# Nemegt Formation

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. 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.

## 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 Barun Goyot Formation. It overlies and sometimes interfingers with the Barun Goyot 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. 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.

## Paleobiota of the Nemegt Formation

Stratigraphic positions are based on Eberth (2018) who correlated localities to their approximate position within the formation.
| Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon | Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited. |

### Amphibians

| Genus | Species | Location | Stratigraphic position | Material | Notes | Image |
| Altanulia | A. alifanovi | Altan Uul II | Middle or Upper | "Partial maxilla." | A discoglossid. Its classification has been criticized. | |

### Crocodylomorphs

| Genus | Species | Location | Stratigraphic position | Material | Notes | Image |
| Paralligator | P. gradilifrons | Nemegt, Nogon Tsav, Ulaan Bulag | Lower | "Partially complete skulls with sparse body elements." | A paralligatorid. | |

### Fish

| Genus | Species | Location | Material | Notes |
| 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. |
| Hiodontidae indet. | Indeterminate | Ulan Khushu | "Partial vertebrae and premaxilla." | A mooneye. |
| Teleostei indet. | Indeterminate | Not specified. | "Vertebral centra found within a specimen of Raptorex." | A teleost. |
| Osteichthyes indet. | Indeterminate | Bügiin Tsav | "Vertebrae and scales found within a specimen of Deinocheirus''." | A bony fish. |

### Flora

| Genus | Species | Location | Material | Notes |
| Araucariaceae indet. | Indeterminate | Altan Uul II, Nemegt | "Petrified wood and trunks." | An araucarian. |
| Monilitheca | M. minuta | Ulaan Bulag | "Two isolated megasporophylls and megaspore casts." | A sporophyll related to quillworts. |
| Lemnaceae indet. | Indeterminate | Tsagan Khushu, Ulaan Bulag | "Fruits." | Duckweed. |
| Potamogeton-like indet. | Indeterminate | Ulaan Bulag | "Shoots and leaves." | Pondweed. |

### Invertebrates

| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Altanicypris | A. bispinifera | Altan Uul IV, Nemegt | Lower | "Complete carapace and left valve." | An ostracod. |
| Altanicypris | A. multispina | Altan Uul IV, Nemegt | Lower | "Nine carapaces and some detached valves." | An ostracod. |
| Altanicypris | A. szczechurae | Altan Uul IV, Nemegt | Lower | "Multiple carapaces and detached valves." | An ostracod. |
| Candona | C. altanulaensis | Altan Uul IV, Bügiin Tsav, Nemegt | | "Multiple specimens with carapaces and valves." | An ostracod. |
| Candona | cf. C. fabaeformis | Altan Uul IV, Nemegt | Lower | "Several carapaces and detached valves." | An ostracod. |
| Candoniella | C. altanica | Altan Uul IV, Bügiin Tsav, Nemegt | | "Multiple specimens with carapaces and valves." | An ostracod. |
| Candoniella | C. mordvilkoi | Altan Uul IV | Lower | "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. |
| Cypridopsis? | Indeterminate | Nemegt | Lower | "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. |
| Cypridea | C. barsboldi | Altan Uul IV, Bügiin Tsav, Nemegt | | "Multiple specimens with carapaces and valves." | An ostracod. |
| Cypridea | C. cavernosa | Altan Uul IV, Bügiin Tsav, Nemegt, Nogon Tsav | | "Multiple specimens with carapaces and valves." | An ostracod. |
| Cypridea | C. rostrata | Bügiin Tsav, Nemegt, Nogon Tsav | | "Caparaces." | An ostracod. |
| Cypridea | Indeterminate | Altan Uul IV, Nemegt | Lower | "Five juvenile carapaces." | An ostracod. |
| Gobiella | G. prima | Altan Uul IV, Nemegt | Lower | "Several carapaces and detached valves." | An ostracod. |
| Khandia | K. stankevitchae | Altan Uul IV, Nemegt | Lower | "Multiple carapaces and valves." | An ostracod. |
| Leiria | Indeterminate | Nemegt | Lower | "Several detached valves." | An ostracod. |
| Leiria | Indeterminate | Nemegt | Lower | "Complete carapace." | An ostracod. |
| Limnocythere sp. | Indeterminate | Nemegt | Lower | "Juvenile carapace." | An ostracod. |
| Lycopterocypris? | cf. L. profunda | Altan Uul IV, Nemegt | Lower | "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. |
| Nemegtia | N. obliquecostae | Altan Uul IV, Nemegt | Lower | "Multiple carapaces and valves." | An ostracod. |
| Nemegtia | 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, Nemegt | Lower | "Multiple carapaces and detached valves." | An ostracod. |
| Rhinocypris sp. | Indeterminate | Nemegt | Lower | "Several carapaces." | An ostracod. |
| Scabriculocypris | S. ingenicus? | Altan Uul IV, Bügiin Tsav, Naran Bulag, Nemegt, Tsagan Khushu | | "Multiple carapaces and valves." | An ostracod. |
| Scabriculocypris | S. rasilis? | Altan Uul IV, Nemegt | Lower | "Several carapaces and detached valves." | An ostracod. |
| Timiriasevia | cf. T. miaogouensis | Altan Uul IV, Nemegt | Lower | "Seven valves." | An ostracod. |
| Timiriasevia | cf. T. opinabilis | Altan Uul IV | Lower | "Adult carapace." | An ostracod. |
| Timiriasevia | T. minuscula | Altan Uul IV, Bügiin Tsav, Nemegt | | "Multiple specimens with carapaces and valves." | An ostracod. |
| Timiriasevia | 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. |

### Mammals

| Genus | Species | Location | Stratigraphic position | Material | Notes | Image |
| Buginbaatar | B. transaltaiensis | Bügiin Tsav | Middle or Upper | "Fragmentary skull." | A multituberculate. | |
| "Gurlin Tsav skull" | Indeterminate | | Middle or | Partial skull | A mysterious metatherian, possibly a sparassodont. | |

### Pterosaurs

| Genus | Species | Location | Stratigraphic position | Material | Notes | Image |
| Unnamed azhdarchid | Indeterminate | Guriliin Tsav | Middle or Upper | "Fragmentary cervical vertebrae." | A giant azhdarchid. | |

### Turtles

| Genus | Species | Location | Stratigraphic position | Material | Notes |
| 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 II | Lower | "Shell fragment." | A nanhsiungchelyid. |
| Nanhsiungchelyidae indet. | Indeterminate | Khuren Tsav | | "Damaged shell." | A nanhsiungchelyid. |
| Nanhsiungchelyidae indet. | Indeterminate | Nemegt | Lower | "Partial plastron." | A nanhsiungchelyid. |
| Mongolemys | M. elegans | Bügiin Tsav, Hermiin Tsav, Tsagan Khushu | | "Skull, shell and body elements from multiple specimens." | 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." | A trionychid. |
| "Trionyx" | "T". gilbentuensis | Gilbentu | | "Fragmentary shell." | A trionychid. |
| "Trionyx" | "T". gobiensis | Altan Uul III, Bamba Khuduk, Nemegt, Tsagan Khushu | | "Fragmentary shell and carapace fragments." | A trionychid. |
| Trionychidae indet. | Indeterminate | Bamba Khuduk, Bügiin Tsav, Guriliin Tsav, Ingeni Khobur | | "Shell and body remains." | A trionychid. Originally identified as Amyda menneri. |

### Dinosaurs
#### Ornithischians
##### Ankylosaurs

| Genus | Species | Location | Stratigraphic position | Material | Notes |
| "Dyoplosaurus" | "D". giganteus | Nemegt | Lower | A series of caudal vertebrae, metatarsals, phalanges, osteoderms, and an undescribed partial tail club. | An ankylosaurid now regarded as nomen dubium. |
| Saichania | S. chulsanensis | Altan Uul IV | Lower | Partial vertebrae, tail club, and osteoderms. | An ankylosaurid also present in the Barun Goyot Formation. |
| Tarchia | T. teresae | Altan Uul IV, Hermiin Tsav I | Lower | A skull, mandibles, vertebrae, tail club, and other undescribed postcranial material. | An ankylosaurid. |
| Tarchia | T. tumanovae | Hermiin Tsav | Lower | "Partial skeleton with complete skull." | An ankylosaurid. |
| Tetrapodosaurus-like indet. | Indeterminate | Bügiin Tsav, Shar Tsav | | "Footprint casts." | Ankylosaurid tracks. |
| Ankylosauridae indet. | Indeterminate | Altan Uul II-III-IV, Bügiin Tsav, Hermiin Tsav I, Khuree Tsav, Nemegt | | A partial dentary, caudal vertebrae, dorsal vertebrae, metatarsals, phalanges, pedal phalanx, humeri, partial pelvis, tail clubs, cervical half-rings, osteoderms, skin impressions, and undescribed juveniles and partial skeletons. | Cranial and postcranial material representing numerous individuals of different ages. |

##### Hadrosaurs

| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Amblydactylus-like indet. | Indeterminate | Bügiin Tsav, Bügiin Tsav II, Guriliin Tsav, Yagaan Khovil | | "Footprint casts." | Three-toed hadrosaurs tracks. Attributed to Saurolophus. |
| Barsboldia | B. sicinskii | Nemegt | Lower | "Partial vertebrae, pelvis, and ribs." | 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." | Hand and feet hadrosaur tracks. Attributed to Saurolophus. |

##### Pachycephalosaurs

| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Homalocephale | H. calathocercos | Nemegt | Lower | "Partial skull and skeleton including hindlimbs." | A pachycephalosaurid. |
| Prenocephale | P. prenes | Guriliin Tsav, Nemegt | Lower Middle | "Complete skull and partial postcranium of several specimens." | A pachycephalosaurid. |
| Prenocephale | P. sp. | Tsagan Khushu, Nemegt | Lower | "complete left squamosal"; large skull | A pachycephalosaurid. |
| Prenocephale | Indeterminate | Bugiin Tsav | Middle | "incomplete frontoparietal dome and fragments of associated braincase bones" | A pachycephalosaurid. |

#### Sauropods

| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Brontopodus | Indeterminate | Nemegt | Lower | "Large footprint and several four-toed tracks." | Sauropod tracks. Attributed to Opisthocoelicaudia. |
| Brontopodus-like indet. | Indeterminate | Bügiin Tsav, Yagaan Khovil | | "Footprint casts." | Sauropod tracks. |
| Nemegtosaurus | N. mongoliensis | Nemegt | Lower | "Skulls and body remains from several specimens." | A titanosaur. |
| Opisthocoelicaudia | O. skarzynskii | Altan Uul IV | Lower | "Nearly complete skeleton without the skull and cervical series." | A titanosaur. |
| Undescribed sauropods | Indeterminate | Altan Uul I-II-II-IV, Bügiin Tsav, Guriliin Tsav, Hermiin Tsav, Khamaryn Khural, Khuree Tsav, Nemegt, Nogon Tsav, Tsagan Khushu, Ulan Khushu | | "Several fragmentary remains." | Sauropod remains that may represent already named taxa. |
| Unnamed ichnotaxon | Indeterminate | Nemegt | | "Footprint casts with skin impressions." | Four-toed sauropod tracks. Attributed to Opisthocoelicaudia. |

#### Theropods
##### Alvarezsaurs

| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Mononykus | M. olecranus | Bügiin Tsav | Middle | "Fragmentary skull, vertebrae, limbs and fragmented pelvis." | An alvarezsaurid. |
| Mononykus | Indeterminate | Altan Uul III | | "Caudal vertebrae and left hindlimb." | An alvarezsaurid. |
| Nemegtonykus | N. citus | Altan Uul III | | "Partially complete skeleton from two specimens lacking the skull." | An alvarezsaurid. |

##### Birds

| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Brodavis | B. mongoliensis | Bügiin Tsav | | "Nearly complete left tarsometatarsus." | A hesperornithine. |
| Gurilynia | G. nessovi | Guriliin Tsav | Middle | "Left coracoid and partial humerii." | An enantiornithine. |
| Judinornis | J. nogontsavensis | Nogon Tsav | | "Dorsal vertebra." | A hesperornithine. |
| Laevisoolithus | L. sochavai | | | "Whole egg with partly broken pole" | Laid by a bird or small theropod. |
| Subtiliolithus | S. microtuberculatus | | | "Eggshell fragments" | |
| Teviornis | T. gobiensis | Guriliin Tsav | Middle | "Partial forelimbs." | A presbyornithid anseriform. |
| Unnamed ornithurans | Indeterminate | Tsagan Khushu | | "A humerus and two tibiotarsi." | Ornithuran remains. |

##### Dromaeosaurs

| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Adasaurus | A. mongoliensis | Bügiin Tsav | Middle | "Partial skull and fragmentary postcrania from two specimens." | A dromaeosaurid. |
| Dromaeosauridae indet. | Indeterminate | Bügiin Tsav | Middle | "Cervical vertebra." | A dromaeosaurid. |
| Dromaeosauridae indet. | Indeterminate | Hermiin Tsav | Lower | "Dorsal vertebra." | A dromaeosaurid. |
| Dromaeosauridae indet. | Indeterminate | Khaichin I | | "Three teeth." | A dromaeosaurid. |
| Dromaeosauridae indet. | Indeterminate | Nemegt | Lower | "Dorsal vertebra and other elements." | A dromaeosaurid. |
| Indeterminate | Indeterminate | Bügiin Tsav | | "Footprints with poor claw marks." | Two-toed theropod tracks. Attributed to either Adasaurus or Zanabazar. |

##### Ornithomimosaurs

| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Anserimimus | A. planinychus | Bügiin Tsav | Middle or Upper | "Partial skeleton lacking the skull." | An ornithomimid. |
| Deinocheirus | D. mirificus | Altan Uul III-IV, Bügiin Tsav | Lower Middle | "Complete skull with virtually complete postcranial remains from three specimens." | A giant deinocheirid. |
| Gallimimus | G. bullatus | Presence in most localities | | "Multiple specimens with nearly complete skull and postcranial elements." | An ornithomimid. |
| Indeterminate | Indeterminate | Bügiin Tsav | | "Nine trackways including an articulated Gallimimus foot." | Three-toed theropod tracks. Attributed to ornithomimids. |
| Unnamed ornithomimid | Indeterminate | Tsagan Khushu | | "Partial vertebral column with fore and hindlimbs." | An ornithomimid. |

##### Oviraptorosaurs

| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Avimimus | A. portentosus | Shar Tsav | | "Skull roof with fragmentary body elements." | An avimimid. Also present in the Djadokhta Formation. |
| Avimimus | A. nemegtensis | Nemegt | Lower | "Fragmented skull with partial skeleton." | An avimimid. |
| Conchoraptor | C. gracilis | Guriliin Tsav | Middle | "Skull and a flock of articulated individuals." | An oviraptorid. Also present in the Barun Goyot Formation. |
| Elmisaurus | E. rarus | Altan Uul II, Nemegt | Lower Middle or Upper | "Fragmentary skull and partially skeletons from several specimens." | A caenagnathid. |
| Elongatoolithidae indet. | Indeterminate | Bügiin Tsav | | "Three eggs containing embryonic remains." | Oviraptorid eggs. |
| Gobiraptor | G. minutus | Altan Uul III | | "Partial crania with complete lower jaws and fragmented postcrania." | An oviraptorid. |
| Indeterminate | Indeterminate | Hermiin Tsav, Shar Tsav, Yagaan Khovil | | "Footprint casts." | Three-toed theropod tracks. Attributed to Avimimus. |
| Nemegtomaia | N. barsboldi | Nemegt | Lower | "Multiple specimens including a nesting specimen and eggs." | An oviraptorid also present in the Barun Goyot Formation. |
| Nomingia | N. gobiensis | Bügiin Tsav, Nemegt | Lower Middle | "Vertebral series with partial hindlimbs and a referred pygostyle." | A caenagnathid. May be synonymous with Elmisaurus. |
| Oksoko | O. avarsan | Bügiin Tsav, Guriliin Tsav | Middle | "Multiple associated skeletons." | An oviraptorid. |
| Rinchenia | R. mongoliensis | Altan Uul II | Middle or Upper | "Skull with nearly complete skeleton." | An oviraptorid. |

##### Therizinosaurs

| Genus | Species | Location | Stratigraphic position | Material | Notes | Image |
| Therizinosaurus | T. cheloniformis | Altan Uul, Hermiin Tsav, Nemegt | Lower | "Fore and hindlimb elements from multiple specimens." | A giant therizinosaurid. | |
| Undescribed therizinosaurids | Indeterminate | Altan Uul, Bügiin Tsav, Guriliin Tsav, Ulan Khushu | | "Partial remains." | Therizinosaurid remains. | |

##### Troodontids

| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Borogovia | B. gracilicrus | Altan Uul IV | Lower | "Partial hindlimbs." | A troodontid. |
| Indeterminate | Indeterminate | Bügiin Tsav | | "Footprints with poor claw marks." | Two-toed theropod tracks. Attributed to either Adasaurus or Zanabazar. |
| Tochisaurus | T. nemegtensis | Nemegt | Lower | "Left metatarsus." | A troodontid. |
| Zanabazar | Z. junior | Bügiin Tsav | Middle | "Skull with fragmentary postcranial skeleton." | A troodontid originally identified as "Saurornithoides junior". |

##### Tyrannosaurs

| Genus | Species | Location | Stratigraphic position | Material | Notes |
| Alioramus | A. remotus | Nogon Tsav | | "Partially preserved skull with very sparse postcranial remains." | An alioramin. |
| Alioramus | A. altai | Tsagan Khushu | | "Nearly complete skull with partial postcrania." | An alioramin. |
| Bagaraatan | B. ostromi | Nemegt | Lower | "Fragmentary lower jaw, hindlimb and caudal vertebrae." | A tyrannosauroid. |
| Indeterminate | Indeterminate | Bügiin Tsav, Bügiin Tsav II | | "Footprint casts." | Three-toed theropod tracks. Attributed to Tarbosaurus. |
| Raptorex | R. kriegsteni | | | "Nearly complete skeleton including the skull." | A controversial genus of tyrannosaurid that could be a juvenile Tarbosaurus. |
| Tarbosaurus | T. bataar | Presence in most localities | | "Nearly complete skeletons from multiple specimens." | A large tyrannosaurid which was the most common large carnivore in the area. |
| 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. |