The Ergilin Dzo Formation (formerly known as the Ardyn Obo Formation), is a

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

in southeastern

Mongolia Mongolia is a landlocked country in East Asia, bordered by Russia to the north and China to the south and southeast. It covers an area of , with a population of 3.5 million, making it the world's List of countries and dependencies by po ...

. It preserves

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

of a wide variety of animals, which date back to the

late Eocene The Priabonian is, in the International Commission on Stratigraphy, ICS's geologic timescale, the latest age (geology), age or the upper stage (stratigraphy), stage of the Eocene epoch (geology), Epoch or series (stratigraphy), Series. It spans ...

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

brontothere Brontotheriidae is a family of extinct mammals belonging to the order Perissodactyla, the order that includes horses, rhinoceroses, and tapirs. Superficially, they looked rather like rhinos with some developing bony nose horns, and were some of ...

s 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

primate Primates is an order (biology), order of mammals, which is further divided into the Strepsirrhini, strepsirrhines, which include lemurs, galagos, and Lorisidae, lorisids; and the Haplorhini, haplorhines, which include Tarsiiformes, tarsiers a ...

s and rarity of

crocodyliform Crocodyliformes is a clade of Crurotarsi, crurotarsan archosaurs, the group often traditionally referred to as "crocodilians". They are the first members of Crocodylomorpha to possess many of the features that define later relatives. They are the ...

s (known from one specimen currently) in this formation may suggest the

paleoenvironment Paleoecology (also spelled palaeoecology) is the study of interactions between organisms and/or interactions between organisms and their environments across geologic timescales. As a discipline, paleoecology interacts with, depends on and informs ...

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 A floodplain or flood plain or bottomlands is an area of land adjacent to a river. Floodplains stretch from the banks of a river channel to the base of the enclosing valley, and experience flooding during periods of high Discharge (hydrolog ...

environment with a

braided stream A braided river (also called braided channel or braided stream) consists of a network of river channels separated by small, often temporary, islands called '' braid bars'' or, in British English usage, '' aits'' or ''eyots''. Braided streams t ...

network formed by fluvial systems.

Fossil content

In their review of the

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

fauna of the formation, Tsubamoto ''et al.'' (2022) found that 77

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

and 104 species of vertebrate have been reported from the Ergilin Dzo Formation, representing 22

orders Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * H ...

and 49

families Family (from ) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictability, structure, and safety as ...

. 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

perissodactyl Perissodactyla (, ), or odd-toed ungulates, is an order of Ungulate, ungulates. The order includes about 17 living species divided into three Family (biology), families: Equidae (wild horse, horses, Asinus, asses, and zebras), Rhinocerotidae ( ...

s being the most diverse group (33 species reported).

Ruminant Ruminants are herbivorous grazing or browsing artiodactyls belonging to the suborder Ruminantia that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microb ...

artiodactyls,

rhinocerotoid Rhinocerotoidea is a superfamily (taxonomy), superfamily of Perissodactyla, perissodactyls that appeared 56 million years ago in the Paleocene. They included four extinct families, the Amynodontidae, the Hyracodontidae, the Paraceratheriidae, an ...

s and

s are the most abundant groups in terms of collected specimen numbers.

Artiodactyls

Cimolestans

Didymoconids

Eulipotyphlans

Ferae

Gliriformes

Mesonychians

Perissodactyls

Reptiles

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

Birds

Crocodyliforms

Squamates

Turtles

Amphibians

Fish

References

{{reflist Geologic formations of Mongolia Paleogene System of Asia Eocene Asia Paleontology in Mongolia