SYSTEMATIC PALEONTOLOGY
DINOSAURIA Owen, 1842
ORNITHISCHIA Seeley, 1887
ORNITHOPODA Marsh, 1881
IGUANODONTIA Dollo, 1888, sensu Sereno, 2005
ANKYLOPOLLEXIA Sereno, 1986,
sensu Sereno, 2005
STYRACOSTERNA Sereno, 1986,
sensu Sereno, 2005
HADROSAURIFORMES Sereno, 1997,
sensu Sereno, 1998
HADROSAUROIDEA Cope, 1870,
sensu Sereno, 2005
3.1. Fully assembled skull of IVPP V 12534, holotype of Equijubus normani, in (A) right lateral and (B) left lateral views. Scale bar equals 10 cm.
EQUIJUBUS NORMANI You, Luo, Shubin, Witmer, Tang, and Tang, 2003 (Figs. 3.1–3.20)
Holotype IVPP V 12534, complete skull and partial postcranium of a single individual.
Diagnosis For genus and species by monotypy. Characters derived from the original diagnosis of You, Luo, et al. (2003) are marked with an asterisk (*). Basal hadrosauroid characterized by four autapomorphies: (1), rostrodorsally curved finger-like process that arises from the maxillary process of the jugal at the jugal-lacrimal contact*; (2), elongate rostral ramus of lacrimal that extends along the dorsal margin of the maxilla and terminates rostral to the apex of the ascending process of the maxilla*; (3), epipophyses present on third cervical vertebra; and (4), hyposphene present on at least dorsal vertebrae 6–8 and 11–15. Also distinguished by the following unique combination of characters: rostral ramus of lacrimal tapers to a point, ventrolateral process of premaxilla contacts prefrontal (also noted by Paul, 2008:201, as lack of contact between the lacrimal and nasal); quadrate gradually curves caudally along its entire length in lateral view (also noted by Paul, 2008:201, as “shaft nearly straight”); and convex dorsal margin of ilium (also noted by Carpenter and Ishida, 2010).
Horizon Middle Grey Unit, Xinminpu Group; Albian, Early Cretaceous (Tang et al., 2001).
Type Locality Gongpoquan Basin, Mazongshan area, Gansu Province, China.
DESCRIPTION
Cranium and Mandible
Although it is nearly complete, the skull of IVPP V 12534 is extremely compressed transversely, obscuring the medial surfaces of most of the cranial elements (Fig. 3.1); the dorsal surface of the skull roof is visible in only right lateral view (Fig. 3.1A). Some pieces, including the predentary, rostral ramus of the right dentary, rostral ends of both premaxillae, the articulated right postorbital and squamosal, and the left paroccipital process can be separated from the rest of the skull and described in greater detail. See You, Luo, et al. (2003:348–350, fig. 1C) for a reconstruction and measurements of the fully assembled skull. See Appendix 3.1 for additional cranial and postcranial measurements of IVPP V 12534.
Premaxilla and Nasal The oral margin of the premaxilla is gently convex and edentulous (Fig. 3.2A, B). The rostral end of the premaxilla is ventrally inflected relative to the ventral margin of the maxilla, such that the oral margin is positioned well below the maxilla (Fig. 3.2A, B). The caudolateral corner of the oral margin curves caudodorsally towards the rostroventral process of the maxilla (Fig. 3.2A). The oral margin of each premaxilla expands laterally to form a transversely broad snout (Fig. 3.2C, D). Each premaxilla bears two rostrocaudally elongate denticles on its oral margin (Fig. 3.2D), as in Mantellisaurus (NHMUK R5764), Ouranosaurus (cast of MNHN GDF 300), and Eolambia (CEUM 35635). Caudal to the denticles, the ventral surface of the premaxilla is flat and somewhat rugose along the interpremaxillary suture (Fig. 3.2D). Dorsal to the oral margin, the lateral surface of the premaxilla is concave, forming the rostral end of the narial fossa (Fig. 3.2A, B).
Caudodorsal to the narial fossa, the premaxilla splits into two processes, the dorsomedial and ventrolateral processes; the point at which these processes diverge forms the rostral margin of the external naris (Fig. 3.2A, B). The dorsomedial process is overlapped laterally by the premaxillary process of the nasal, which forms the dorsal margin of the external naris (Figs. 3.2A, B, 3.3, 3.4). The dorsomedial processes of the left and right premaxillae meet along the interpremaxillary suture (Fig. 3.2C). The ventrolateral process forms the ventral margin of the external naris and contacts the maxilla along its ventral margin, curving over the rostroventral process of the maxilla and extending caudally to contact the lacrimal and prefrontal (Figs. 3.2A, 3.3, 3.4). The ventrolateral process also contacts the prefrontal in Hippodraco (McDonald, Kirkland, et al., 2010), Theiophytalia (Brill and Carpenter, 2006), Dakotadon (Weishampel and Bjork, 1989), Iguanodon bernissartensis (Norman, 1980), Mantellisaurus (Norman, 1986), and Jinzhousaurus (Barrett et al., 2009), but does not in Altirhinus (Norman, 1998) or Xuwulong (You et al., 2011). The ventrolateral process tapers towards its caudal end, as in Hippodraco (McDonald, Kirkland, et al., 2010), Dakotadon (Weishampel and Bjork, 1989), Iguanodon (Norman, 1980), and Mantellisaurus (Norman, 1986) in contrast to the dorsoventrally expanded ventrolateral processes of more derived hadrosauroids such as Eolambia (Kirkland, 1998; Head, 2001) and Probactrosaurus gobiensis (Norman, 2002) (Figs. 3.3, 3.4). The ventrolateral process contacts the nasal along its dorsal margin (