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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 the angular, it appears that this bone was largely obscured by the surangular when the jaw is viewed laterally, as in derived hadrosauroids, including hadrosaurids (Horner et al., 2004).

7.14. Left dentary of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) lateral; (B) medial; and (C) dorsal views. Scale bars equal 10 cm. Abbreviations: a.p, alveolar parapet; cor.p, coronoid process; dst, diastema between first dentary tooth and predentary; gr, groove extending across alveolar foramina; pd.s, sutural surface for predentary; rep.cr, replacement dentary crowns; spl.s, sutural surface for splenial; sym, dentary symphysis; t.r, tooth row.