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The Djadokhta Formation was named by Berkey and Morris (1927), based on the fossiliferous sandstone-dominant beds at Bayan Zak (= Flaming Cliffs) in the central part of the Gobi Desert. It is this unit that has yielded a rich dinosaur fauna, mammals, and dinosaur eggs within nest structures. Although its depositional environments were originally considered lacustrine (Verzilin, 1979, 1982), it is now clear that they mainly represent eolian environments (Eberth, 1993; Fastovsky et al., 1997; Loope et al., 1998). Other Campanian-age localities that consist of eolian beds rich in dinosaur fossils are also known in the Gobi Desert of both Mongolia and China, and have been correlated with the Djadokhta Formation on the basis of similarities in vertebrate faunal composition and depositional environments (Eberth, 1993; Jerzykiewicz et al., 1993). Below, we briefly review the major Djadokhta localities of Mongolia.

Bayn Dzak Bayn Dzak is situated 15 km east-northeast of Bulgan Sum in the South Gobi Aimag (Fig. 7.1). Known as the “Flaming Cliffs,” this locality was originally described by Roy Chapman Andrews of the Central Asiatic Expedition, American Museum of Natural History, in the 1920s and has subsequently been described by other researchers (see Dashzeveg et al., 2005). The fossiliferous beds at this locality consist of eolian sandstones, fluvial-eolian sandstones with paleosol layers, and red-colored mudstone strata of lacustrine origin. Although many specimens, including Protoceratops andrewsi, Pinacosaurus grangeri, Oviraptor philoceratops, turtles, and lizards, as well as dinosaur eggs and nests (elongatoolithid form), have been found in these beds, no ornithopod specimens have been reported from this locality to date (Dashzeveg et al., 2005). This locality has also produced the holotypes of Velociraptor mongoliensis and Saurornithoides mongoliensis (Osborn, 1924). The fauna was recently summarized by Dashzeveg et al. (2005), who cited Oviraptor (= Rinchenia) mongoliensis as present at this locality. However, Rinchenia mongoliensis is known only from the Maastrichtian Nemegt Formation (Osmólska et al., 2004), whereas the holotype of Oviraptor philoceratops was collected at Bayn Dzak (Osborn, 1924).

Tögrögiin Shiree Locality Tögrögiin Shiree (Tugrikin-Shireh of Fastovsky et al., 1997), discovered by the joint Polish-Mongolian Expeditions in the late 1960s and early 1970s, is situated 26 km northwest of Bulgan Sum of the South Gobi Aimag (Fig. 7.1). The fossiliferous beds at this locality consist mainly of fine- to medium-grained sandstones with large-scale cross stratification (foresets inclined more than 30°). Trace fossils (simple and branching vertical sand pipes) and dinosaur footprints are abundant on the bedding planes of these beds. These sediments have been interpreted as originating as eolian dune sands (Fastovsky et al., 1997). The combined thickness of the eolian sandstone beds is 50 m (Watabe and Suzuki, 2000). Small, presumed infant individuals of a hadrosauroid have been found in the eolian beds at this locality (Barsbold and Perle, 1983). These specimens will be described elsewhere. An abundance of vertebrate fossils have been found from the eolian beds, including Velociraptor, Protoceratops, Shuvuuia (an alvarezsaurid), lizards, multituberculate mammals, an enantiornithine bird, and dinosaur eggs and nests (Mikhailov, 1991) (elongatoolithid and Protoceratopsidovum forms [Mikhailov et al., 1994]). The fauna was summarized by Dashzeveg et al. (2005), who did not recognize any ornithischians from the locality. However, skeletal remains of Protoceratops andrewsi are common at this locality (Osmólska, 1993; Fastovsky et al., 1997; Fastovsky et al., 2011)

7.1. Map of Mongolia showing the major Djadokhta Formation localities in the Gobi Desert. Numbers correspond to the following localities: (1) Alag Teeg; (2) Tögrögiin Shiree; (3) Bayn Dzak; (4) Üüdin Sair; (5) Dzamyn Khond; (6) Abdrant Nuru; (7) Ukhaa Tolgod; (8) Khoolson; (9) Hermiin Tsav-II; (10) Shilüüt Uul.

Alag Teeg The specimens of Plesiohadros djadokhtaensis nov. gen. et sp. reported herein were found at Alag Teeg (Fig. 7.1), situated 3 km north of Tögrögiin Shiree (Tverdokhlebov and Tsybin, 1974). Alluvial and paludal facies dominate at this locality, thus differing from most other correlative localities (D. Eberth, pers. comm., 2012). Eolian sandstone beds with large-scale cross stratification and invertebrate trace fossils overlie the fluvial beds and consist of white and red sandstone, and red and brown mudstone layers. Total thickness of the fluvial part of the section at Alag Teeg is 33 m (Suzuki and Watabe, 2000; Watabe and Suzuki, 2000).

The cranial and postcranial bones of P. djadokhtaensis were discovered in mudstone beds in the lowermost part of the geological section at the locality. An articulated pes referred to this species was discovered at a different site. The pes was buried in a mudstone layer with its dorsoventral long axis vertical, and thus perpendicular to the bedding plane, which suggests that the animal was killed in a “mud trap” (Watabe and Tsogtbaatar, 2007).

Üüdin Sair Üüdin Sair is situated 35 km southwest of Tögrögiin Shiree. The section consists of a lower portion of large-scale inclined strata comprising alternating sandstone and mudstone layers, and an upper portion of fine-grained reddish sandstone with large-scale cross stratification. The lower portion is interpreted as the remains of large-scale point bar deposits and the upper portion is interpreted as consisting of eolian dune deposits. Although there are many vertebrate fossils at the locality, including protoceratopsids (a large form, Udanoceratops tschizhovi, and several smaller forms), Pinacosaurus, mammals, lizards, turtles, and dinosaur eggs and nests (elongatoolithid form), no ornithopod specimens have so far been recognized in these collections.

Dzamyn Khond Dzamyn Khond, 9 km southwest of Üüdin Sair (Fig. 7.1), is a small outcrop consisting of fine-grained red sandstones with cross stratification. The beds are considered to be eolian dune deposits. An unnamed oviraptorine (represented by the well-preserved skeleton IGM 100/42, Clark et al., 2002), Pinacosaurus, lizards, and mammals have been found from a small outcrop here; and different kinds of dinosaur eggs (at least three forms, belonging to the elongatoolithid form) and nests have also been found.

Ukhaa Tolgod Ukhaa Tolgod, located 220 km southwest of Bayn Dzak (Fig. 7.1), provides one of the most diverse of all Djadokhta-age assemblages (Dingus et al., 2008). The fossiliferous strata consist of fine-grained, cross-stratified red sandstones, considered to be eolian dune deposits. Worked by the Mongolian Academy of Sciences–American Museum of Natural History expeditions, this locality constitutes one of the most recently discovered of the Djadokhta-age sites to be exploited, and has already yielded upward of a dozen new species within the last two decades. Khaan, Tsaagan, and numerous other theropods; Pinacosaurus; Protoceratops; lizards; turtles; mammals; as well as several kinds of dinosaur eggs and nests have been recovered at Ukhaa Tolgod, as summarized recently by Dingus et al. (2008).