The Hsanda Gol Formation is a

geological 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 ...

located in the Tsagan Nor Basin of central

Mongolia Mongolia; Mongolian script: , , ; lit. "Mongol Nation" or "State of Mongolia" () is a landlocked country in East Asia, bordered by Russia to the north and China to the south. It covers an area of , with a population of just 3.3 million ...

that dates to the

Rupelian The Rupelian is, in the geologic timescale, the older of two ages or the lower of two stages of the Oligocene Epoch/ Series. It spans the time between . It is preceded by the Priabonian Stage (part of the Eocene) and is followed by the Chattian ...

age of the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but t ...

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided ...

. It formed in continental

desert A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About on ...

-like conditions and is notable for its fossil mammals, first excavated by

Roy Chapman Andrews Roy Chapman Andrews (January 26, 1884 – March 11, 1960) was an American explorer, adventurer and naturalist who became the director of the American Museum of Natural History. He led a series of expeditions through the politically disturbed ...

and the

American Museum of Natural History The American Museum of Natural History (abbreviated as AMNH) is a natural history museum on the Upper West Side of Manhattan in New York City. In Theodore Roosevelt Park, across the street from Central Park, the museum complex comprises 26 int ...

in the 1920s. Hsanda Gol is included in the Hsandagolian

Asian Land Mammal Age The Asian land mammal ages, acronym ALMA, establish a geologic timescale for prehistoric Asian fauna beginning 58.7 Mya during the Paleogene and continuing through to the Miocene ( Aquitanian) (23.03 Ma). These periods are referred to as ages, sta ...

, to which it gives its name.

Geology

The 2600 feet thick Hsanda Gol formation is covered in various places by

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

-age variegated clays, sands, and gravels. It rests on top of the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name), is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 145& ...

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 The Ergilian age is a period of geologic time (37.2–33.9 Ma) within the Late Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene ...

age to the Late

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', ...

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 Paleomagnetism (or palaeomagnetismsee ), is the study of magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.'' Certain magnetic minerals in roc ...

in underlying and overlaying

lava flow Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

s 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 Ephemerality (from the Greek word , meaning 'lasting only one day') is the concept of things being transitory, existing only briefly. Academically, the term ephemeral constitutionally describes a diverse assortment of things and experiences, f ...

rivers. Some taxa lived through this change including hyaenodonts and indricotheres, but others, such as

brontothere Brontotheriidae is a family (biology), family of extinct mammals belonging to the order Perissodactyla, the order that includes horses, rhinoceroses, and tapirs. Superficially, they looked rather like Rhinoceros, rhinos, although they were actua ...

amynodont Amynodontidae ("defensive tooth") is a family (biology), family of extinct perissodactyls related to true rhinoceroses. They are commonly portrayed as semiaquatic hippo-like rhinos but this description only fits members of the Metamynodontini; ...

s, 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

chalicothere Chalicotheres (from Greek '' chalix'', "gravel" and '' therion'', "beast") are an extinct clade of herbivorous, odd-toed ungulate (perissodactyl) mammals that lived in North America, Eurasia, and Africa from the Middle Eocene until the Early Ple ...

Paleobiota

Hsanda Gol's paleobiota resembles current desert mammal faunas, including the one found in the same area today. It is composed largely of

rodent Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...

s and

lagomorph The lagomorphs are the members of the taxonomic order Lagomorpha, of which there are two living families: the Leporidae (hares and rabbits) and the Ochotonidae (pikas). The name of the order is derived from the Ancient Greek ''lagos'' (λαγ� ...

s with high-crowned teeth adapted to chew hard vegetation; several of these species are

fossorial A fossorial () animal is one adapted to digging which lives primarily, but not solely, underground. Some examples are badgers, naked mole-rats, clams, meerkats, and mole salamanders, as well as many beetles, wasps, and bees. Prehistoric evid ...

and none

arboreal Arboreal locomotion is the locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. The habitats pose nu ...

, indicating that there were no areas with high tree density. The only large

browsing Browsing is a kind of orienting strategy. It is supposed to identify something of relevance for the browsing organism. When used about human beings it is a metaphor taken from the animal kingdom. It is used, for example, about people browsing o ...

ungulate Ungulates ( ) are members of the diverse clade Ungulata which primarily consists of large mammals with hooves. These include odd-toed ungulates such as horses, rhinoceroses, and tapirs; and even-toed ungulates such as cattle, pigs, giraffes, ...

, ''

Paraceratherium ''Paraceratherium'' is an extinct genus of hornless rhinoceros. It is one of the largest terrestrial mammals that has existed and lived from the early to late Oligocene epoch (34–23 million years ago). The first fossils were discovered ...

'', was probably transient and there were no medium-large herbivores. Predators were diverse but generally small, with the largest (''

Hyaenodon ''Hyaenodon'' ("hyena-tooth") is an extinct genus of carnivorous hyaenodont mammals from tribe Hyaenodontini, within subfamily Hyaenodontinae in family Hyaenodontidae,Malcolm C. McKenna, Susan K. Bell (1997."Classification of Mammals: Above the ...

'') being

wolf The wolf (''Canis lupus''; : wolves), also known as the gray wolf or grey wolf, is a large canine native to Eurasia and North America. More than thirty subspecies of ''Canis lupus'' have been recognized, and gray wolves, as popularly un ...

-sized, and were adapted to hunt by ambush. Landscape would appear as a sparse plain with few tall trees near rivers or where

groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit ...

could be found otherwise. Pallinology studies in contemporary sites of

China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...

and Mongolia are consistent with an arid-adapted woody

scrubland Shrubland, scrubland, scrub, brush, or bush is a plant community characterized by vegetation dominated by shrubs, often also including grasses, herbs, and geophytes. Shrubland may either occur naturally or be the result of human activity. It ...

dominated by

Mormon tea ''Ephedra'' is a genus of gymnosperm shrubs. The various species of ''Ephedra'' are widespread in many arid regions of the world, ranging across southwestern North America, southern Europe, northern Africa, southwest and central Asia, northe ...

and salt-tolerant nitre bush. The few trees were broad-leaved and deciduous, related to modern elm,

birch A birch is a thin-leaved deciduous hardwood tree of the genus ''Betula'' (), in the family Betulaceae, which also includes alders, hazels, and hornbeams. It is closely related to the beech- oak family Fagaceae. The genus ''Betula'' cont ...

, and oak.Prothero, Donald R.. Life of the Past : Rhinoceros Giants: the Paleobiology of the Indricotheres, Indiana University Press, 2013. ProQuest Ebook Central, https://ebookcentral.proquest.com/lib/universidadcomplutense-ebooks/detail.action?docID=1161291.

Amphibians

Reptiles

Mammals

Leptictids

Erinaceids

Rodents

Lagomorphs

Carnivorans

Perissodactyls

Artiodactyls

Creodonts

References

{{reflist ^ http://www.fossilworks.org/cgi-bin/bridge.pl?a=collectionSearch&taxon_no=336714&max_interval=Oligocene&country=Mongolia&is_real_user=1&basic=yes&type=view&match_subgenera=1 Geologic formations of Mongolia Paleogene System of Asia Oligocene Asia Rupelian Stage Paleontology in Mongolia