You, H.-L., Z.-X. Luo, N. H. Shubin, L. M. Witmer, Z.-L. Tang, and F. Tang. 2003. The earliest-known duck-billed dinosaur from deposits of late Early Cretaceous age in northwest China and hadrosaur evolution. Cretaceous Research 24:347–355.
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A New Hadrosauroid (Plesiohadros djadokhtaensis) from the Late Cretaceous Djadokhtan Fauna of Southern Mongolia
Khishigjav Tsogtbaatar, David B. Weishampel, David C. Evans, and Mahito Watabe
ABSTRACT
The Djadokhta Formation (Campanian) has been intensely surveyed and sampled for its fossil vertebrate fauna for almost a century. Its dinosaur fauna is historically defined as being dominated by the ornithischians Protoceratops and Pinacosaurus, as well as oviraptorid theropods, but recent collecting efforts continue to yield new taxa. Here we describe and name the first ornithopod from this unit. The material consists of an almost complete skull and associated limb material collected from the Alag Teeg locality. Plesiohadros djadokhtaensis nov. gen. et nov. sp. is characterized by a unique combination of plesiomorphic and derived characters within Hadrosauroidea, and an autapomorphically rugose, raised rim around the rostrodoral margin of the orbit. In order to place P. djadokhtaensis into its paleoecological and stratigraphic context within these Upper Cretaceous rocks of Mongolia, we also provide a review of the geological setting of the Djadokhta Formation in the Gobi Desert. Plesiohadros is the largest member of the Djadokhta fauna, and the only known hadrosauroid from the Campanian of Mongolia. An updated phylogenetic analysis indicates that P. djadokhtaensis is a derived hadrosauroid proximate to Hadrosauridae (sensu Sereno). Plesiohadros is posited as the sister taxon to a clade that includes Lophorhothon and Hadrosauridae, and is more closely related to Hadrosauridae than to Bactrosaurus, Telmatosaurus, or Tethyhadros. The complete replacement of non-hadrosaurid hadrosauroids with true hadrosaurids in the Maastrichtian of Mongolia highlights the complex pattern of faunal interchange between Asian and North America in the latest Cretaceous.
INTRODUCTION
After nearly 100 years of scientific exploitation, the Upper Cretaceous rocks of the Gobi Desert in southern Mongolia continue to yield new and unusual dinosaur taxa. In the last decade alone over a dozen new taxa have been named – including the theropods Tsaagan, Mahakala, and Kol; and the ceratopsians Bainoceratops, Yamaceratops, and Gobiceratops (Norell et al., 2006; Turner et al., 2007, 2009; Tereschenko and Alifanov, 2003; Makovicky and Norell, 2006; Alifanov, 2008, respectively). Ornithopods are considerably less diverse in these deposits than in contemporaneous Upper Cretaceous dinosaur-bearing units of western North America and Europe (Horner et al., 2004; Weishampel and Jainu, 2011; Dalla Vecchia, this volume), which are, in general, dominated by hadrosaurids. Only one new ornithopod taxon, Haya griva, has been identified from the Late Cretaceous of Mongolia in the last three decades (Makovicky et al., 2011).
In this chapter, we describe a new genus and species of hadrosauroid ornithopod, Plesiohadros djadokhtaensis, from the Alag Teeg locality, southern Gobi Desert, Mongolia. Plesiohadros is the second record of an ornithopod from the Djadokhta Formation (Barsbold and Perle, 1983), and the only named hadrosauroid taxon from Campanian-age rocks in Mongolia. In order to place Plesiohadros djadokhtaensis into its paleoecological and stratigraphic context within these Upper Cretaceous rocks of Mongolia, we outline the geological setting of the Djadokhta Formation at Alag Teeg and elsewhere in the Gobi Desert, review the different vertebrate assemblages of Djadokhta age, and assess the chronostratigraphy of the Djadokhta Formation. P. djadokhtaensis is then described, and its systematic relationships within Hadrosauroidea are assessed. Finally, we comment on the biostratrigraphy and biogeographic implications of this new form within the context of our updated hadrosauroid phylogeny.
Institutional Abbreviations MPC, Mongolian Paleontology Centre, Ulan Baatar, Mongolia; ROM, Royal Ontario Museum, Toronto, Ontario; MNHN GDF, Muséum national d’Histoire naturelle, Paris, France.
GEOLOGICAL SETTING
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;