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