Nemegt Formation

The Nemegt Formation (or Nemegtskaya Svita) is a geological formation in the Gobi desert of Mongolia, dating from the Late Cretaceous. It overlies and sometimes interfingers with the Barun Goyot Formation. Interfingering has been noted at the stratotype (Red Walls) and Khermeen Tsav. It consists of river channel sediments and contains fossils of fish, turtles, crocodilians, and a diverse fauna of dinosaurs, including birds. The climate associated with it was wetter than when preceding formations were deposited; there seems to have existed at least some degree of forest cover. Fossilized trunks have been also found.

There has been no absolute dating of the Nemegt Formation. It is, however, almost certainly early Maastrichtian c 71-69 Ma. Gradzinski and others considered a Campanian age possible but more recent research indicates otherwise. A Campanian age no longer seems credible, in as much as the Alagteegian (or lower Djadokhtan, at the locality "Chuluut Uul") has been radiometrically dated at about 73.5 Ma or even younger (a more recent K/Ar date is 71.6 +/- 1.6 Ma). The c 73.5 (or perhaps 72) Ma Alagteegian is separated from the Nemegt by the "classic" Djadokhtan (e.g. Bayan Dzag), later Djadohktan (represented by Ukhaa Tolgod) and Barungoyotian (Khulsan). All these intervening horizons almost certainly represent more than the 1.5 million years between the dated Alagteegian level and the onset of Maastrichtian time (72.1 million Ma according to current dating). Ergo the Nemegt is entirely Maastrichtian. See also Shuvalov, Sochava and Martinsson The Age of Dinosaurs in Russia and Mongolia. The presence of Saurolophus further supports an early Maastrichtian age as the same genus occurs in the early Maastrichtian Horseshoe Canyon formation.

Description

The Nemegt Formation (the upper beds) are composed of shales and sandstones that were deposited by ancient lakes, streams, and flood plains. The Altan Ula locality was described by Michael Novacek (1996) as "a canyon carved out of a very rich series of sedimentary rocks" with "steep cliffs and narrow washes". The presence of petrified wood, and the remains of Araucariaceae conifers indicate that the forests of the Nemegt were thickly wooded, with a high canopy formed by tall conifer trees. When examined, the rock facies of this formation suggest the presence of stream and river channels, mudflats, and shallow lakes. Sediments also indicate that there existed a rich habitat, offering diverse food in abundant amounts that could sustain massive Cretaceous dinosaurs.^[1]

Flora and fauna of the Nemegt Formation

Coelurosaurs

Oviraptorosaurs

Genus	Species	Location	Stratigraphic Position	Material	Notes	Images
Avimimus	Avimimus portentosus				Avimimids.	Avimimus Rinchenia
Elmisaurus	Elmisaurus rarus			"[Three] pedes [and a] manus."[2]	A caenagnathid.
Conchoraptor	C. gracilis	Omnogov	Red Beds of Hermiin Tsav	"Partial skeleton with skull, [four] partial skulls, [and] numerous skull and postcranial fragments."[2]
Nemegtomaia	Nemegtomaia barsboldi				An oviraptorid.
Nomingia	N. gobiensis			"Partial postcranium."[3]
Rinchenia	Rinchenia mongoliensis			"Skull and fragmentary postcranium."[3]	An oviraptorid.

Paravians

Genus	Species	Location	Stratigraphic Position	Material	Notes	Images
Adasaurus[4]	Adasaurus mongoliensis[4]			"Partial skull and fragmentary postcrania."[5]	A dromaeosaurid.	Zanabazar
Borogovia	Borogovia gracilicrus			"Partial hindlimbs."[6]	A troodontid.
Gurilynia	Gurilynia nessovi			"Shoulder girdle and forelimb elements."[7]	An enantiornithine.
Judinornis	Judinornis nogontsavensis			"Vertebra."[8]	A hesperornithine.
Saurornithoides	S. junior				Reclassified as Zanabazar junior
Teviornis	Teviornis gobiensis				An anseriform related to modern ducks and geese.
Tochisaurus	Tochisaurus nemegtensis			A metatarsus.[6]	A troodontid.
Zanabazar[9]	Zanabazar junior[4]			"Skull with fragmentary postcranial skeleton."[6]	A troodontid.

Other coelurosaurs

Genus	Species	Location	Stratigraphic Position	Material	Notes	Images
Alioramus[4]	Alioramus remotus[4]				A tyrannosaur, actually present in the Beds of Nogoon Tsav. Nemegt fossils reclassified as A. altai.	Alioramus Deinocheirus Mononykus
	Alioramus altai				A tyrannosaur.
Anserimimus	Anserimimus planinychus			"Incomplete postcranium."[10]	An ornithomimid.
Bagaraatan	Bagaraatan ostromi			"Fragmentary skull and associated postcranium."[11]	A coelurosaur or uncertain classification.
Deinocheirus[4]	Deinocheirus mirificus[4]			"Forelimb elements, skull. Two new specimens together provide a nearly complete skeleton."[10]	A gigantic ornithomimosaur.
Gallimimus[4]	Gallimimus bullatus[4]			"[Three] nearly complete skeletons, complete postcranium, skull with associated frgamentary postcranium, [and] other fragmentary postcrania.",[10] and two full skeletons "[12]	An ornithomimid.
Mononykus	Mononykus olecranus				An alvarezsaurid.
Raptorex[13]	Raptorex kreigsteni[13]				A tyrannosaurid, may be a juvenile Tarbosaurus.[14]
Tarbosaurus[4]	Tarbosaurus bataar[4]			"A skull and [Two] skeletons [15]	A tyrannosaur. Articulated skeletons are unusually common here.
Therizinosaurus[4]	Therizinosaurus cheloniformis[4]	Omnogov, Mongolia	White Beds of Hermiin Tsav	"Forelimb elements [and a] referred hindlimb."[16]	A giant therizinosaur.

Invertebrates

Mammals

Ornithischians

Genus	Species	Location	Stratigraphic Position	Material	Notes	Images
Barsboldia	Barsboldia sicinskii			"Sacrum [and] pelvis."[17]	A hadrosaurid.	Homalocephale.
Dyoplosaurus	Dyoplosaurus giganteus				An ankylosaur. Formerly synonymized with Tarchia kielanae, now considered a nomen dubium.[18]
Homalocephale	Homalocephale calathoceros				A pachycephalosaur.
Prenocephale	P. prenes			"Complete skull with associated partial postcranium."[19]
Saichania	Saichania chulsanensis			"Complete skull"	An ankylosaur, also found in the Barun Goyot Formation.
Saurolophus	Saurolophus angustirostris			"At least [fifteen] specimens, including articulated skull and postcranial skeleton."[17]	A hadrosaurid.
Tarchia	Tarchia teresae[20]				An ankylosaur.

Sauropods

Genus	Species	Location	Stratigraphic Position	Material	Notes	Images
Nemegtosaurus	Nemegtosaurus mongoliensis			"Skull."[21]	A titanosaur.	Nemegtosaurus
Opisthocoelicaudia	Opisthocoelicaudia skarzynskii			"Skeleton lacking skull and cervical series."[22]	A titanosaur.

Fossil Eggs

References

See also

• Barun Goyot Formation
• Djadochta Formation
• List of fossil sites
• Nemegt Basin
• List of dinosaur-bearing rock formations