Hsanda Gol Formation

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,[2][3] 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.[4]

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.[5]

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.[6][7]

Carnivorans

Carnivorans of the Hsanda Gol Formation
Genus	Species	Location	Stratigraphic Position	Abundance	Notes	Images
Amphicticeps	A. shackelfordi	SW Loh Loh			A miacid similar to the beech marten.	Nimravus cranium. Palaeogale cranium.
Amphicynodon	A. teilhardi	Loh			A small primitive bear similar to a mustelid.
?Cynodictis	?C. elegans ?C. constans	Loh			A small bear dog comparable to a red fox.
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.
Palaeogale	P. ulyesses P. parvula	East Loh Loh Tatal Gol Field 538			Mustelids comparable to the marbled polecat and the least weasel.
Palaeoprionodon	P. gracilis	SW Loh			A feliform.
Cf. Plesictis	Indeterminate	Tatal Gol			A larger mustelid comparable to a wolverine.
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.[8]

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.	Paraceratherium restored next to Hyaenodon.

Artiodactyls

Artiodactyls of the Hsanda Gol Formation
Genus	Species	Location	Stratigraphic Position	Abundance	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	P. asiaticus	Tatal Gol			A hypsodont bovid possibly indicative of the presence of grass.
Pseudomeryx	P. gobiensis	Tatal Gol			A gelocid.

Creodonts

Creodonts of the Hsanda Gol Formation
Genus	Species	Location	Stratigraphic Position	Abundance	Notes	Images
Hyaenodon	H.pervagus H. aymardi H. ambiguus 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 horridus, a similar sized North American relative.