Hadrosaurs. David A. Eberth
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.
Dentition
Maxillary teeth in the holotype of P. djadokhtaensis are not well preserved, but they are preserved in place and provide important information on the anatomy of individual teeth (Fig. 7.16). Unfortunately, the caudal end of the tooth row is not preserved and the total number of tooth positions is therefore unknown; however, the number of tooth positions in the maxillary tooth row is at least 20, with a single functional tooth and one replacement tooth per position. Each tooth is straight and symmetrical and has a well-developed primary carina on the buccal surface. Very slight secondary ridges are present on mesial and distal to the primary ridge on some, but not all, teeth.
7.15. Right surangular of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) lateral; (B) dorsal; and (C) medial views. Scale bars equal 5 cm. Abbreviations: an.f, angular facet; ar.f, articular facet; cot, mandibular cotyle; ret, retroarticular process; spl.f, splenial facet.
The dentary tooth row, consisting of more than 22 tooth positions, is straight in occlusal view (Fig. 7.17). The dentary crowns are relatively broad and slightly asymmetrical in lingual view, and the mid-dentary tooth crown in Fig. 7.17 is approximately twice as tall as it is wide. The apex of each tooth is slightly offset mesially. Ornamentation on the enamel face of the crown consists of a primary ridge, and one or two accessory ridges mesial and distal to the primary ridge. The mesial secondary ridge occurs on most crowns, and is more prominent than the distal secondary ridge, when present, on the same tooth. Small denticles are present along the slightly raised mesial and distal margins of the tooth crown. At least one tooth is functional and there are at least two replacement teeth per tooth position, but precise counts of functional and replacement teeth are difficult determine due to poor preservation. The dentary teeth closely resemble those of Bactrosaurus johnsoni in terms of their overall proportions and ornamentation of the enamel face (Godefroit et al., 1998).
7.16. Left maxillary teeth of Plesiohadros djadokhtaensis (MPC-D100/745) in (A, B) lateral; and (C, D) medial views. (A, C) photographs and (B, D). interpretive line drawings. Scale bars equal 5 cm. Abbreviations: d.sh, distal shoulder; m.sh, mesial shoulder; oc.s, occlusal surfaces; pr, primary ridge; rep.cr, replacement maxillary crowns.
Hyoid The long slender right hyoid is slightly curved and twisted along its length (Fig. 7.18). Rostrally blunt, it gently flares into a plate-like caudal end.
7.17. Left dentary teeth of Plesiohadros djadokhtaensis (MPC-D100/745) in medial view. (A) photograph and (B) interpretive line drawing. Scale bar equals 5 cm. Abbreviations: pr, primary ridge; sr, secondary ridge.
7.18. Right hyoid of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) dorsal; (B) lateral; and (C) medial views. Scale bars equal 10 cm.
Postcranium
The atlas and the manus were found in direct association with the holotype skull, and are therefore interpreted as part of that individual. The articulated ankle and foot (MPC-D100/751) was found in that same formation at the same locality as the holotype, and it pertains to a similarly sized large hadrosauroid. MPC-D100/751 is referred to Plesiohadros djadokhtaensis on the basis of stratigraphic and geographic proximity to the holotype.
7.19. Atlas of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) cranial; (B) caudal; (C) dorsal; (D) ventral; and (E) right lateral views. Scale bars equal 5 cm. Abbreviations: at.rf, atlantal rib facet; cc.j, craniocervical joint surface; ic, intercentrum; na, neural arch.
Atlas The atlas is complete, consisting of paired unfused pleurocentra and a single neurocentrum (Fig. 7.19). The pleurocentra form the lateral and dorsal margins of the neural canal; they meet, but do not fuse on the dorsal midline. The internal surface of the ventrally positioned neurocentrum marks where the dens of the axis articulates with the atlas. The atlantal rib (Fig. 7.20) widens proximally to articulate with the diapophysis of the atlas.
Manus The left manus of P. djadokhtaensis is preserved (Fig. 7.21). Digit I, including its metacarpal, is not known. Metacarpals II–V are slender and long (midshaft width < 0.15 length) and metacarpals II, III, and IV align with each other proximally. Metacarpal V is conical and appears to bear two phalanges. The penultimate phalanx of digit III is wedge shaped, and digit IV terminates in a hoof-shaped ungual.
Tibia All that is known of the tibia of P. djadokhtaensis is its distal quarter from the left side (Fig. 7.22). Here the medial malleolus and ventral surface articulate with the astragalus. The remainder of the cranial surface of the tibia forms the facet for the fibula.
7.20. Atlantal rib of Plesiohadros djadokhtaensis (MPC-D100/745) in (A) medial; and (B) lateral views. Scale bars equal 5 cm. Abbreviations: ct, merged capitulum and tuberculum.
Fibula Only the distal fibula is preserved. Medially, it forms an articular facet for the lateral malleolus of the tibia (Fig. 7.22). The distal end of the fibula appears to be only moderately expanded, as seen in most hadrosauroids, and does not exhibit the enlarged distal end as described for Parasaurolophus cyrtocristatus (Brett-Surman and Wagner, 2007).
Astragalus The astragalus was found in articulation with the distal tibia (Fig. 7.22). It is typically hadrosauroid in morphology, cupped proximally for the tibia and very broadly convex where it articulates with the metatarsus. The cranial ascending process is short and shifted to the medial side, as in hadrosaurids (Horner et al., 2004; Brett-Surman and Wagner, 2007).
Pes