Nemegt Formation

Geological formation in Mongolia

The Nemegt Formation (also known as Nemegtskaya Svita) is a geological formation in the Gobi Desert of Mongolia, dating to the Late Cretaceous. The formation consists of river channel sediments and contains fossils of fish, turtles, crocodilians, and a diverse fauna of dinosaurs, including birds.

The Nemegt Formation is composed of mudstones and sandstones that were deposited by ancient lakes, streams, and flood plains. The Altan Uul locality was described by Michael Novacek as "a canyon carved out of a very rich series of sedimentary rocks" with "steep cliffs and narrow washes". 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. These 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. There has been no absolute dating of the Nemegt Formation. Historically the Nemegt has been considered late Campanian to Maastrichtian, based on comparisons of fossils present, but no exact dating has been performed. The age for the underlying Baruungoyot Formation (= Svita) has been suggested as Santonian to Campanian, and Shuvalov (2000) found K Ar dating of basalts that they referred to the mid and upper Baruungoyot to be 75 to 80 million years old.

Genus Species Location Stratigraphic position Material Notes Image Paralligator P. gradilifrons An osteoglossomorph. Includes previously reported material. A mooneye. Teleostei indet. Indeterminate Not specified. "Vertebral centra found within a specimen of Raptorex." A sporophyll related to quillworts. Lemnaceae indet. Indeterminate Tsagan Khushu, Ulaan Bulag "Fruits." Duckweed. Potamogeton like indet. Indeterminate Ulaan Bulag "Shoots and leaves." An ostracod. A. multispina Altan Uul IV, NemegtLower "Nine carapaces and some detached valves." An ostracod. Cyclocypris C. transitoria Bügiin Tsav, Nemegt "Seven adult carapaces." B. transaltaiensis A giant azhdarchid.

Genus Species Location Stratigraphic position Material Notes Image Gobiapalone G. breviplastra Alak Shand Khuduk, Altan Uul I, Bügiin Tsav, Bügiin Tsav II, Nemegt, Ulan Khushu "Shell and body elements from multiple specimens." A trionychid. Gravemys G. barsboldi Bamba Khuduk, Hermiin Tsav, Ingeni Khobur, Tsagan Khushu "Partial to complete shells." A lindholmemydid. Nanhsiungchelyidae indet. Indeterminate Hermiin Tsav IILower "Shell fragment." A nanhsiungchelyid. Indeterminate Khuren Tsav "Damaged shell." A lindholmemydid. Mongolochelys M. efremovi Altan Uul II, Altan Uul III, Bügiin Tsav, Guriliin Tsav, Hermiin Tsav, Nemegt, Tsagan Khushu "Skull, shell and body elements from multiple specimens." A sichuanchelyid. Nemegtemys N. conflata Bügiin Tsav, Nemegt "Partial plastron elements." "D". giganteus An ankylosaurid now regarded as nomen dubium. Saichania S. chulsanensis Altan Uul IV, Hermiin Tsav ILower A skull, mandibles, vertebrae, tail club, and other undescribed postcranial material. Hermiin TsavLower B. sicinskii A hadrosaurid. Hadropodus Indeterminate Nemegt "Numerous footprints." Hadrosaurid tracks. Attributed to Saurolophus. Saurolophus S. angustirostris Presence in most localities "Multiple specimens, including articulated skulls, postcranial skeletons and mass death assemblage." A saurolophine hadrosaurid which was the most common hadrosaur in the area. Unnamed ichnotaxon Indeterminate Nemegt "Numerous footprint casts with trackways." H. calathocercosA pachycephalosaurid. P. sp. N. mongoliensis A titanosaur. Opisthocoelicaudia O. skarzynskii M. olecranus An alvarezsaurid. Indeterminate Altan Uul III "Caudal vertebrae and left hindlimb." N. citus B. mongoliensis G. nessovi J. nogontsavensis L. sochavai "Whole egg with partly broken pole" T. gobiensis Ornithuran remains.

Genus Species Location Stratigraphic position Material Notes Image Adasaurus A. mongoliensis A dromaeosaurid. Dromaeosauridae indet. Indeterminate Bügiin TsavMiddle or Upper "Cervical vertebra. "A dromaeosaurid. Indeterminate Hermiin Tsav Lower "Dorsal vertebra." Indeterminate Khaichin I "Three teeth." Indeterminate NemegtLower "Dorsal vertebra and other elements." Indeterminate Indeterminate Bügiin Tsav "Footprints with poor claw marks." Two toed theropod tracks. Attributed to either Adasaurus or Zanabazar.

Genus Species Location Stratigraphic position Material Notes Image Anserimimus A. planinychus An ornithomimid. Deinocheirus D. mirificus A giant deinocheirid. Gallimimus G. bullatus An ornithomimid. Indeterminate Indeterminate Bügiin Tsav "Nine trackways including an articulated Gallimimus foot." Indeterminate Tsagan Khushu "Partial vertebral column with fore and hindlimbs." A. portentosus An oviraptorid. Also present in the Barun Goyot Formation. Elmisaurus E. rarus A caenagnathid. Elongatoolithidae indet. Indeterminate Bügiin Tsav "Three eggs containing embryonic remains." Oviraptorid eggs. Gobiraptor G. minutus Three toed theropod tracks. Attributed to Avimimus. Nemegtomaia N. barsboldi An oviraptorid also present in the Barun Goyot Formation. Nomingia N. gobiensis O. avarsan Bügiin Tsav, Guriliin TsavMiddle or Upper "Multiple associated skeletons." R. mongoliensis An oviraptorid.

Genus Species Location Stratigraphic position Material Notes Image Therizinosaurus T. cheloniformis A giant therizinosaurid. Undescribed therizinosaurids Indeterminate Altan Uul, Bügiin Tsav, Guriliin Tsav, Ulan Khushu "Partial remains." Therizinosaurid remains.

Genus Species Location Stratigraphic position Material Notes Image Borogovia B. gracilicrus T. nemegtensis Z. junior Bügiin TsavMiddle or Upper "Skull with fragmentary postcranial skeleton." A. remotus Tsagan Khushu "Nearly complete skull with partial postcrania." B. ostromiR. kriegsteni Tarbosaurus T. bataar Presence in most localities"Nearly complete skeletons from multiple specimens." Unnamed ichnotaxon Indeterminate Nemegt "Footprint casts with skin impressions." Three toed theropod tracks. Attributed to Tarbosaurus. Tyrannosauripodidae indet. Indeterminate Nemegt "Five tracks." Three toed tyrannosaurid tracks. Attributed to either Alioramus or Tarbosaurus.