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DINOSAURIA Owen, 1842

ORNITHISCHIA Seeley, 1887

ORNITHOPOD Marsh, 1881

IGUANODONTIA Sereno, 1986

HADROSAUROIDEA sensu Sereno, 2005

PLESIOHADROS, gen. nov.

Type Species PLESIOHADROS DJADOKHTAENSIS sp. nov.

7.2. Photomosaic schematic of the reassembled skull of Plesiohadros djadokhtaensis (MPC-D100/745). (A) in right lateral view; (B) interpretive line drawing of schematic. Abbreviations: br.c, braincase; cor, coronoid process; cot, glenoid for the lateral quadrate condyle; d, dentary; f, frontal; j, jugal; mx(L), maxilla (left); orb, orbit; p, parietal; pd, predentary; pmx(L), premaxilla (left); po, postorbital; poc, paroccipital process; prf, prefrontal; q, quadrate; qj, quadratojugal; ret, retroarticular process; sq, squamosal. Scale bar equals 10 cm.

7.3. Partial left premaxilla of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) lateral; (B) dorsal; (C) rostral; and (D) ventral views. Scale bars equal 5 cm. Abbreviations: ex.l, external lip; for, foramen; in.l, internal lip; om, oral margin; rf.rm, reflected rim.

Etymology Plesios (“near”); hadros (“hadrosaurid”); in reference to its systematic position as a proximate sister taxon to Hadrosauridae.

Diagnosis As per the type and only species.

Plesiohadros djadokhtaensis sp. nov.

Etymology From the Djadokhta Formation.

Holotype MPC-D100/745: partial skull, lower jaw, cervical vertebra, hyoid, metacarpals, and phalanges.

Referred Material MPC-D100/751: tibia, fibula, astragalus, metatarsals, phalanges.

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.