Ordos Basin.

Paleobiota
The Ermaying Formation is notable for its diversity of well-preserved tetrapods.

The upper member occupies most of a biozone historically known as the Sinokannemeyeria fauna. This fossil assemblage has more recently been termed the Sinokannemeyeria-Shansisuchus Assemblage Zone, including approximately coeval sediments of the Kelamayi Formation in Xinjiang. The biozone also extends up to the early part of the Tongchuan Formation.

Tetrapod burrows are known from the formation, occupying both large and small size classes. The larger burrow is reniform (kidney-shaped) in cross section, about 13 cm in height and 30 cm in width. It shallowly slopes down when seen from the side and smoothly undulates when seen from above. Scatches and grooves are readily visible on the inside. The burrow-maker was medium-sized animal, likely a juvenile dicynodont. The smaller burrows are low tapered chambers with incomplete or collapsed entry ramps. They may have been dug out by procolophonids or juvenile cynodonts.

Synapsids
Apart from the taxa listed here, fossils of an unnamed genus of kannemeyeriiform dicynodont have been found in the Upper Ermaying Formation at the Sanjiao site in Shanxi Province. Not counting Shansiodon, this unnamed form is the third "kannemeyeriid" genus known from the Upper Member, as it is distinct from both Parakannemeyeria and Sinokannemeyeria.