# Hsanda Gol Formation

The Hsanda Gol Formation is a geological formation located in the Tsagan Nor Basin of central Mongolia that dates to the Rupelian age of the Oligocene epoch. It formed in continental desert-like conditions and is notable for its fossil mammals, first excavated by Roy Chapman Andrews and the American Museum of Natural History in the 1920s. Hsanda Gol is included in the Hsandagolian Asian Land Mammal Age, to which it gives its name.

## Geology

The 2600 feet thick Hsanda Gol formation is covered in various places by Miocene-age variegated clays, sands, and gravels. It rests on top of the Early Cretaceous Hühteeg Svita formation. The lower portions of the formation are composed of yellow conglomerate, with the higher areas being dominated by red clays, silts and sands, as well as lava and basalt flows.
Hsanda Gol was traditionally assigned to the Middle Oligocene, but after the reassignation of the preceding Ergilian age to the Late Eocene it has now been rendered early Oligocene. It is followed after a faunal hiatus by the overlaying Loh Formation which extends from the Late Oligocene into the Miocene and also contains fossil mammals. Paleomagnetism in underlying and overlaying lava flows establish the beginning of the formation at the end of the magnetic Chron C13n 33.4 million years ago, and its end at the Chron C12n, 31 million years ago.
The Hsanda Gol levels rest directly over, and are continuous with the earlier Houldjin Gravels which are sometimes counted as part of the formation, but were rather deposited during the latest Eocene under wetter and more energetic conditions. In general, the Houldjin Gravels and the Hsanda Gol formation show a transition from more humid, forested conditions to an arid climate with presence of dunes and ephemeral rivers. Some taxa lived through this change including hyaenodonts and indricotheres, but others, such as brontotheres, amynodonts, rhinocerotids and entelodonts went extinct and are as a result absent from Hsanda Gol proper. The following Loh Formation, on the other hand, represents a return to more benign conditions as indicated by the reappearance of rhinocerotids and the entry of the first chalicotheres.

## Paleobiota

Hsanda Gol's paleobiota resembles current desert mammal faunas, including the one found in the same area today. It is composed largely of rodents and lagomorphs with high-crowned teeth adapted to chew hard vegetation; several of these species are fossorial and none arboreal, indicating that there were no areas with high tree density. The only large browsing ungulate, Paraceratherium, was probably transient and there were no medium-large herbivores. Predators were diverse but generally small, with the largest (Hyaenodon) being wolf-sized, and were adapted to hunt by ambush. Landscape would appear as a sparse plain with few tall trees near rivers or where groundwater could be found otherwise. Pallinology studies in contemporary sites of China and Mongolia are consistent with an arid-adapted woody scrubland dominated by Mormon tea and salt-tolerant nitre bush. The few trees were broad-leaved and deciduous, related to modern elm, birch, and oak.

### Amphibians

Amphibians of the Hsanda Gol Formation :
| Genus | Species | Location | Stratigraphic Position | Abundance | Notes |
| Macropelobates | M. osborni | | | | A frog. |

### Reptiles

Reptiles of the Hsanda Gol Formation :
| Genus | Species | Location | Stratigraphic Position | Abundance | Notes |
| Crythiosaurus | C. mongoliensis | | | | An amphisbaenid. |

### Mammals
#### Didymoconids

Didymoconids of the Hsanda Gol Formation :
| Genus | Species | Location | Stratigraphic Position | Abundance | Notes |
| Didymoconus | D. berkeyi | West Tatal Gol Loh Tatal Gol Field 538 | | | D. berkeyi is the only species present west of Tatal Gol, and D. colgatei is the only present at Field 538. They appear together in the others. Though not believed to have modern descendants or ecological equivalents, they present an enlargement of the mastoid region convergent with kangaroo rats and sengis found in semi-arid areas. |
| Didymoconus | D. colgatei | West Tatal Gol Loh Tatal Gol Field 538 | | | D. berkeyi is the only species present west of Tatal Gol, and D. colgatei is the only present at Field 538. They appear together in the others. Though not believed to have modern descendants or ecological equivalents, they present an enlargement of the mastoid region convergent with kangaroo rats and sengis found in semi-arid areas. |

#### Erinaceids

Erinaceids of the Hsanda Gol Formation :
| Genus | Species | Location | Notes |
| Amphechinus | A. rectus | Loh | Ecologically similar to the modern long-eared hedgehog. |
| Exallerix | E. hsandagolensis | Tatal Gol(?) | Without modern ecological equivalent. |
| Palaeoscaptor | P. acridens | Northeast Kholobolchi Nor Loh Field 538 | Also similar to the long-eared hedgehog. |
| Tupaiodon | T. morrisi ?T. minutus | Loh (both) | Similar to the bicolored shrew. |

#### Rodents

Rodents of the Hsanda Gol Formation :
| Genus | Species | Location | Abundance | Notes |
| Cricetops | C. dormitor dormitor C. d. elephantus | SW Loh East Loh NE Kholobolchi Nor Loh Tatal Gol Field 538 | Most common mammal, accounting for around one third of all jaws and teeth in the formation. | A cricetid similar to a gerbil or vole. Both subspecies appear together except northeast from Kholobolchi Nor, where only C. d. dormitor is present. |
| Cyclomylus | C. lohensis | Loh Tatal Gol Field 538 | | A fossorial cylindrodontid anatomically similar to a mole-rat, but ecologically closer to the Tarbagan marmot. Synonymous with Pseudotsaganomys. |
| Eumys | E. asiaticus | East Loh Loh Tatal Gol Field 538 | | A cricetid ecologically similar to a wild house mouse. |
| Karakoromys | K. decessus | SW Loh Loh Tatal Gol | | A gundi ecologically similar to Kozlov's pygmy jerboa. |
| ?Leptotataromys | Indeterminate | Loh Tatal Gol | | A gundi possibly synonymous with Karakoromys. |
| ?Plesispermophilus | ?P. lohiculus | Tatal Gol | | A sciuromorph related to Paramys. |
| cf. Prosciurus | Indeterminate | Tatal Gol | | A relative of Plesispermophilus. |
| Pseudocylindrodon | Indeterminate | Tatal Gol | | A cylindrodontid. |
| Selenomys | S. mimicus | SW Loh East Loh Loh Tatal Gol | | A fossorial relative of the mountain beaver. Possibly ecologically equivalent to the jerboas of the genus Allactaga. |
| Tataromys | T. deflexus T. plicidens T. sigmodon | West Tatal Gold NE Kholobolchi Nor Loh Tatal Gol Field 538 | | Gundis. T. sigmodon appears only at Loh, and T. deflexus only at and west of Tatal Gol. |
| Tachyoryctoides | T. obrutschewi T. pachygnathus | West Tatal Gol (both) | | Synonym of Aralomys. A fossorial relative of bamboo rats. |
| Tsaganomys | T. altaicus | West Tatal Gol East Loh NE Kholobolchi Nor Loh Tatal Gol Field 538 | Most abundant genus after Cricetops. | A fossorial cylindrodontid similar to a mole-rat and possibly indicative of sandy areas. |

#### Lagomorphs

Lagomorphs of the Hsanda Gol Formation :
| Genus | Species | Location | Notes |
| Desmatolagus | D. gobiensis | SW Loh East Loh NE Kholobolchi Nor Loh Tatal Gol Field 538 | Pikas. "D." robustus may belong to a different genus. SW Loh only contains D. gobiensis; NE Kholobolchi Nor only "D." robustus. D. gobiensis may be ecologically similar to Pallas's pika and "D." robustus to the desert hare. |
| Desmatolagus | "D." robustus | SW Loh East Loh NE Kholobolchi Nor Loh Tatal Gol Field 538 | Pikas. "D." robustus may belong to a different genus. SW Loh only contains D. gobiensis; NE Kholobolchi Nor only "D." robustus. D. gobiensis may be ecologically similar to Pallas's pika and "D." robustus to the desert hare. |
| ?Gobiolagus | ?G. teilhardi | Tatal Gol | A leporid. |
| Ochotonolagus | O. argyropuloi | Tatal Gol | A pika similar to the Daurian pika. |
| Procaprolagus | P. maximus | Tatal Gol (all) | Leporids. |
| Procaprolagus | P. mongolicus | Tatal Gol (all) | Leporids. |
| Procaprolagus | P. orlovi | Tatal Gol (all) | Leporids. |
| Procaprolagus | P. vetustus | Tatal Gol (all) | Leporids. |
| Procaprolagus | P. youngi | Tatal Gol (all) | Leporids. |
| Sinolagomys | S. tatalgolicus | Tatal Gol | A pika. |

#### Carnivorans

Carnivorans of the Hsanda Gol Formation :
| Genus | Species | Location | Abundance | Notes | Images |
| Amphicticeps | A. shackelfordi | SW Loh Loh | | An amphicynodontid. | Nimravus cranium. Palaeogale cranium. |
| Amphicynodon | A. teilhardi | Loh | | A small primitive bear similar to a mustelid. | Nimravus cranium. Palaeogale cranium. |
| ?Cynodictis | ?C. elegans ?C. constans | Loh | | A small bear dog comparable to a red fox. | Nimravus cranium. Palaeogale cranium. |
| Nimravus | Indeterminate | Tatal Gol | | A small nimravid or "false sabertooth". There are at least two different species represented, both unnamed: a smaller one similar to a wildcat and a larger one resembling a Eurasian lynx. | Nimravus cranium. Palaeogale cranium. |
| Palaeogale | P. ulyesses P. parvula | East Loh Loh Tatal Gol Field 538 | | Mustelids comparable to the marbled polecat and the least weasel. | Nimravus cranium. Palaeogale cranium. |
| Palaeoprionodon | P. gracilis | SW Loh | | A feliform. | Nimravus cranium. Palaeogale cranium. |
| Cf. Plesictis | Indeterminate | Tatal Gol | | A larger mustelid comparable to a wolverine. | Nimravus cranium. Palaeogale cranium. |
| Proailurus | Indeterminate | Tatal Gol | One lower jaw fragment. | An early felid better known from Europe and North America. Some authors have instead assigned this material to the nimravid Eofelis or the more advanced felid Pseudaelurus, based on dental features. | Nimravus cranium. Palaeogale cranium. |

#### Perissodactyls

Perissodactyls of the Hsanda Gol Formation :
| Genus | Species | Location | Stratigraphic Position | Abundance | Notes | Images |
| Paraceratherium | P. transouralicum | SW Loh Loh Tatal Gol | | | A giant browsing rhinoceros of the family Hyracodontidae, originally described as Indricotherium grangeri, and one of the largest land mammals of all time. Despite its massive size (up to 6 meters tall), it could have occupied a niche similar to the Bactrian camel at Hsanda Gol. | Life restoration of P. transouralicum |

#### Artiodactyls

Artiodactyls of the Hsanda Gol Formation :
| Genus | Species | Location | Notes | Image |
| Eumeryx | E. culminis | SW Loh East Loh Loh Tatal Gol Field 538 | A gelocid pecoran related to the chevrotain, but ecologically comparable to the goitered gazelle. | Palaeohypsodontus teeth. |
| Cf. Miomeryx | Indeterminate | | A gelocid. | Palaeohypsodontus teeth. |
| Palaeohypsodontus | P. asiaticus | Tatal Gol | A hypsodont bovid possibly indicative of the presence of grass. | Palaeohypsodontus teeth. |
| Pseudomeryx | P. gobiensis | Tatal Gol | A gelocid. | Palaeohypsodontus teeth. |

#### Creodonts

Creodonts of the Hsanda Gol Formation :
| Genus | Species | Location | Notes | Images |
| Hyaenodon | H. ambiguus | West Tatal Gol SW Loh NE Kholobolchi Nor Loh Tatal Gol Field 538 | The largest carnivore in the formation, a predatory creodont similar to a grey wolf that lived in many different Holarctic environments. H. pervagus is the only species found outside of Tatal Gol. | Restoration of Hyaenodon gigas, largest species of Hyaenodon. |
| Hyaenodon | H. aymardi | West Tatal Gol SW Loh NE Kholobolchi Nor Loh Tatal Gol Field 538 | The largest carnivore in the formation, a predatory creodont similar to a grey wolf that lived in many different Holarctic environments. H. pervagus is the only species found outside of Tatal Gol. | Restoration of Hyaenodon gigas, largest species of Hyaenodon. |
| Hyaenodon | H. compressus | West Tatal Gol SW Loh NE Kholobolchi Nor Loh Tatal Gol Field 538 | The largest carnivore in the formation, a predatory creodont similar to a grey wolf that lived in many different Holarctic environments. H. pervagus is the only species found outside of Tatal Gol. | Restoration of Hyaenodon gigas, largest species of Hyaenodon. |
| Hyaenodon | H. gigas | West Tatal Gol SW Loh NE Kholobolchi Nor Loh Tatal Gol Field 538 | The largest carnivore in the formation, a predatory creodont similar to a grey wolf that lived in many different Holarctic environments. H. pervagus is the only species found outside of Tatal Gol. | Restoration of Hyaenodon gigas, largest species of Hyaenodon. |
| Hyaenodon | H. pervagus | West Tatal Gol SW Loh NE Kholobolchi Nor Loh Tatal Gol Field 538 | The largest carnivore in the formation, a predatory creodont similar to a grey wolf that lived in many different Holarctic environments. H. pervagus is the only species found outside of Tatal Gol. | Restoration of Hyaenodon gigas, largest species of Hyaenodon. |