Hadrosaurs. David A. Eberth
(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).
Sedimentological Environments of the Djadokhta Formation
Fastovsky et al. (1997) demonstrated the widespread existence of eolian depositional environments in the Djadokhta Formation at Tögrögiin Shiree, more specifically an erg (= sand sea) with interconnected eolian dunes. Fluvial environments occurred in patches between the dunes (at Alag Teeg and Bayn Dzak). Based on the rich discoveries of vertebrate fossils including dinosaurs (and their eggs, nests, and infant individuals) in Djadokhta eolian beds, it suggests the possibility that the eolian (arid) environments of the Djadokhta age were inhabited by dinosaurs and other vertebrates. Ornithopod specimens found at Alag Teeg are the only records of this taxonomic group in the Djadokhta Formation, making Plesiohadros djadokhtaensis an apparently minor component of the dinosaur fauna in this overall arid setting. However, the eolian depositional environments and lithology seem to preferentially preserve the remains of small vertebrates such as small-sized theropods, lizards, and mammals. But the occurrence of aquatic vertebrates, such as turtles and crocodiles, is rare. The dominance of eolian deposits at these vertebrate fossil bearing localities suggests that the faunal composition is preservationally biased in favor of small body sizes.
Age of the Djadokhta Formation
The time interval represented by the Djadokhta Formation is difficult to estimate. Based on patterns of mammalian biostratigraphy, exposures of the Djadokhta Formation throughout the region have been suggested to be Campanian in age (Kielan-Jaworowska, 1974; ~80 Ma, Jerzykiewicz and Russell, 1991; Jerzykiewicz et al., 1993). However, because the lower and upper boundaries of the Djadokhta Formation cannot be observed in outcrops at the type and other localities, it is unknown if a Campanian age applies to the entire formation throughout the region. The thick eolian redbeds that dominate these localities do not lend themselves easily to age assessments using paleomagnetostratigraphic means (J. Hicks, pers. comm.); however, recent paleomagnetostratigraphic work by Dashzeveg et al. (2005) suggests that the Byan Dzak sediments were deposited during the rapid succession of polarity changes during the Late Campanian (~71–75 Ma).
Djadokhta-like outcrops are also present at Khulsan, Nemegt, and Khermeen Tsav in the Nemegt Basin. The upper boundary with the Nemegt Formation can be observed at Nemegt. At Khermeen Tsav, the upper and lower boundaries with fluvial beds are observed. The upper boundary of the formation is transitional with the overlying Nemegt Formation, where Djadokhta-like redbeds are referred to as the Baruungoyot Formation (Eberth et al., 2009).
SYSTEMATIC PALEONTOLOGY
DINOSAURIA Owen, 1842
ORNITHISCHIA Seeley, 1887
ORNITHOPOD Marsh, 1881
IGUANODONTIA Sereno, 1986
HADROSAUROIDEA sensu Sereno, 2005
PLESIOHADROS, gen. nov.
Type Species PLESIOHADROS DJADOKHTAENSIS sp. nov.
7.2. Photomosaic schematic of the reassembled skull of Plesiohadros djadokhtaensis (MPC-D100/745). (A) in right lateral view; (B) interpretive line drawing of schematic. Abbreviations: br.c, braincase; cor, coronoid process; cot, glenoid for the lateral quadrate condyle; d, dentary; f, frontal; j, jugal; mx(L), maxilla (left); orb, orbit; p, parietal; pd, predentary; pmx(L), premaxilla (left); po, postorbital; poc, paroccipital process; prf, prefrontal; q, quadrate; qj, quadratojugal; ret, retroarticular process; sq, squamosal. Scale bar equals 10 cm.
7.3. Partial left premaxilla of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) lateral; (B) dorsal; (C) rostral; and (D) ventral views. Scale bars equal 5 cm. Abbreviations: ex.l, external lip; for, foramen; in.l, internal lip; om, oral margin; rf.rm, reflected rim.
Etymology Plesios (“near”); hadros (“hadrosaurid”); in reference to its systematic position as a proximate sister taxon to Hadrosauridae.
Diagnosis As per the type and only species.
Plesiohadros djadokhtaensis sp. nov.
Etymology From the Djadokhta Formation.
Holotype MPC-D100/745: partial skull, lower jaw, cervical vertebra, hyoid, metacarpals, and phalanges.
Referred Material MPC-D100/751: tibia, fibula, astragalus,