In 1963, the Mongolian paleontologist Demberelyin Dashzeveg reported the discovery of a new fossiliferous locality of the Djadochta Formation: Tugriken Shireh. During the 1960s to 1970s, Polish-Mongolian and Russian-Mongolian paleontological expeditions collected new, partial to complete specimens of Protoceratops and Velociraptor at this locality, making these dinosaur species a common occurrence in Tugriken Shireh. Some of the most notable excavations made at Tugriken Shireh include the Fighting Dinosaurs (Protoceratops and Velociraptor locked in combat), and abundant articulated, in situ (in the original pose), and sometimes complete skeletons of Protoceratops.
During the 1980s, a joint Soviet-Mongolian paleontological expedition discovered several Mesozoic fossil-rich localities in the Gobi Desert of Mongolia. Among these sites, Udyn Sayr was discovered and examined by the expedition, regarding its age as Late Cretaceous. This new locality was predominantly rich in avimimid fossils, with a lesser abundance of mammal and other dinosaur fossils.
In 1993 teams of a collaborative Mongolian-North American expedition (supported by the Mongolian Academy of Sciences and American Museum of Natural History) discovered a new fossil locality of the Djadochta Formation called Ukhaa Tolgod (meaning "Brown Hills"). Like previous localities, Ukhaa Tolgod has yielded a prominent abundance of well-preserved fossils, including high concentrations of mammal, dinosaur, lizard, and egg remains. A vast majority of specimens from this locality are usually found in near-complete articulation. Overall, when compared to other Mesozoic fossil sites, the diversity of fossils in Ukhaa Tolgod is unusually high.
Bayn Dzak (also spelled Bain Dzak, Bayanzag, Bayn Zag, Bayan Zag, or Shabarakh Usu; locally known as Flaming Cliffs): It is dominated by reddish-orange sandstones and well-sorted, unbedded, and medium-grained sands. The thickness of the strata at the Flaming Cliffs at least more than 30 m (98 ft). Less abundant lithology of Bayn Dzak includes cemented and poorly-cemented siltstones, mudstones, and grayish conglomerates. The latter are better exposed at western escarpments of the Flaming Cliffs. Bayn Dzak is about 90 m (300 ft) in total thickness and can be divided into two sections: alternations of horizontally-bedded sandstone and mudstone in the lowermost part, and sandstone-dominated successions in the upper or main part.
Tugriken Shireh (also spelled Tugrik, Toogreeg, Toogreek, Tugreek, Tugrug, Turgrugyin, Tugrugeen, Tögrögiin, Tugrikiin, or Tugrikin): This locality is about 30 m (98 ft) in thickness and characterized by poorly cemented, fine-grained sandstones that have colors varying from pink to yellowish-white. The predominant mineral is quartz, and lesser common minerals are represented by feldspars and lithic fragments. Both cross-stratified and structureless sandstones are scattered across Tugriken Shireh.
Udyn Sayr (also spelled Udan Sayr, Udan Sair, Ulaan Sair, or Üüden Sair): Sediments of this locality are exposed across a region of more than 60 km2 (23 sq mi). It is divided into lower (thickness of at least more than 10 m (33 ft)) and upper (thickness of about 50 m (160 ft)) beds. The lower beds are fluvial originated and dominated by sandstones and mudstones. The upper beds are likely of aeolian origin and consist of reddish, cross-stratified and structureless sandstones.
Ukhaa Tolgod (also spelled Oka Tolga): The strata exposed at Ukhaa Tolgod is dominated by reddish sandstones, with some sandstones containing small amounts of conglomeratic lenses and/or cobbles and pebbles. Conglomerate itself is are in this site, and to a lesser level are mudstones and siltstones, which are thin and laterally restricted. Cross-stratified and fine-structured sandstones are particularly abundant at Ukhaa Tolgod.
Zamyn Khondt (also spelled Dzamyn Khondt, Zamin Khond, or Dzamin Khond): This locality is characterized by reddish, well-sorted, and fine-grained sandstones with calcareous concretions. Some aeolian beds are present and are finely stratified to massive, having a visible thickness of about 20 m (66 ft).
The Djadochta Formation is separated into a lower Bayn Dzak Member and an upper Turgrugyin Member, which represent very similar depositional environments. Further strata from the Bayn Dzak Member includes that of the Ukhaa Tolgod locality, and its overall age is regarded also within the Campanian.
Based on the superposition of the members, the Tugrugyin Member overlies the Bayn Dzak Member making it somewhat younger, which indicates that the Bayn Dzak paleofauna lived somewhat earlier than that from Tugriken Shireh. However, it is not yet understood the precise temporal difference: Localities within the Djadochta Formation are considered to represent a sequence of progressively younger sediments and thereby paleofaunas. Ukhaa Tolgod may be younger than both Bayn Dzak and Tugriken Shireh. Based on their fossil record and strata, Udyn Sayr and Zamyn Khondt have been correlated with other Djadokhta localities, though fossils of Udyn Sayr may indicate that this locality is younger than Bayn Dzak and Tugriken Shireh.
Examinations on the strata of the Alag Teg (also spelled Alag Teeg or Alag Teer) locality, once considered part of this formation, indicates that it belongs to a different geological formation: the Alagteeg Formation, which is slightly older than the overlying Djadochta Formation. Based on sediments and stratigraphic relationships, the lower part of the Bayn Dzak locality is correlated with the Alag Teg locality, making both sections part of the Alagteeg Formation. The upper or main part of the former locality is considered part of the Djadochta Formation itself, as it shares similar lithology and stratigraphic relationships with Tugriken Shireh.
Examples of the Djadochta Formation preservation: articulated Citipati (top) and Protoceratops (bottom) specimens
A vast majority of articulated specimens from the Djadochta Formation are found in unstructured sandstones, indicating burial in situ by high-energy sand-bearing events. Some buried Protoceratops individuals are preserved in distinctive postures involving the body and head arched upwards, suggesting that the animals died in the process of trying to free themselves from the body of sand, where they eventually fossilized. As they were unable to escape burial, the sandy mass prevented carcasses from being scavenged by vertebrates. Most of these "buried" specimens are found with bite traces and large borings (tunnel-like holes made by small invertebrates) on bone joints areas and other surfaces, indicating that after death they were largely scavenged by invertebrates, such as skin beetles.
It has been suggested that the repeated occurrence of these feeding traces at limb joints may reflect that the responsible scavengers focused on collagen at the joint cartilage of dried dinosaur carcasses as a source of nitrogen, which was very low in the arid Djadochta Formation environments.
Examinations at the fossil preservation and sediments of Ukhaa Tolgod indicates that preserved animals were buried alive by catastrophic dune collapses. It is thought to have occurred when sand dunes became oversaturated with water resulting in their sudden downfall; heavy rainfall events likely acted as the triggering mechanism for this collapse. Examples from the Ukhaa Tolgod preservation include Citipati (brooding adults entombed atop nests and eggs);Khaan (a pair in close proximity likely killed by a single collapse event); and Saichangurvel (individual buried alive by a muddy dune).
Articulated Protoceratops from Tugriken Shireh. This dinosaur is one of the most common occurrences in the Djadochta Formation
Among fossils, Protoceratops is extremely common in Djadochta localities. Bayn Dzak is reported as one of the localities with the highest concentration of Protoceratops fossils and has been noted as the "Protoceratops fauna". Adjacent to Bayn Dzak, at Tugriken Shireh, Protoceratops is also abundant. Other common dinosaur components of the paleofauna include Pinacosaurus and Velociraptor. Small vertebrates like lizards and mammals are rather abundant and diverse, with Adamisaurus and Kryptobaatar being the most abundant representatives. The paleofauna of the Djadochta Formation is very similar in composition to the nearby and coeval-regarded Bayan Mandahu Formation of Inner Mongolia. The two formations share many of the same genera, but differ in species. For instance, the most common mammal in Djadochta is Kryptobaatar dashzevegi, while in Bayan Mandahu it is the closely related K. mandahuensis. Similarly, the dinosaur fauna of Djadochta includes Protoceratops andrewsi and Velociraptor mongoliensis, which Bayan Mandahu yields P. hellenikorhinus and V. osmolskae.
Although fossil plants are extremely rare in the Djadochta Formation, the great abundancy of herbivorousProtoceratops at the arid-deposited Tugriken Shireh locality indicates that it had a moderate coverage of bushes or other low-growing plants.
The relatively low paleobiodiversity and climate settings of the Djadochta suggest that these conditions contributed to stressed paleoenvironments. Most of the fossil occurrences in the formation are occupied by Protoceratops, and small to medium-sized ankylosaurs, oviraptorids, and dromaeosaurids make much of the overall paleofauna. Large-bodied animals are absent or extremely rare in the formation. Comparisons with the Nemegt Formation further reflects stressed paleoenvironments. In contrast to Djadochta, Nemegt has yielded an extensive diversity of large dinosaur taxa, such as Deinocheirus, Nemegtosaurus, Saurolophus, Tarbosaurus, or Therizinosaurus. Most of these taxa are herbivorous, which combined with the mesic (well-watered) settings of the Nemegt Formation allowed the development of giant herbivores, in contrast to the stressed Djadochta Formation. Another indicative of stressed paleoenvironments is the almost non-existent amount of fully aquatic animals. Turtles are rarely recovered, and most are terrestrial such as Zangerlia.
^ abcdHasegawa, H.; Tada, R.; Ichinnorov, N.; Minjin, C. (2009). "Lithostratigraphy and depositional environments of the Upper Cretaceous Djadokhta Formation, Ulan Nuur basin, southern Mongolia, and its paleoclimatic implication". Journal of Asian Earth Sciences. 35 (1): 13−26. Bibcode:2009JAESc..35...13H. doi:10.1016/j.jseaes.2008.11.010.
^ abcJerzykiewicz, T.; Currie, P. J.; Eberth, D. A.; Johnston, P. A.; Koster, E. H.; Zheng, J.-J. (1993). "Djadokhta Formation correlative strata in Chinese Inner Mongolia: an overview of the stratigraphy, sedimentary geology, and paleontology and comparisons with the type locality in the pre-Altai Gobi". Canadian Journal of Earth Sciences. 30 (10): 2180−2195. Bibcode:1993CaJES..30.2180J. doi:10.1139/e93-190.
^Horovitz, I. (2003). "Postcranial skeleton of Ukhaatherium nessovi (Eutheria, Mammalia) from the Late Cretaceous of Mongolia". Journal of Vertebrate Paleontology. 23 (4): 857−868. doi:10.1671/2399-10. JSTOR4524387. S2CID85809847.
^Suzuki, Chiappe, Dyke, Watabe, Barsbold and Tsogtbaatar, 2002. A new specimen of Shuvuuia deserti Chiappe et al., 1998, from the Mongolian Late Cretaceous with a discussion of the relationships of alvarezsaurids to other theropod dinosaurs. Contributions in Science. 494, 1-18.
^ abLongrich & Currie (2009). Albertonykus borealis, a new alvarezsaur (Dinosauria: Theropoda) from the Early Maastrichtian of Alberta, Canada: Implications for the systematics and ecology of the Alvarezsauridae. Cretaceous Research. 30(1), 239-252.
^Chiappe, L.M., Norell, M. A., and Clark, J. M. (1998). "The skull of a relative of the stem-group bird Mononykus." Nature, 392 (6673): 275-278.
^ abDufeau, 2003. The cranial anatomy of the theropod dinosaur Shuvuuia deserti (Coelurosauria: Alvarezsauridae), and its bearing upon coelurosaurian phylogeny. Masters Thesis, The University of Texas at Austin. 275 pp.
^Pei, 2015. New paravian fossils from the Mesozoic of east Asia and their bearing on the phylogeny of the Coelurosauria. PhD thesis, Columbia University. 545 pp.
^Saitta, E., Fletcher, I., Martin, P.G., Pittman, M., Kaye, T., True, L., Norell, M., Abbott, G., Summons, R., Penkman, K., & Vinther, J. (2018). Preservation of feather fibers from the Late Cretaceous dinosaur Shuvuuia deserti raises concern about immunohistochemical analyses on fossils. Organic Geochemistry. 125: 142-151.
^Norell, Chiappe and Clark, (1993). New limb on the avian family tree. Natural History. September, 38-43.
^Penkalski, P.; Tumanova, T. (2017). "The cranial morphology and taxonomic status of Tarchia (Dinosauria: Ankylosauridae) from the Upper Cretaceous of Mongolia". Cretaceous Research. 70: 117−12. doi:10.1016/j.cretres.2016.10.004.
^Gilmore, C. W. (1933). "Two new dinosaurian reptiles from Mongolia with notes on some fragmentary specimens". American Museum Novitates (679): 1–20. hdl:2246/2076.
^ abArbour, V. M.; Currie, P. J. (2013). "The taxonomic identity of a nearly complete ankylosaurid dinosaur skeleton from the Gobi Desert of Mongolia". Cretaceous Research. 46: 24−30. doi:10.1016/j.cretres.2013.08.008.
^Clarke, Julia A., Norell, Mark A. (2002). "The morphology and phylogenetic position of Apsaravis ukhaana from the Late Cretaceous of Mongolia". American Museum Novitates, No. 3387, American Museum of Natural History, New York, NY.
^Chiappe, L. M.; Suzuki, S.; Dyke, G. J.; Watabe, M.; Tsogtbaatar, K.; Barsbold, R. (2007). "A new Enantiornithine bird from the Late Cretaceous of the Gobi desert". Journal of Systematic Palaeontology. 5 (2): 193−208. doi:10.1017/S1477201906001969. S2CID85391743.
^Chiappe, L, M.; Norell M. A.; Clark, J. M. (2001). A new skull of Gobipteryx minuta (Aves: Enantiornithes) from the Cretaceous of the Gobi Desert. American Museum Novitates. 3346, 1-15.
^Handa, N.; Watabe, M.; Tsogtbaatar, K. (2012). "New Specimens of Protoceratops (Dinosauria: Neoceratopsia) from the Upper Cretaceous in Udyn Sayr, Southern Gobi Area, Mongolia". Paleontological Research. 16 (3): 179−198. doi:10.2517/1342-8144-16.3.179. S2CID130903035.
^Norell and Makovicky, 1997. Important features of the dromaeosaur skeleton: Information from a new specimen. American Museum Novitates. 3215, 28 pp.
^Barsbold and Osmólska, 1999. The skull of Velociraptor (Theropoda) from the Late Cretaceous of Mongolia. Acta Palaeontologica Polonica. 44(2), 189-219.
^Norell and Makovicky, 1999. Important features of the dromaeosaurid skeleton II: Information from newly collected specimens of Velociraptor mongoliensis. American Museum Novitates. 3282, 45 pp.
^Watabe and Tsogtbaatar, 2004. Report on the Japan - Mongolia Joint Paleontological Expedition to the Gobi desert, 2000. Hayashibara Museum of Natural Sciences Research Bulletin. 2, 45-67.
^Turner, Makovicky and Norell, 2007. Feather quill knobs in the dinosaur Velociraptor. Science. 317, 1721.
^David Hone; Jonah Choiniere; Corwin Sullivan; Xing Xu; Michael Pittman; Qingwei Tan (2010). New evidence for a trophic relationship between the dinosaurs Velociraptor and Protoceratops. , 291(3-4), 0−492. doi:10.1016/j.palaeo.2010.03.028.
^Watabe and Suzuki, (2000). Report on the Japan - Mongolia Joint Paleontological Expedition to the Gobi desert, 1996. Hayashibara Museum of Natural Sciences Research Bulletin. 1, 58-68.
^Barsbold, R.; Perle, A. (1983). "On taphonomy of joint burial of juvenile dinosaurs and some aspects of their ecology". Transactions of the Joint Soviet-Mongolian Paleontological Expedition (in Russian). 24: 121−125.
^Watabe, M.; Suzuki, S.; Tsogtbaatar, K. (2006). "Geological and geographical distribution of bird-like theropod, Avimimus in Mongolia". Journal of Vertebrate Paleontology. 26 (supp. 003): 136A−137A. doi:10.1080/02724634.2006.10010069. S2CID220413406.
^Pei, R.; Norell, M. A. (2011). "A new troodontid (Dinosauria: Theropoda) from the Late Cretaceous Djadokhta Formation of Mongolia". Journal of Vertebrate Paleontology. 31 (supp. 002): 172A. doi:10.1080/02724634.2011.10635174.