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. V2, 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.
Lower Jaw
Predentary In dorsal view, the gracile predentary is shovel shaped with a gently rounded rostral margin (Fig. 7.13). In ventral view, the predentary is more rectangular than in Protohadros (Head, 1998), Gilmoreosaurus (Prieto-Márquez and Norell, 2010) and Bactrosaurus johnsoni (Godefroit et al., 1998), although it is not as pronounced in this respect as most hadrosaurids. There are numerous denticles and neurovascular foramina across the entire rostral margin. Denticles are organized into doublets: pairs of relatively small, rounded, and triangular denticles project dorsally from a common base along the oral surface of the predentary, and would have fit into a continuous transverse slot on an underside of the premaxilla. The median denticle doublet is the largest of the series, and there are five more doublet sets on left side, where the complete series is preserved. The ventral process is robust and likely bilobed, but only the left side is preserved. There is a short median dorsal process that is triangular in dorsal view. Large lateral processes contact the external surface of the dentary rostral to the tooth row.
Dentary The left dentary is long and moderately high (Fig. 7.14). The symphyseal process turns medially to end in a linear and roughly horizontal symphysis. The diastema between the predentary articulation and the first dentary tooth is extremely long, approximately one-third the length of the tooth row. The oral margin of the dentary rostral to the tooth row is moderately down-turned, and the body of the dentary is deep below the middle of the tooth row. The coronoid process is stout, oriented approximately 100° from the long axis of the dentary. The terminus of the coronoid process is taller than wide, and has distinct rostral and caudal expansions (Fig. 7.14A). On the caudal aspect of the coronoid process, there is no facet for the coronoid bone. The most caudoventral aspect of the dentary ends behind the apex of the coronoid process; here and on the caudal aspect of the coronoid process, the dentary articulates with the surangular. The tooth row is generally poorly preserved, but the caudal end of the tooth row terminates beyond the rostral margin of the coronoid process, as in Jeyawati rugoculus (McDonald et al., 2010) and Bactrosaurus johnsoni (Godefroit et al., 1998). Although it is difficult to determine the exact number of individual tooth files, there appear to be more than 32 in the dentary.
7.13. Predentary of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) rostral; (B) left lateral; (C) dorsal; and (D) ventral views. Scale bars equal 5 cm. Abbreviations: cusp, denticulations on oral margin; d.p, dorsal process; l.p, lateral process; md.p, median dorsal process; pd.f, predentary foramina; v.gr, vascular groove; v.pr, ventral process.
Surangular The surangular is poorly preserved (Fig. 7.15). Only a small part of the thin rostral process that contacts the coronoid process is preserved. The robust central region of the surangular forms most of the mandibular glenoid, but is also fragmented and incomplete. It is unknown whether or not a surangular foramen on the lateral body of the bone was present. Caudally, the surangular forms the lateral portion of the stout, up-turned, and medially curved retroarticular process. The retroarticular process preserves articulation facets with the small articular distally, the splenial medially, and the angular ventrally. Given the ventrally positioned contact surface for