Ergilin Dzo Formation

The Ergilin Dzo Formation (formerly known as the Ardyn Obo Formation), is a geologic formation in southeastern Mongolia. It preserves fossils of a wide variety of animals, which date back to the late Eocene epoch. The type fauna of the Ergilian Asian land mammal age is defined by the mammalian fossils of this formation.

Depositional environment
Based on the presence of brontotheres and the abundance of herbivorous low-crowned mammals, the Ergilin Dzo Formation has been estimated to be humid and warm/hot with a relatively-closed area. However, the absence of primates and rarity of crocodyliforms (known from one specimen currently) in this formation may suggest the paleoenvironment had open areas and was more arid than similarly aged faunal assemblages from elsewhere in Eurasia. Sedimentary analyses suggest the Ergilin Dzo Formation was a floodplain environment with a braided stream network formed by fluvial systems.

Fossil content
In their review of the vertebrate fauna of the formation, Tsubamoto et al. (2022) found that 77 genera and 104 species of vertebrate have been reported from the Ergilin Dzo Formation, representing 22 orders and 49 families. Fossils of mammals, reptiles (including birds), fish and amphibians have all been recovered from the formation since the early 20th Century.

Mammals
81 mammal species have been found in the Ergilin Dzo Formation, with perissodactyls being the most diverse group (33 species reported). Ruminant artiodactyls, rhinocerotoids and brontotheres are the most abundant groups in terms of collected specimen numbers.

Reptiles
Reptiles are represented by 20 known species in the formation, with 11 of these being birds.