# 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.

#### Artiodactyls

Artiodactyls reported from the Ergilin Dzo Formation :
| Genus | Species | Presence | Material | Notes |
| Anthracotheriidae indet. | Indeterminate | Ergilin Dzo locality. | Left astragalus. | An anthracothere. |
| cf. Bothriodon | cf. B. sp. | Ergilin Dzo & Khoer Dhan localities. | Jaw & limb elements. | An anthracothere. |
| Brachyhyops | B. trofimovi | Khoer Dzhan locality. | Jaw elements. | An entelodontid. |
| Brachyhyops | B. sp. | Ergilin Dzo locality. | Right maxilla fragment. | An entelodontid. |
| Entelodon | E. gobiensis | Khoer Dzhan & Ergilin Dzo localities. | | An entelodontid. |
| Eumeryx | E. sp. | | | A deer. |
| Gobiomeryx | G. dubius | | | A leptomerycid. |
| Lophiomeryx | L. angarae | | | A lophiomerycid. |
| Lophiomeryx | L. sp. | | | A lophiomerycid. |
| Miomeryx | M. altaicus | | | A leptomerycid. |
| Miomeryx | M. sp. | | | A leptomerycid. |
| Praetragulus | P. electus | Khoer Dzhan locality. | Numerous jaw & limb elements. | A hypertragulid. |
| Praetragulus | P. gobiae | Ergilin Dzo locality. | Jaw elements. | A hypertragulid. |

#### Cimolestans

Cimolestans reported from the Formation :
| Genus | Species | Presence | Stratigraphic member | Material | Notes | Images |
| Gobiopithecus | G. khan | Khoer Dzhan locality. | Ergilin Member. | Mandibles. | A pantolestid. | |

#### Didymoconids

Didymoconids reported from the Ergilin Dzo Formation :
| Genus | Species | Presence | Stratigraphic member | Material | Notes | Images |
| Ardynictis | A. furunculus | | | Skull & jaws. | | |
| Ergilictis | E. reshetovi | Ergilin Dzo locality. | | Jaw elements. | | |

#### Eulipotyphlans

Eulipotyphlans reported from the Ergilin Dzo Formation :
| Genus | Species | Presence | Stratigraphic member | Material | Notes |
| Amphechinus | A. sp. | Ergilin Dzo locality. | Sevkhul Member. | Left dentary fragment. | A hedgehog. |
| Ictopidium? | I.? sp. | | | | A changlelestid. |
| Oligochenus | O. grandis | Ergilin Dzo locality. | Sevkhul Member. | Right dentary fragment. | A gymnure. |
| Palaeoscaptor? | P.? sp. | | | | A hedgehog. |
| Soricinae indet. | Indeterminate | | | | A shrew. |
| Tupaiodontinae indet. | Indeterminate | Ergilin Dzo locality. | Ergilin Member. | Premolar. | A changlelestid. |

#### Ferae

Ferae reported from the Ergolin Dzo Formation :
| Genus | Species | Presence | Material | Notes |
| Alagtsavbaatar | A. indigenus | Alag Tsav & Khoer Dzhan localities. | Jaw & limb elements. | A small feliform. |
| Amphicyonidae indet. | Indeterminate | Ergilin Dzo locality. | Tooth. | A bear-dog. |
| Asiavorator | A. gracilis | Ergilin Dzo locality. | Right mandible fragment. | A small feliform. |
| Eofelis | E. sp. | Khoer Dzhan locality. | Jaw elements. | A nimravid. |
| Hyaenodon | H. chunkhtensis | Khoer Dzhan locality. | Left mandibular corpus. | A hyaenodont. |
| Hyaenodon | H. eminus | Khoer Dzhan & Ergilin Dzo localities. | Jaw elements. | A hyaenodont. |
| Hyaenodon | H. gigas | Khoer Dzhan locality. | | A hyaenodont. |
| Hyaenodon | H. incertus | Khoer Dzhan & Ergilin Dzo localities. | | A hyaenodont. |
| Hyaenodon | H. mongoliensis | Khoer Dzhan & Ergilin Dzo localities. | | A hyaenodont. |
| Hyaenodon | H. pervagus | Khoer Dzhan & Ergilin Dzo localities. | | A hyaenodont. |
| Hyaenodon | H. pumilus | | Mandibular ramus. | A hyaenodont. |
| Nimravus | N. mongoliensis | Khoer Dzhan locality. | Jaw & limb elements. | A nimravid. |
| Proviverrinae indet. | Indeterminate | Khoer Dzhan locality. | Tooth. | A hyaenodont. |

#### Gliriformes

Gliriformes reported from the Ergilin Dzo Formation :
| Genus | Species | Presence | Material | Notes |
| Ardynomys | A. olseni | | Lower jaw. | A rodent. |
| Ardynomys | A. silentiumis | | | A rodent. |
| Ardynomys | A. sp. | | | A rodent. |
| Desmatolagus | D. ardynense | | | A lagomorph. |
| Desmatolagus | D. gobiensis | | | A lagomorph. |
| Desmatolagus | D. robustus | | Lower jaws. | A lagomorph. |
| Eucricetodon | E. sp. | | | A rodent. |
| Lagomorpha indet. | Indeterminate | | | A lagomorph. |
| Zofiagale | Z. ergilensis | Ergilin Dzo locality. | Right dentary. | An anagalid. |

#### Mesonychians

Mesonychians reported from the Ergilin Dzo Formation :
| Genus | Species | Presence | Stratigraphic member | Material | Notes | Images |
| Metahapalodectes | M. sp. | | | | A hapalodectid. | |
| Mongolestes | M. hadrodens | | | | A mesonychid. | |

#### Perissodactyls

Perissodactyls reported from the Ergilin Dzo Formation :
| Genus | Species | Presence | Stratigraphic member | Material | Notes |
| cf. Allacerops | cf. A. sp. | Khoer Dzhan. | | Jaw elements. | An eggysodontid. |
| Amynodon | A. sp. | | | | An amynodont. |
| Amynodontopsis | A. lunanensis | | | | An amynodont. |
| Ardynia | A. praecox | Khoer Dzhan. | Sevkhul & Ergilin members. | Jaw elements. | A hyracodontid. |
| Ardynia | A. sp. | Khoer Dzhan. | Sevkhul & Ergilin members. | Jaw fragments. | A hyracodontid. |
| Armania | A. asiana | Khoer Dzhan & Ergilin Dzo. | Sevkhul & Ergilin Members. | Jaw elements. | A hyracodontid. |
| Brontotheriidae indet. | Indeterminate | | | | A brontothere. |
| Cadurcodon | C. ardynensis | | | | An amynodont. |
| Cadurcodon | C. sp. | | | | An amynodont. |
| Cadurcotherium | C. progressus | | | | An amynodont. |
| cf. Deperetella | cf. D. sp. | | | | A deperetellid. |
| Embolotherium | E. andrewsi | | | Skull. | A brontothere. |
| Embolotherium | E. grangeri | Khoer Dzhan. | | Partial skull. | A brontothere. |
| Embolotherium | E. sp. | | | | A brontothere. |
| Eomoropus | E. sp. | | | | An eomoropid. |
| Eubrontotherium | E. clarnoensis | Ergilin Dzo. | | Partial skeleton. | A brontothere. |
| Juxia | J. borissiaki | Ergilin Dzo locality. | Ergilin Member. | Jaws & teeth. | A paraceratheriid. |
| cf. Nasamplus | cf. N. progressus | | | | A brontothere. |
| Paraceratheriidae indet. | Indeterminate | Khoer Dzhan. | Ergilin Member. | Teeth. | Initially reported as Urtinotherium (or Paraceratherium) sp. |
| Paraceratherium | P. parvum | | | | A paraceratheriid. |
| Paracolodon | P. inceptus | | | | A helaletid. |
| Paracolodon | P. sp. | | | | A helaletid. |
| Prohyracodon | P. meridionalis | Khoer Dzhan. | Sevkhul Member. | Jaw elements. | A hyracodontid. |
| Prohyracodon | P. obrutschewi | | | | A hyracodontid. |
| Prohyracodon | ?P. parvum | Khoer Dzhan. | Zangut-Sevkhul member. | Jaw elements. | A hyracodontid. |
| Protembolotherium | P. efremovi | Ergilin Dzo. | | Skull elements. | A brontothere. |
| Ronzotherium | R. brevirostris | | | | A rhino. |
| Ronzotherium | R. orientale | Ergilin Dzo, Bayan Tsav ravine & Khoer Dzhan. | Ergilin Member. | Jaw elements. | A rhino. |
| Ronzotherium | R. sp. | Khoer Dzhan. | Ergilin Member. | Fragment of mandibular ramus. | A rhino. |
| Schizotherium | S. avitum | | | | A chalicothere. |
| Sharamynodon | S. mongoliensis | | | | An amynodont. |
| Teleolophus | T. magnus | | | | A deperetellid. |
| cf. Zaisanamynodon | cf. Z. sp. | | | | An amynodont. |

### Reptiles

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

#### Birds

Birds reported from the Ergilin Dzo Formation :
| Genus | Species | Presence | Material | Notes |
| Accipitridae indet. | Indeterminate | Khoer Dzhan. | Leg bones. | An accipitrid. |
| Anatidae indet. | Indeterminate | Khoer Dzhan. | | An anatid. |
| Ardeidae indet. | Indeterminate | Khoer Dzhan. | | A heron. |
| Cygninae indet. | Indeterminate | Khoer Dzhan. | | A swan. |
| Eogrus | E. sp. | Ergilin Dzo. | | An eogruid. |
| Ergilornis | E. minor | Ergilin Dzo. | | An eogruid. |
| Ergilornis | E. rapidus | Ergilin Dzo. | | An eogruid. |
| Ergilornis | E. sp. | | | An eogruid. |
| Lari indet. | Indeterminate | Khoer Dzhan. | Skull fragment. | A shorebird. |
| Rallidae indet. | Indeterminate | Khoer Dzhan. | | A rail. |
| Sonogrus | S. gregalis | | | An eogruid. |

#### Crocodyliforms

Crocodyliforms reported from the Ergilin Dzo Formation :
| Genus | Species | Presence | Stratigraphic member | Material | Notes | Images |
| Crocodyliformes indet. | Indeterminate | Ergilin Dzo. | | Tooth. | | |

#### Squamates

Squamates reported from the Ergilin Dzo Formation :
| Genus | Species | Presence | Stratigraphic member | Material | Notes | Images |
| Agamidae indet. | Indeterminate | | | | An agamid lizard. | |
| Lacertilia indet. | Indeterminate | | | | A lizard. | |

#### Turtles

Turtles reported from the Ergilin Dzo Formation :
| Genus | Species | Notes |
| Chrysemys | C. sp. | |
| Emydidae gen. et. sp. indet. | Indeterminate | A terrapin. |
| Ergilemys | E. insolitus | A tortoise. |
| ?Melanochelys | ?M. elongata | |
| Platysternidae gen. et. sp. indet. | Indeterminate | |
| Trionychidae gen. et. sp. indet. | Indeterminate | A softshell turtle. |

### Amphibians

Amphibians reported from the Ergilin Dzo Formation :
| Genus | Species | Presence | Stratigraphic member | Material | Notes | Images |
| Urodela gen. et. sp. indet. | Indeterminate | | | | A salamander. | |

### Fish

Fish reported from the Formation :
| Genus | Species | Presence | Stratigraphic member | Material | Notes | Images |
| Amia | A. sp. | | | | A bowfin. | |
| Parasilurus | P. sp. | | | | A catfish. | |