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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 (Cuthbertson and Holmes, 2010), and Gryposaurus notabilis (ROM 873). The jugal contributes extensively to the margin of the infratemporal fenestra, where an acute angle marks the junction between the postorbital process and the body of the jugal, as in hadrosaurids. A shallow depression on the medial surface of the caudal jugal process marks the articulation with the quadratojugal.

Quadratojugal The thin right quadratojugal of Plesiohadros is roughly pentagonal in lateral view (Fig. 7.6). Externally it is marked by the facet for the jugal and internally by a long sinuous articulation, which receives the scalloped rostral margin of the quadrate.

Prefrontal The right prefrontal, where it forms the rostrodorsal margin of the orbit, is flat, triangular, and more than three times as long as wide in dorsal view (Fig. 7.7). Articulation with the lacrimal (the latter bone is not preserved) occurs within a crescentic fossa on the medial surface of the prefrontal. The orbital rim bears numerous vertical ridges that may correspond to fusion with one or more supraorbital elements. Contact with the frontal is along the complex medial and ventral surface. The most unusual aspect of the prefrontal is that the rostrodorsal margin of the orbit flares dorsolaterally to form a rugose, everted, wing-like margin, which is unusual in hadrosauroids.

Frontal The right frontal is more completely preserved than the left. As preserved, the somewhat triangular right frontal is 50% longer than wide (Fig. 7.8). In dorsal view, there are two facets for the nasal (medially) and prefrontal (laterally) at the rostral end of the otherwise flat frontal. Laterally, the frontal forms a small part of the orbital margin, between its prefrontal and postorbital contact surfaces. Contact with the parietal is difficult to interpret due to poor preservation, but appears to consist of a somewhat sinuous, transverse butt joint. The interfrontal suture is undulatory and more complex caudally than rostrally. The head of the laterosphenoid fits into a small, well-defined fossa that spans the undersurface of frontal-postorbital suture. The impression of the olfactory bulbs and tracts (cranial nerve [c.n.] I) are found on the ventral surface of the frontal. In overall morphology, the frontal is similar to that of Bactrosaurus johnsoni (Godefroit et al., 1998), Levnesovia (Sues and Averianov, 2009), and Jintasaurus (You and Li, 2009), although it is somewhat narrower relative to its total length.

7.8. Braincase of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) right lateral; (B) ventral; (C) dorsal; and (D) caudal views. Scale bars equal 10 cm. Abbreviations: boc, basioccipital; bpt, body of basisphenoid; bptp, basipterygoid process; bsp, basisphenoid; bst, basal tubera; exo, exoccipital; f, frontal; fm, foramen magnum; fr.s, interfrontal suture; lsp, laterosphenoid; n.s, suture for nasal; op, opisthotic; orb.m, orbital margin; osp, orbitosphenoid; p, parietal; poc, paroccipital process; po.s, suture for postorbital; prf.s, suture for prefrontal; pro, prootic; prs, parasphenoid; soc, supraoccipital; V, VIII, X, foramina for cranial nerves.

Postorbital The tri-radiate postorbital has a long caudal (squamosal) process that overlaps the reciprocal process of the squamosal in a scarf joint (Fig. 7.9). The ventral (jugal) process appears short, but is incomplete distally. The caudal margin of the ventral process is nearly co-linear with the ventral margin of the caudal process. The caudal process appears to be relatively long, as in Jintasaurus (You and Li, 2009), Bactrosaurus johnsoni (Godefroit et al., 1998), Levnesovia (Sues and Averianov, 2009), and most hadrosaurids. The distal end of the caudal process is bifurcated at its contact with the squamosal. The stout medial process makes an extensive, highly interdigitated joint with the frontal. The orbital margin is relatively smooth compared to that of the hadrosauroids Jeyawati, Eolambia, and Protohadros (McDonald et al., 2010).

7.9. Right postorbital of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) lateral; (B) dorsal; (C) ventral; and (D) medial views. Scale bars equal 5 cm. Abbreviations: fr.s, sutural surface for frontal; ju.p, jugal process; orb.m, orbital margin; sq.p, squamosal process; sq.s, sutural surface for squamosal; stf.m, margin of supratemporal fenestra.

Squamosal The rostral (postorbital) process of the squamosal is relatively long and straight in lateral view (Fig. 7.10). The rostral process is dorsoventrally tall and makes a unilobed contact with the caudal process of the postorbital. The medial process extends toward the midline, but it is not known how much of the caudal aspect of the parietal it concealed on the occiput. The cotylus for articulation with the head of the quadrate is well developed and located along the ventral margin of the squamosal body. It is framed by a short, robust prequadrate process and a pendant postquadrate process that contacts the paroccipital process caudally.

Quadrate The right quadrate of P. djadokhtaensis is tall, approximately 50% of the skull length (Fig. 7.11). The dorsal condyle of the quadrate is incomplete on its caudal aspect; therefore it is unknown whether this surface bore a strong buttress that met the postquadrate process of the squamosal. The middle of the paraquadratic notch, which is reduced to a poorly defined embayment, occurs 65% down the shaft of the quadrate. In ventral view, the ventral condyle of the quadrate narrows slightly in the middle, giving a bicondylar morphology to this joint. The lateral condyle is expanded rostrocaudally so that the condyles appear subtriangular in ventral view; furthermore the lateral condyle is rostrocaudally longer than the medial, as in Bactrosaurus johnsoni (Godefroit et al., 1998), Gilmoreosaurus mongoliensis (Prieto-Márquez and Norell, 2010), and all hadrosaurids. However, the lateral condyle lacks the distinctive enlarged, globular morphology characteristic of hadrosaurids (Horner et al., 2004). The pterygoid ala, roughly triangular in shape, extends rostromedially to where it would contact the quadrate ala of the pterygoid.

Pterygoid Only a partial right pterygoid is known for P. djadokhtaensis (Fig. 7.12). On the medial surface of the quadrate ramus is a prominent, nearly linear buttress whose distal limit articulated with the medial side of the body of the quadrate. The U-shaped articulation for the basipterygoid is visible at the rostral terminus of this buttress. Articulations with the ectopterygoid and maxilla are found on the rostral and ventral margins of the pterygoid.

Parietal The parietal is a single, median, hourglass-shaped element. It contacts the frontals rostrally, the postorbital rostrolaterally, the caudolateral braincase wall ventrally, and the supraoccipital caudoventrally. Caudally, the parietal narrows abruptly to form the long and prominent sagittal crest, which is only slightly down-warped along its length, unlike the strongly downwarped morphology of lambeosaurines (Horner et al., 2004). The length of the sagittal crest is more than half the length of the supratemporal fenestra.

Supraoccipital The supraoccipital is triangular in caudal view (Fig. 7.8). It occupies the midline between the overlying parietal and underlying opisthotic/exoccipital complex ventrally. The caudal surface of the supraoccipital is nearly vertical. The ventral margin of the supraoccipital is horizontal and developed as a strong ridge along its suture with the opisthotic/exoccipital complex.

Basioccipital In caudal view, the basioccipital is reniform where it forms the occipital condyle (Fig. 7.8). Here it contributes approximately one third to the lower margin of the foramen magnum. Large exoccipital condyloids articulate with the lateral aspect of the occipital condyle. There is only a short collum, approximately the length of the occipital condyle in ventral view. Rostral to the collum, the basioccipital broadly contributes to the caudal half of the basal tubera, where it complexly joins with the basisphenoid.

Basisphenoid The basisphenoid forms the rostral half of the basal tubera, making a complex transverse suture with the basioccipital (Fig. 7.8). Between the tubera is the continuation of the groove separating the basioccipital portion of the tubera, which spans the basisphenoid-basioccipital joint. The basipterygoid processes project approximately 5 cm (average of right and left) in a slightly ventrolateral direction. The basipterygoid processes appear to relatively longer, diverge less strongly, and are directed more ventrally than in Levnesovia (Sues and Averianov, 2009) and Bactrosaurus johnsoni (Godefroit et al., 1998), but these features could be accentuated by mediolateral crushing on the holotype braincase. The synovial facet for the pterygoid is on the rostrolateral terminus of the basipterygoid process. The basisphenoid contributes to the ventral margin of the trigeminal foramen. A horizontal groove marks the passage of the ophthalmic division of the trigeminal nerve (c.n. V₁), while a vertical groove marks the bony wall of the maxillary and mandibular divisions of the trigeminal nerve (c.n. V_{2, 3}). Immediately caudal to and paralleling the channel for the maxillary and mandibular divisions of the trigeminal nerve is a groove for hyomandibular branch of facial nerve (c.n. VII). Only a small part of the parasphenoid is preserved ahead of the body of the basisphenoid.

Lateral Wall of Braincase The orbitosphenoid is fragmentary in MPC-D100/745, which provides no information except the location of what appears to be the optic canal (c.n. II). The rostral contact with the presphenoid is not preserved and the presphenoid is missing. However, its caudal contact with the laterosphenoid is visible as an undulatory joint. The head of the laterosphenoid is broad as it contacts the undersurface of the frontal and postorbital (Fig. 7.8). Otherwise, it is gently curved where it forms the lateral wall of the braincase. It contributes the rostrodorsal margin of the large trigeminal foramen, approximately where the laterosphenoid meets the basisphenoid along their horizontal contact.

7.10. Right squamosal of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) lateral; (B) medial; and (C) ventral views. Scale bars equal 5 cm. Abbreviations: paro.s, sutural surface for paroccipital process of exoccipital/opisthotic complex; po.p, postorbital process; po.s, sutural surface for postorbital; pr.p, prequadratic process; qu.c, quadrate cotylus.

7.11. Right quadrate of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) lateral; (B) rostral; (C) caudal; (D) dorsal; and (E) ventral views. Scale bars equal 5 cm. Abbreviations: dqj.f, dorsal quadratojugal facet; md.c, mandibular condyle; pt.w, pterygoid wing; sq.c, squamosal condyle; qj.n, quadratojugal notch; vqj.f, ventral quadratojugal facet.

7.12. Right pterygoid of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) lateral; and (B) caudomedial views. Scale bars equal 5 cm. Abbreviations: dq.p, dorsal quadrate process; b.art, basicranial articulation; mx.s, sutural surface for maxilla; vq.p, ventral quadrate process.

The prootic makes an oblique scarf joint over the opisthotic as it forms the middle element of the lateral side of the braincase. Rostrally, it contacts the laterosphenoid in a simple butt joint, where it also contributes the dorsocaudal margin of the large trigeminal foramen (c.n. V). Its lateral surface continues as the crista prootica from the opisthotic. At the base of the prootic, near its junction with the opisthotic, is the foramen ovale, which is overlain by the footplate of the stapes (not preserved).

The fused opisthotic and exoccipital form the paroccipital process, which is long, ventrally pendant, and slightly rostrally curved. In caudal view, the presumed exoccipital portion of the complex forms relatively large condyloids that abut either side of the occipital condyle. Foramina for the glossopharyngeal (c.n. IX), vagus (c.n. X), spinal accessory (c.n. XI), and hypoglossal (c.n. XII) nerves are obscured by crushing in this region.