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.
Description
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 as indicated by petrified fossil trunks. When examined, the rock facies of the upper section consist of braided meanders, and oxbow lakes. Alteration of sediment grades indicates frequent hydrodynamic condition changes (seasonal rain). Siltstone pellets indicate desiccation at the channel margins in river low-stand periods. Bank erosion features would form through river channel migration. Generally rivers consisted of low flow regimes with sporadic upper flow regimes indicated by exotic pebbles. The lower sections of the formation represent fluvial settings in a seasonally wet and dry climate. The zone of the upper Baruungoyot and lower Nemegt Formations was deposited under a time of increasing wetness. The middle sections of the formation indicate paludal and lacustrine settings deposited under low energy conditions,with an increase in drought conditions as evidence by caliches and evidence of sand dunes being blown in. The upper part of the formation shows a return to fluvial settings and a wetter climate than the rocks below it.A study of oxygen-18 isotopes from theropod teeth show seasonal patterns similar to present day continental climates. Mean annual temperatures are estimated to be around with seasonal precipitation patterns compared to meteorological data from Shijiazhuang, China; suggesting a monsoonal, cold semi-arid climate. That same study indicated that due to the greenhouse conditions of the Cretaceous, the East Asian monsoon was strong enough to reach into Mongolia. The Nemegt environment was dominated by araucarian conifers, but it also supported a diverse array of other plants, including ginkgos and a wide variety of angiosperms—ranging from plane trees (sycamores) to duckweeds. The groundcover included quillwort like plants as well as sedges (members of the group Cyperaceae). Grass is also known from the formation, and as in the Laramie Formation of North America, reed grasses (primarily Phragmites) were present. The area also supported Bald Cypress. The area probably resembled the modern day Okavango Delta in Botswana.
Stratigraphy
The most recent stratigraphy divides the Nemegt Formation into three informal members. The lower member is dominated by fluvial deposits, while middle and upper members consist of alluvial plain, paludal, lacustrine, and fluvial deposits. The colour of the sediments is usually light grey to tan in colour in comparison to the typically red colour of the underlying Baruungoyot Formation. It overlies and sometimes interfingers with the Baruungoyot Formation. Interfingering has been noted at the stratotype (Red Walls) and Hermiin Tsav. 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. U-Pb dating from teeth of Tarbosaurus suggested that the deposition of the middle Nemegt Formation happened before 66.7 ± 2.5 Ma, during the Maastrichtian age, though whether the depositional age of the formation dates back to the late Campanian remains unresolved.
While it has been generally assumed that the Djadochta, Baruungoyot, and Nemegt formations were layered in succession, some geological analysis suggested interfingering rather than superposition of these features. Thus these formations could represent different parts of a single interconnected ecosystem, with the Djadochta reflecting arid dune habitats dominated by small animals, the Baruungoyot representing transitional semi-arid zones, and the Nemegt capturing wetter river plains that supported large dinosaurs.
Paleobiota of the Nemegt Formation Stratigraphic positions are based on Eberth (2018) who correlated localities to their approximate position within the formation.
Flora Genus Species Location Stratigraphic position Material Notes ImageAraucariaceaeIndeterminateAltan Ula II"Petrified wood and trunks"Araucarian conifer, probably the dominant flora of the formation.ArundoA. sp. Nogon Tsav"Leaf impressions."Cane Grass.CarpinusC. altanensis Atlan Uul"Fruits."A Hornbeam birch Cyperaceae indet.Indeterminate Ulaan Bulag"Scattered stem and leaf impressions."A grass-like monocot.GinkgoG. altanensisAltan Uul"Leaf impressions."Ginkgophyte.G. sp. Altan Uul"Leaf impressions."Ginkgophyte.LemnaL. chushuensisTsagan Khushu"Fruit."Duckweed.Lemnaceae indet. IndeterminateTsagan Khushu, Ulaan Bulag"Fruits."Duckweed. Monilitheca M. minuta Ulaan Bulag "Two isolated megasporophylls and megaspore casts." A sporophyll related to quillworts.NelumboN. nogontzavensisNogon Tsav"Leaf and fruit impressions."Water Lotus.N. sp. Atlan Uul"Leaf impressions."Water Lotus.NilssoniaN. sp. Atlan Uul"Leaf impressions."Leaves assigned to Cycadophyta or their own order, Nilssoniales. Nyssoidea N. mongolica Bugiin TsavMiddle or Upper "Fruits." Likely not in the Nyssaceae family. Also in the Bayanshiree formation.PhragmitesP. sp. Atlan Uul, Nogon Tsav"Leaf and stem impressions."Reed Grass.PlatanusP. sp. Altan Uul"Leaf impressions."Sycamore. Potamogeton-like Indeterminate Ulaan Bulag "Shoots, leaves, and fruits." Pondweed. Silicified Wood Indeterminate Altan Uul II, Nemegt "Petrified wood and trunks." Indeterminate fossil trees. TaxodiumT. sp. Paleocne border locality, likely Tsagan Khushu"Leaf impressions."Cypress.TrochodendroidesT. sp. Altan Uul, and Paleocne border locality, likely Tsagan Khushu"Leaf impressions."A Katsura relative. QuereuxiaQ. sp. Atlan Uul, Nogon Tsav""Leaf impressions."A Water caltrop look-alike. ZingiberopsisZ. sp. Nogon Tsav"Leaf impressions."Ginger relative.
Invertebrates Genus Species Location Stratigraphic position Material Notes Image Altanicypris A. bispinifera Altan Uul IV, NemegtLower "Complete carapace and left valve." An ostracod. A. multispina Altan Uul IV, NemegtLower "Nine carapaces and some detached valves." An ostracod. A. szczechurae Altan Uul IV, NemegtLower "Multiple carapaces and detached valves." An ostracod. Candona C. altanulaensis Altan Uul IV, Bügiin Tsav, Nemegt "Multiple specimens with carapaces and valves." An ostracod. cf. C. fabaeformis Altan Uul IV, NemegtLower "Several carapaces and detached valves." An ostracod. Candoniella C. altanica Altan Uul IV, Bügiin Tsav, Nemegt "Multiple specimens with carapaces and valves." An ostracod. C. mordvilkoi Altan Uul IVLower "Three adult carapaces." An ostracod. Cyclocypris C. transitoria Bügiin Tsav, Nemegt "Seven adult carapaces." An ostracod. Cypria C. elata Bügiin Tsav, Hermiin Tsav "Several carapaces and valves." An ostracod. Cypridopsis? C. bugintsavicus Altan Uul IV, Bügiin Tsav, Nemegt, Tsagan Khushu "Ten adult carapaces." An ostracod. Indeterminate NemegtLower "Several complete carapaces." An ostracod. Cypris? C. ectypa Altan Uul IV, Bügiin Tsav, Nemegt, Ulan Bulag "Complete carapaces and some detached valves." An ostracod. Cypridea cf. C. punctilataeformis Nemegt "One adult carapace." An ostracod. C. barsboldi Altan Uul IV, Bügiin Tsav, Nemegt "Multiple specimens with carapaces and valves." An ostracod. C. cavernosa Altan Uul IV, Bügiin Tsav, Nemegt, Nogon Tsav "Multiple specimens with carapaces and valves." An ostracod. C. rostrata Bügiin Tsav, Nemegt, Nogon Tsav "Caparaces." An ostracod. Indeterminate Altan Uul IV, NemegtLower "Five juvenile carapaces." An ostracod. Gobiella G. prima Altan Uul IV, NemegtLower "Several carapaces and detached valves." An ostracod. Khandia K. stankevitchae Altan Uul IV, NemegtLower "Multiple carapaces and valves." An ostracod. Leiria Indeterminate NemegtLower "Several detached valves." An ostracod. Indeterminate NemegtLower "Complete carapace." An ostracod. Limnocythere sp. Indeterminate NemegtLower "Juvenile carapace." An ostracod. Lycopterocypris? cf. L. profunda Altan Uul IV, NemegtLower "Multiple complete carapaces and left valve." An ostracod. Mongolianella M. palmosa? Altan Uul IV, Bügiin Tsav, Nemegt "Several adult carapaces." An ostracod. Mongolocypris M. distributa Altan Uul IV, Bügiin Tsav, Nemegt, Tsagan Khushu "Multiple specimens with complete carapaces and valves." An ostracod. Nemegtia N. biformata Altan Uul IV, Bügiin Tsav, Nemegt, Tsagan Khushu "Multiple complete carapaces and several detached valves." An ostracod. N. obliquecostae Altan Uul IV, NemegtLower "Multiple carapaces and valves." An ostracod. N. reticulata Altan Uul IV, Bügiin Tsav, Nemegt, Nogon Tsav, Tsagan Khushu "Several carapaces and detached valves." An ostracod. Paracypridea? P. mongolica Altan Uul IV, NemegtLower "Multiple carapaces and detached valves." An ostracod. Rhinocypris sp. Indeterminate NemegtLower "Several carapaces." An ostracod. Scabriculocypris S. ingenicus? Altan Uul IV, Bügiin Tsav, Naran Bulag, Nemegt, Tsagan Khushu "Multiple carapaces and valves." An ostracod. S. rasilis? Altan Uul IV, NemegtLower "Several carapaces and detached valves." An ostracod. Timiriasevia cf. T. miaogouensis Altan Uul IV, NemegtLower "Seven valves." An ostracod. cf. T. opinabilis Altan Uul IVLower "Adult carapace." An ostracod. T. minuscula Altan Uul IV, Bügiin Tsav, Nemegt "Multiple specimens with carapaces and valves." An ostracod. T. naranbulakensis Altan Uul IV, Bügiin Tsav, Nemegt "Four carapaces and one right valve." An ostracod. Ziziphocypris Z. costata Altan Uul IV, Bügiin Tsav, Nemegt "Fourteen carapaces." An ostracod.
Fish Genus Species Location Stratigraphic position Material Notes Image Harenaichthys H. lui Altan Uul, Bügiin Tsav, Hermiin Tsav, Ulan Khushu "Partial skull parts, isolated and articulated centra, and articulated caudal fin." An osteoglossomorph. Includes previously reported material. Osteichthyes indet. Indeterminate Bügiin Tsav "Vertebrae and scales found within a specimen of Deinocheirus''." A bony fish.
Amphibians Genus Species Location Stratigraphic position Material Notes Image Altanulia A. alifanovi Altan Uul IIMiddle or Upper Partial maxilla A discoglossid frog. Its classification has been criticized. Anura indet. Indeterminate Guriliin Tsav Maxilla fragment A frog of uncertain affinities. May be attributable to Altanulia. Gobiates G. sp. Guriliin Tsav Right frontoparietal and maxilla, left ilium A gobiatid frog
Mammals Genus Species Location Stratigraphic position Material Notes Image Buginbaatar B. transaltaiensis Bügiin TsavMiddle or Upper "Fragmentary skull." A multituberculate. "Gurlin Tsav skull" IndeterminateMiddle or Partial skull A mysterious metatherian, possibly a sparassodont.Cimolomyidae indet.IndeterminateGuriliin TsavMiddle to UpperHumerusShares affinities to taxa from the Hell Creek Formation.
Crocodylomorphs Genus Species Location Stratigraphic position Material Notes Image Paralligator P. gradilifrons Nemegt, Nogon Tsav, Ulaan BulagLower "Partially complete skulls with sparse body elements." A paralligatorid.
Pterosaurs Genus Species Location Stratigraphic position Material Notes Image Unnamed azhdarchid Indeterminate Guriliin TsavMiddle or Upper "Fragmentary cervical vertebrae." A giant azhdarchid.
Turtles