Hadrosaurs. David A. Eberth
rel="nofollow" href="#ulink_5a24bada-8165-5575-9f7e-7688794d782b">Figs. 3.3, 3.4). The ventrolateral process of the premaxilla and the nasal contribute equally to the caudal margin of the external naris.
The nasal contacts the premaxilla rostrally and ventrally, the prefrontal ventrally, the frontal caudally, and its counterpart medially (Figs. 3.3, 3.4). Rostral to its contact with the ventrolateral process of the premaxilla, the ventral margin of the nasal curves rostrodorsally toward the dorsomedial process of the premaxilla, forming the dorsal margin and part of the caudal margin of the external naris. The sutures between the nasals and frontals are difficult to discern due to damage and compression of the skull; their shape is not certain (Figs. 3.3, 3.4).
3.2. Articulated premaxillae of IVPP V 12534, holotype of Equijubus normani. (A) right premaxilla in lateral view; (B) left premaxilla in lateral view; (C) premaxillae in dorsal view; (D) same in ventral view. Abbreviations: bf, bone fragment; dmp, dorsomedial process; en, external naris; ips, interpremaxillary suture; md, marginal denticles; nf, narial fossa; om, oral margin; pmxp, premaxillary process of nasal; rvp, rostroventral process of right maxilla; vlp, ventrolateral process. Scale bar equals 10 cm.
Maxilla, Palatine, and Pterygoid The rostroventral process of the maxilla curves rostroventrally to contact the ventrolateral process of the premaxilla (Figs. 3.2A, 3.3, 3.4). There is a short diastema between the rostral end of the rostroventral process and the first alveolus. The presence of a rostrodorsal process cannot be ascertained due to the articulation of the premaxillae with the maxillae of IVPP V 12534. The ventral margin of the maxilla is gently concave in lateral view (Figs. 3.3, 3.4). The lateral surface of the maxilla is pierced by several deep, irregularly distributed neurovascular foramina dorsal to the tooth row and near the dorsal margin of the maxilla (Figs. 3.3A, 3.4A), as in Dakotadon (SDSM 8656; Weishampel and Bjork, 1989).
The ascending process of the maxilla is rostrocaudally broad and triangular, and contacts the ventrolateral process of the premaxilla and the lacrimal along its rostral margin, and the maxillary process of the jugal along its caudal margin (Figs. 3.3, 3.4). The antorbital fossa is not visible in lateral view. The jugal process of the maxilla is finger-like and projects caudolaterally to lock into a recess on the medial surface of the maxillary process of the jugal (Fig. 3.5), as in Iguanodon bernissartensis (Norman, 1980), Mantellisaurus (Norman, 1986), Altirhinus (Norman, 1998), Probactrosaurus gobiensis (Norman, 2002), and Eolambia (Kirkland, 1998; CEUM 34356).
The right palatine and right and left pterygoids are partially visible through the orbits and infratemporal fenestrae of IVPP V 12534 (Figs. 3.3, 3.4). However, due to damage and the overlap of other cranial elements, little can be determined regarding their morphology. The caudal alar processes of the right pterygoid can be identified and are in contact with the braincase and the medial wing of the right quadrate (Fig. 3.4).
3.3. Skull of IVPP V 12534, holotype of Equijubus normani. (A) skull in left lateral view; (B) tracing of the skull in left lateral view. Solid black lines indicate sutures and contacts between cranial bones; dashed lines indicate uncertain sutures and contacts. Predentary, rostral ramus of right dentary, rostral ends of the premaxillae, articulated right postorbital and squamosal, and the left paroccipital process have been removed. Abbreviations: bc, braincase; boc, basioccipital; cp, coronoid process of the left dentary; en, external naris; flp, finger-like process of the maxillary process of the left jugal; fo, fenestra ovalis; hy, hyoid; lan, left angular; ld, left dentary; lexo, left exoccipital; lf, left frontal; lj, left jugal; ll, left lacrimal; lmx, left maxilla; lna, left nasal; lpmx, left premaxilla; lpo, left postorbital; lpoc, left paroccipital process; lprf, left prefrontal; lpt, left pterygoid; lsa, left surangular; mf, metotic foramen; oc, occipital condyle; p, parietal; rd, right dentary; rf, right frontal; rq, right quadrate; rsa, right surangular; rvp, rostroventral process of left maxilla; V, VII, and XII, foramina for cranial nerves V, VII, and XII. Scale bars equal 10 cm.
3.4. Skull of IVPP V 12534, holotype of Equijubus normani. (A) skull in right lateral view; (B) tracing of the skull in right lateral view. Solid black lines indicate sutures and contacts between cranial bones; dashed lines indicate uncertain sutures and contacts. Predentary, rostral ramus of right dentary, rostral ends of the premaxillae, and the left paroccipital process have been removed. The articulated right postorbital and squamosal were left in place. Abbreviations: bc, braincase; cap, caudal alar process of the right pterygoid; cp, coronoid process of the right dentary; en, external naris; flp, finger-like process of the maxillary process of the right jugal; lf, left frontal; lmx, left maxilla (rostroventral process); lna, left nasal; lpo, left postorbital; lprf, left prefrontal; lpt, left pterygoid; p, parietal; pqf, paraquadrate foramen; ran, right angular; rd, right dentary; rf, right frontal; rj, right jugal; rl, right lacrimal; rmx, right maxilla; rna, right nasal; rpal, right palatine; rpmx, right premaxilla (ventrolateral process); rpo, right postorbital; rprf, right prefrontal; rpt, right pterygoid; rq, right quadrate; rqj, right quadratojugal; rsa, right surangular; rsq, right squamosal; vb, vertical buttress; vfj, ventral flange of right jugal. Scale bars equal 10 cm.
3.5. Left maxilla-jugal articulation of IVPP V 12534, holotype of Equijubus normani. Abbreviations: jp, jugal process of maxilla; mxp, maxillary process of jugal. Scale bar equals 10 cm.
Jugal The maxillary process of the jugal tapers to a point and extends rostrally to meet the apex of the ascending process of the maxilla (Figs. 3.3, 3.4). The contact between the maxillary process and the lacrimal is unusually complex; a finger-like process arises from the maxillary process and appears to overlap the lateral surface of the lacrimal (Figs. 3.3, 3.4). This feature is present on both sides of the skull and has not been observed in any other iguanodontian, and therefore is an autapomorphy of Equijubus normani. Many other basal iguanodonts exhibit a tapering maxillary process, such as Mantellisaurus (NHMUK R11521), Altirhinus (Norman, 1998), Jinzhousaurus (Barrett et al., 2009), and Xuwulong (You et al., 2011),