TheDjadochta formation(sometimestranscribedand also known asDjadokhta,Djadokata,orDzhadokhtskaya) is a highlyfossiliferousgeological formationin Central Asia,Gobi Desert,dating from theLate Cretaceousperiod, about 75 million to 71 million years ago. Thetype localityis the Bayn Dzak locality, famously known as theFlaming Cliffs.Reptile (including dinosaur) and mammal remains are among the fossils recovered from the formation.
The Djadochta formation was first documented and explored—though only a single locality—duringpaleontologicalexpeditions of theAmerican Museum of Natural Historyin 1922–1925, which were part of theCentral Asiatic expeditions.The expeditions were led byRoy Chapman Andrews,in company ofWalter Willis Grangeras chief paleontologist and field team. The team did extensive exploration at the Bayn Dzak (formerly Shabarakh Usu) region, which they nicknamedflaming cliffs,given that at sunset the sediments of this locality had a characteristic reddish color. Notable finds included the first known fossils ofOviraptor,Protoceratops,Saurornithoides,andVelociraptor,the first confirmed dinosaur eggs (a partial nest ofOviraptor), as well as fossil mammals. Some of these were briefly described byHenry Fairfield Osbornduring the ongoing years of the expeditions. In 1927 the formation was formally described and established by Berkey and Morris, with Bayn Dzak as thetype locality.[1][2]
In 1963 the Mongolian paleontologist Demberelyin Dashzeveg reported the discovery of a new fossiliferous locality of the Djadochta Formation: Tugriken Shireh.[3]During the 1960s to 1970s, Polish-Mongolian and Russian-Mongolian paleontological expeditions collected new, partial to complete specimens ofProtoceratopsandVelociraptorat this locality, making these dinosaur species a common occurrence in Tugriken Shireh.[4]Some of the most notable excavations made at Tugriken Shireh include theFighting Dinosaurs(ProtoceratopsandVelociraptorlocked in combat),[5][6]and abundant articulated,in situ(in the original pose), and sometimes complete skeletons ofProtoceratops.[7][8]
During the 1980s a jointSoviet-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 inavimimidfossils, with a lesser abundance of mammal and other dinosaur fossils.[4]
In 1993 a collaborative expedition by the Mongolian Academy of Sciences and the American Museum of Natural History discovered a new fossil site within the Djadochta Formation, named Ukhaa Tolgod, which translates to "Brown Hills." This site has yielded a significant number of well-preserved fossils, including those of mammals, dinosaurs, lizards, and eggs. Most specimens are found in near-complete articulation, indicating excellent preservation conditions. Compared to other Mesozoic fossil sites, Ukhaa Tolgod stands out for its high fossil diversity.[9][10]
The modern-day Djadochta Formation is set in an arid habitat of sand dunes with little freshwater apart from oases andarroyos,in the Gobi Desert. The dominantlithologyof the Djadochta Formation is represented by non-marine,cementedreddish-orange and pale orange to light gray, medium to fine-grained sands andsandstones,which include minor deposits ofcalcareousconcretionsand orange-brown siltyclay.Less abundantsedimentationcomprisesconglomerates,siltstones,fluvial(water-deposited) sandstones, andmudstones.The entire thickness of the formation in theUlan Nur Basinis at least 80 m (260 ft). Severalaeolian processes(wind works) indicate the presence of large straight-crested dune-like structures, and smallerbarchan(crescent-shaped) and parabolic (poorly U-shaped) dunes across the formation.[11][12][10]Reddish sandstones are observed in numerous localities.[12][10]
Bayn Dzak(also spelled Bain Dzak, Bayanzag, Bayn Zag, Bayan Zag, or Shabarakh Usu; locally known asFlaming 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.[13][12]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.[14]
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 isquartz,and lesser common minerals are represented by feldspars and lithic fragments. Bothcross-stratifiedand structureless sandstones are scattered across Tugriken Shireh.[15][12]
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.[16]
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 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.[17][10]
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).[16]
The Djadochta formation occurs in theLate Cretaceousperiod of theCampanianstage.Magnetostratigraphicdatings from the Bayn Dzak and Tugriken Shireh localities suggest that the Djadochta Formation was deposited during a time of rapidly changingpolarityat about 75 million to 71million years ago.[12]
The Djadochta formation is separated into a lower Bayn Dzak Member and an upper Turgrugyin Member, which represent very similar depositional environments.[12]Furtherstratafrom the Bayn Dzak Member includes that of the Ukhaa Tolgod locality, and its overall age is regarded also within the Campanian.[10]
Based on thesuperpositionof the members, the Tugrugyin Memberoverliesthe 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:[12]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.[18]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.[19]
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: theAlagteeg 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.[14]
Based on strata and rockfacies(such as sandstones andcaliche) of the formation and coeval units (Bayan Mandahu) it is currently agreed thatsedimentsof the Djadochta Formationwere depositedbywind activityin arid paleoenvironments comprising sand dunes with a warmsemi-aridclimate.[20][12][14]Fluvial sedimentation at the Ukhaa Tolgod locality indicates the presence of short-livedwater bodiesduring the times of the formation, which also contributed to its deposition.[10]
Examples of the Djadochta Formation preservation: articulatedCitipati(top) andProtoceratops(bottom) specimens
A vast majority of articulated specimens from the Djadochta Formation are found in unstructured sandstones, indicating burialin situby high-energy sand-bearing events. Some buriedProtoceratopsindividuals 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 asskin beetles.[21][22][23]
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.[24]
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.[9][25][10]Examples from the Ukhaa Tolgod preservation includeCitipati(brooding adults entombed atop nests and eggs);[26][27]Khaan(a pair in close proximity likely killed by a single collapse event);[28]andSaichangurvel(individual buried alive by a muddy dune).[29]
Among fossils,Protoceratopsis extremely common in Djadochta localities. Bayn Dzak is reported as one of the localities with the highest concentration ofProtoceratopsfossils and has been noted as the "Protoceratopsfauna ".[30]Adjacent to Bayn Dzak, at Tugriken Shireh,Protoceratopsis also abundant.[15]Other common dinosaur components of the paleofauna includePinacosaurusandVelociraptor.[11]Small vertebrates like lizards and mammals are rather abundant and diverse, withAdamisaurusandKryptobaatarbeing the most abundant representatives.[29][31][30]The paleofauna of the Djadochta Formation is very similar in composition to the nearby and coeval-regardedBayan Mandahu FormationofInner Mongolia.The two formations share many of the same genera, but differ in species. For instance, the most common mammal in Djadochta isKryptobaatar dashzevegi,while in Bayan Mandahu it is the closely relatedK. mandahuensis.Similarly, the dinosaur fauna of Djadochta includesProtoceratops andrewsiandVelociraptor mongoliensis,which Bayan Mandahu yieldsP. hellenikorhinusandV. osmolskae.[20][32]
Although fossil plants are extremely rare in the Djadochta Formation, the great abundancy ofherbivorousProtoceratopsat the arid-deposited Tugriken Shireh locality indicates that it had a moderate coverage ofbushesor other low-growing plants.[15]
The relatively lowpaleobiodiversityand climate settings of the Djadochta suggest that these conditions contributed tostressedpaleoenvironments. Most of the fossil occurrences in the formation are occupied byProtoceratops,and small to medium-sizedankylosaurs,oviraptorids,anddromaeosauridsmake much of the overallpaleofauna.Large-bodiedanimals are absent or extremely rare in the formation. Comparisons with theNemegt Formationfurther reflects stressed paleoenvironments. In contrast to Djadochta, Nemegt has yielded an extensive diversity of large dinosaur taxa, such asDeinocheirus,Nemegtosaurus,Saurolophus,Tarbosaurus,orTherizinosaurus.Most of these taxa are herbivorous, which combined with themesic(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 fullyaquatic animals.Turtles are rarely recovered, and most are terrestrial such asZangerlia.[30]It is suggested that most of the fragmented hadrosaur, tyrannosaur and sauropod remains across the formation likely belong to non-endemic,passing by species.[30]
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