Locality and Horizon Djadokhta Formation, ?Upper Campanian, Upper Cretaceous. MPC-D100/745 was collected in sediments exposed in the northern hills of Alag Teeg, Gobi Desert, South Gobi Aimag. MPC-D100/751 was collected in exposures of the southern main cliff of the same general locality.
Diagnosis Large non-hadrosaurid hadrosauroid characterized by the following unique combination of characters: ornamentation on lingual surface of the dentary teeth includes only one or two accessory ridges located mesial and distal to primary carina; length of diastema between first dentary tooth and predentary on the dentary extremely long, equal to approximately one-third of tooth row; caudal extent of dentary tooth row does not extend beyond the apex of the coronoid process; reflected rim around oral margin of the premaxilla; premaxillary bill constricted abruptly behind the oral margin; poorly defined quadratojugal embayment of the quadrate; acute angle between postorbital bar and jugular bar on the infratemporal fenestra; and proximal end of metacarpal III aligned with that of metacarpals II and IV. Plesiohadros is also characterized by one autapomorphy: prefrontal flares dorsolaterally to form a rugose, everted, winglike rim around rostrodorsal orbital margin.
DESCRIPTION
Based on our reconstruction (Fig. 7.2), the length of the skull of Plesiohadros djadokhtaensis is approximately 820 mm, with a height through the quadrates of 420 mm and a width across the paroccipital processes of 180 mm. These are approximate dimensions given that the specimen is highly weathered and fractured. The region of the external naris, the antorbital fenestrae and fossa, and the orbit are not preserved. The shape of the supratemporal fenestra is unknown, but it is at least 120 mm long. The infratemporal fenestra is much shorter than tall and ovate in lateral view, judging from the shape of the postorbital process of the jugal (see below). The occiput is triangular and narrow dorsally. The ventral part of the quadrates splay distinctly laterally. The foramen magnum is 36 mm tall and 53 mm wide.
7.4. Rostral half of left maxilla of Plesiohadros djadokhtaensis (MPC-D100/745) in lateral (top) and medial (bottom) views. Scale bars equal 5 cm. Abbreviations: a.f, alveolar foramina; a.p, alveolar parapet; pmx.s, sutural surface for premaxilla; rm.p, rostromedial process; rl.p, rostrolateral process.
7.5. Right jugal of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) lateral; (B) medial; (C) dorsal; and (D) rostral views. Scale bars equal 10 cm. Abbreviations: cd.p, caudal process; fvf, free ventral flange on jugal; itf, infratemporal fenestra; mx.f, maxillary facet; mx.p, maxillary process; or.m, orbital margin; po.f, postorbital facet; po.p, postorbital process; qj.f, facet for the rostral process of the quadratojugal.
7.6. Right quadratojugal of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) lateral; (B) medial views. Scale bars equal 5 cm. Abbreviations: ju.f, facet for the caudal process of jugal; qu.s, sutural surface for quadrate.
Although most of the skull of Plesiohadros djadokhtaensis is known, thus far, no nasal, palatine, ectopterygoid, palpebral, sclerotic elements, and stapes have been recovered. Most of the axial skeleton, forelimb (with the exception of the manus), and the entire pelvic girdle and femur are not preserved.
Premaxilla Only the most rostral portion of the left premaxilla is preserved (Fig. 7.3). The oral margin is reflected to form a distinct prenarial fossa. The degree of reflection is more similar to that seen in the saurolophines Gryposaurus notabilis (ROM 873) and Prosaurolophus maximus (ROM 787) than to the weakly reflected oral margin of non-hadrosaurid hadrosauroids including Bactrosaurus johnsoni (Godefroit et al., 1998), Protohadros byrdi (Head, 1998), Jinzhousaurus yangi (Barrett et al., 2009), and Tethyshadros insularis (Dalla Vecchia, 2009). In dorsal view, the rostral margin is broadly arcuate and constricted abruptly behind the oral margin in dorsal view. There is no obvious primary or accessory premaxillary foramina, as are present in saurolophines (Horner et al., 2004). The oral margin has a double-layer morphology consisting of an external denticle-bearing layer that can be seen externally and an internal palatal layer of thickened bone that is set back slightly from the oral margin and separated from the denticulate layer by a deep sulcus bearing neurovascular foramina. This morphology is present in all saurolophines and lambeosaurines, as well as Bactrosaurus johnsoni (Prieto-Márquez, 2011a; Campione et al., 2013) and Ouranosaurus nigeriensis (MNHN GDF 300). The nasal process and caudolateral process are not preserved.
Maxilla The left maxilla is highly fractured but virtually complete (Fig. 7.4). It has a roughly isosceles shape, being half as high as it is long. The rostromedial process is separated from the somewhat larger rostrolateral process by a deep embayment. Behind this region is a flat articular surface leading to the dorsal process that accommodated the caudolateral premaxillary process. The ventral margin of the jugal articulation is somewhat sigmoidal. The ectopterygoid shelf is relatively prominent, though highly fractured, and is relatively short and caudoventrally oriented. The length of the ectopterygoid shelf relative to the total length of the maxilla is similar to that in Gilmoreosaurus mongoliensis (Prieto-Márquez and Norell, 2010), Bactrosaurus johnsoni (Godefroit et al., 1998), and Levnesovia transoxiana (Sues and Averianov, 2009), but is considerably shorter than that of hadrosaurids. An arcuate row of alveolar foramina, connected by a neurovascular groove, is on the medial maxillary surface, which otherwise is flat. There are at least 18 tooth positions (5–6 tooth positions/5 cm) arranged in a linear fashion (although this may be due to the lateral crushing of the element). This is fewer than typically present in lambeosaurines and saurolophines of equivalent tooth row length (Horner et al., 2004).
7.7. Right prefrontal of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) lateral; (B) medial; and (C) dorsal views. Scale bars equal 5 cm. Abbreviations: fr.s, sutural surface for frontal; lac.s, sutural surface for lacrimal; nas.s, sutural surface for nasal; or.m, orbital margin.
Jugal The right jugal is shallowly concave medially, and slightly higher than long as preserved (the rostral process incomplete; Fig. 7.5). Only the caudal margin of the maxillary process is preserved. Contact with the ectopterygoid in this area appears to be absent, while articulation with the palatine is enlarged. This condition characterizes saurolophines and lambeosaurines, as well as Bactrosaurus johnsoni (Godefroit et al., 1998), Levnesovia transoxiana (Sues and Averianov, 2009), and Telmatosaurus transsylvanicus (Weishampel et al., 1993), whereas the ectopterygoid contacts the jugal in Protohadros (Head, 1998), Eolambia caroljonesa (McDonald et al., 2012), and non-hadrosauroid ornithopods. The postorbital process extends 90 mm above the base of the orbit and gives this margin a strongly obtuse angle. The ventral flange of the jugal is pronounced beneath the infratemporal fenestra, as in saurolophines and lambeosaurines, but unlike the smoothly rounded ventral profile of the jugal in Tethyshadros (Dalla Vecchia, 2009), Protohadros (Head, 1998), Equijubus normani (You et al., 2003), and Fukuisaurus (Kobayashi and Azuma, 2003). The ventral margin of the caudal blade is shallowly concave. The caudal process is relatively narrow, as in Tethyshadros (Dalla Vecchia, 2009) and Bactrosaurus johnsoni (Godefroit et al., 1998), as well as the saurolophines Maiasaura peeblesorum (ROM 44770), Brachylophosaurus canadensis