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Plesiohadros djadokhtaensis is characterized by a unique combination of plesiomorphic and derived characters relative to more basal iguanodontians and hadrosaurids, as well as an autapomorphic prefrontal that flares dorsolaterally to form a rugose, everted, wing-like rim around the rostrodorsal orbital margin, which may have even supported a small horn-like keratinous projection in life. Similar circumorbital rugosity has been described on the postorbital of the hadrosauroid Jeyawati from the Turonian of western North America (McDonald et al., 2010).

Plesiohadros djadokhtaensis is the first hadrosauroid identified from the fossiliferous Djadokhta Formation in Mongolia, and the first new member of this clade named from the Late Cretaceous of Mongolia in three decades. Remarkably, this is only the second report of remains referable to Hadrosauroidea from these deposits, despite extensive sampling and research (Dashzeveg et al., 2005; Dingus et al., 2008). The only previous report of hadrosauroid material from this formation was the brief note on numerous articulated infant hadrosauroid skeletons from the Tögrögiin Shiree locality by Barsbold and Perle (1983). Together, these represent the only Campanian occurrences hadrosauroids in Mongolia. In the Maastrichtian, two species of hadrosaurids are recognized: the common Saurolophus angustirostris and the rare Barsboldia sicinskii, both from the Nemegt Formation (Bell, 2011; Prieto-Márquez, 2011b). However, hadrosaurids are both taxonomically diverse and numerically abundant in the Campanian–Maastricthian of North America (Horner et al., 2004; Campione and Evans, 2011), as well as in the Amur region of Eastern Russia and adjacent China (e.g., Godefroit, Bolotsky, and Lauters, 2012; Bolotsky et al., this volume) and possibly the European archipelago (Weishampel and Jainu, 2011; Dalla Vecchia, this volume). It thus appears that hadrosauroids were less diverse and less abundant in the latest Cretaceous of Mongolia than in many other parts of Laurasia during the Campanian and Maastrichtian.

7.24. Strict consensus tree of ten most-parsimonious trees (tree length = 309). See text for explanation of phylogenetic analysis and tree statistics. Numbers in black text to the left of the nodes are Bremer Decay (left) and bootstrap (right) values, whereas numbers in gray text to the right of the nodes indicate the number of synapomorphies that support the node. Only bootstrap values in excess of 50% are reported.

Plesiohadros is the largest member of the Djadokhta fauna. With a reconstructed skull length of approximately 820 mm, Plesiohadros likely attained the body size of a typical hadrosaurid, such as Hypacrosaurus (Evans, 2010) or Brachylophosaurus (Cuthbertson and Holmes, 2010), which have comparable skull lengths and reach total body lengths in excess of 10 m and estimated body masses of at least 2 tons (Paul, 1997). The Djadokhta fauna is characterized by the presence of the small-bodied ceratopsian Protoceratops andrewsi, as well as a number of small theropod taxa including Oviraptor and Velociraptor; apart from Plesiohadros, the largest members of the fauna are the ankylosaur Pinacosaurus and an unnamed tyrannosaur (Longrich, 2010).

The probable late Campanian age of Plesiohadros djadokhtaensis is significant, as this new taxon represents the youngest occurrence on a non-hadrosaurid hadrosauroid from Mongolia, after which it appears that non-hadrosaurid hadrosauroids were replaced by the immigration of true hadrosaurids into the area, including Saurolophus and Barsboldia, which likely originated after a dispersal event from western North America to Asia via Beringia (Prieto-Márquez, 2010a). At present, there are no dinosaur fossil–bearing localities in either China or Mongolia where true hadrosaurids and non-hadrosaurid hadrosauroids co-occur.

The Nemegt Formation is considered to represent a time when relatively more humid climates prevailed in the Gobi region of southern Mongolia and northern China (e.g., Jerzykiewicz and Russell, 1991). In contrast, the Djadokhta and Baruungoyot formations were deposited when more arid climatic conditions prevailed in the same area. Accordingly, the disappearance of derived non-hadrosaurid hadrosauroids in Mongolian Late Cretaceous (post–Djadokhta age) might have been a response to the shift from arid to more mesic climatic conditions. There is no fossil record of hadrosauroids from the Baruungoyot Formation; the pattern of turnover of ornithopod assemblages from hadrosauroid to hadrosaurid is not clearly shown in the Gobi desert. Likewise, the immigration of derived hadrosaurids from North America to Mongolia (Central Asia) may be related to this regional shift from more arid to more mesic climates.

As in Mongolia, there are no definitive occurrences of non-hadrosaurid hadrosauroids in the hadrosaur-dominated assemblages of the Maastrichtian of North America (Campione and Evans, 2011; Evans et al., 2011; Campione et al., 2013), suggesting that non-hadrosaurid hadrosauroids were completely replaced by hadrosaurids in Laurasia by the Maastrichtian, except where they (e.g., Telmatosaurus transsylvanicus and Tethyshadros insularis) persisted as island relicts in the European archipelago (Weishampel et al., 1993; Dalla Vecchia, 2009; Weishampel and Jainu, 2011). The other exception to this might be in the Iren Dabasu Formation in Inner Mongolia (China). The stratigraphy and sedimentology of the Iren Dabasu Formation, which hosts the derived non-hadrosaurid hadrosauroids Gilmoreosaurus mongoliensis and Bactrosaurus johnsoni (Gilmore and Granger, 1933; Prieto-Márquez and Norell, 2010; Prieto-Márquez, 2011a), has recently been restudied by Van Itterbeeck et al. (2005). Although originally considered Senonian in age (Currie and Eberth, 1993), Van Itterbeeck et al. (2005) concluded that the Iren Dabasu Formation is most probably latest Campanian–early Maastrichtian in age, based on their interpretation of the microfossil assemblage, particularly four species of charophytes and eight species of ostracods. If correct, these would be the youngest occurrence of non-hadrosaurid hadrosauroids in Asia, although Sues and Averianov (2009) argued that the data presented by Van Itterbeeck et al. (2005) suggesting a Maastrichtian age for the Iren Dabasu Formation is equivocal.

Although the age and occurrence of Plesiohadros suggest that hadrosaurids replaced derived hadrosauroids in Mongolia in the early Maastrichtian, they do not reveal whether Hadrosauridae originated in North America (Prieto-Márquez, 2010a) or Asia (Godefroit et al., 2008). The occurrence and phylogenetic position of P. djadokhtaensis, together with other hadrosauroids in the context of this phylogenetic analysis, supports the hypothesis that Eurasia played a key role in hadrosaurid origins (Russell, 1993). However, the new data provided by P. djadokhtaensis suggests that the dispersal event took place in the common ancestor of the enigmatic North American form Lophorhothon atopus and Hadrosauridae (using Fitch optimization of continental areas on the phylogeny in Fig. 7.24). This predates the origin of Hadrosauridae proper, at least under most definitions proposed in the last two decades (Godefroit et al., 1998; Horner et al., 2004; Sereno, 2005; Sues and Averianov, 2009), but because Hadrosaurus foulkii is not included in this analysis, it cannot be evaluated for the definition of Prieto-Márquez (2010b). Furthermore, there are hadrosaurids in North America (e.g., Gryposaurus latidens) and Asia (e.g., Aralosaurus, Jaxartosaurus) that are older than the late Campanian age of Plesiohadros (Prieto-Márquez, 2010a). However, this study does support the hypothesis that some Maastrichtian hadrosaurids of Asia likely represent immigration events from North America (e.g., Prieto-Márquez, 2010a). It is important to note that the phylogeny presented here is not complete in terms of taxon sampling, which is likely to affect the general biogeographic conclusions presented here. However, it is becoming increasingly clear that the Late Cretaceous faunal interchange between Asia and North America may be more complex than previously considered (Xu et al., 2010; Prieto-Márquez, 2010a; Evans et al., 2013), and more phylogenetic work on the Mongolian dinosaurs will help to clarify general patterns of Late Cretaceous biogeography.