==== Front PeerJPeerJPeerJPeerJPeerJ2167-8359PeerJ Inc. San Francisco, USA 187210.7717/peerj.1872PaleontologyTaxonomyZoologyHistologyAnatomy and osteohistology of the basal hadrosaurid dinosaur Eotrachodon from the uppermost Santonian (Cretaceous) of southern Appalachia Prieto-Márquez Albert tobrien@example.com Gregory M. 2Ebersole Jun A. 31 School of Earth Sciences, University of Bristol, Bristol, UK2 Department of Biological Science, Florida State University, Tallahassee, Florida, USA3 McWane Science Center, Birmingham, Alabama, USANoto Christopher 14 4 2016 2016 4 e187276-50-7144 9 3 2016 © 2016 Prieto-Márquez et al.2016Prieto-Márquez, Erickson & EbersoleThis is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction and adaptation in any medium and for any purpose provided that it is properly attributed. For attribution, the original author(s), title, publication source (PeerJ) and either DOI or URL of the article must be cited.The cranial and postcranial anatomy of the basal hadrosaurid dinosaur Eotrachodon orientalis, from the uppermost Santonian of southern Appalachia (southeastern U.S.A.), is described in detail. This animal is the only known pre-Campanian non-lambeosaurine hadrosaurid, and the most complete hadrosauroid known from Appalachia. E. orientalis possesses a mosaic of plesiomorphic and derived characters in the context of Hadrosauroidea. Characters shared with basal hadrosauroids include a short and sloping maxillary ectopterygoid shelf, caudally prominent maxillary jugal process, one functional tooth per alveolus on the maxillary occlusal plane, a jugal rostral process with a shallow caudodorsal margin and medioventrally facing articular facet, a vertical dentary coronoid process with a poorly expanded apex, and tooth crowns with accessory ridges. Derived characters shared with other hadrosaurids include a circumnarial depression compartmented into three fossae (as in brachylophosaurins and Edmontosaurus), a thin everted premaxillary oral margin (as in Gryposaurus, Prosaurolophus, and Saurolophus), and a maxilla with a deep and rostrocaudally extensive rostrodorsal region with a steeply sloping premaxillary margin (as in Gryposaurus). Eotrachodon orientalis differs primarily from the other hadrosauroid from the Mooreville Chalk of Alabama, Lophorhothon atopus, in having a slender and crestless nasal whose caudodorsal margin is not invaded by the circumnarial depression. Hadrosaurus foulkii, the only other known hadrosaurid from Appalachia, is distinct from E. orientalis in having dentary teeth lacking accessory ridges and a dorsally curved shaft of the ischium. A histological section of the tibia of the E. orientalis holotype (MSC 7949) suggests that this individual was actively growing at the time of death and, thus, had the potential to become a larger animal later in development. AnatomyEvolutionDinosaurHadrosauridCretaceousAppalachiaHistologyNSFEAR 0959029Marie Curie Intra-European Fellowship for Career Development (European Commission, Research and Innovation)This research was partially funded by NSF Grant EAR 0959029 presented to GME and a Marie Curie Intra-European Fellowship for Career Development (European Commission, Research and Innovation) presented to APM. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. ==== Body Introduction Hadrosaurids are facultatively bipedal ornithopod dinosaurs characterized by densely packed dental batteries (Erickson et al., 2012), mediolaterally expanded rostra (Morris, 1970), and hypertrophied nasal passages that are often associated to elaborated supracranial crests (Evans, 2006; Prieto-Márquez, 2010a). During the Late Cretaceous (Santonian–late Maastrichtian) these animals became a major component of the terrestrial vertebrate faunas of Eurasia, the Americas, and Antarctica (Prieto-Márquez, 2010b). Recently, a new genus and species of hadrosaurid, Eotrachodon orientalis Prieto-Márquez, Erickson & Ebersole (2016) was erected from uppermost Santonian strata of Alabama, southeastern United States. During the Late Cretaceous, this region was located in Appalachia, a continent that was separated from the western Laramidia by the Western Interior Seaway (Gates, Prieto-Márquez & Zanno, 2012). Autapomorphies of E. orientalis include: 1) a tripartite circumnarial depression that is divided longitudinally into dorsal and ventral fossae, with the latter subdivided into caudoventral and lightly incised rostroventral fossae; 2) dorsal fossa of circumnarial depression above bony naris extending further caudally than caudal extent of caudoventral fossa; 3) caudodorsal region of circumnarial depression above bony naris excavating lateral nasal surface, deeply rostrally but gradually diminishing caudally; 4) premaxillary lateral process abruptly deflected ventrally, and forming a 165° angle with long axis of circumnarial depression; 5) maxilla with subtriangular joint surface for jugal that is more laterally than dorsally-facing and prominent dorsal jugal tubercle projected caudally; and finally 6) a steeply down-warped sagittal crest of the parietal substantially elevated above the temporal bar (Prieto-Márquez, Erickson & Ebersole, 2016). Eotrachodon orientalis is critical for understanding the early evolution of the clade hadrosaurids because it: 1) is the first known pre-Campanian non-lambeosaurine hadrosaurid; 2) is the most complete Appalachian hadrosauroid known and the only hadrosaurid from this landmass for which the skull has been recovered, (displaying diagnostic elements never before found in the other hadrosauroid taxa from the region); 3) occupies a basal position within the Hadrosauridae, sister taxon to the Saurolophidae (the clade consisting of the Saurolophinae and the Lambeosaurinae; Prieto-Márquez, Erickson & Ebersole, 2016); and 4) documents the acquisition of a derived circumnarial structure (Prieto-Márquez & Wagner, 2014) as early as the late Santonian, prior to the evolution of the fully derived feeding apparatus that characterizes saurolophid hadrosaurids and before the split between the hollow-crested lambeosaurines and the solid-crested and unadorned saurolophines (Prieto-Márquez, Erickson & Ebersole, 2016). Originally, Prieto-Márquez, Erickson & Ebersole (2016) provided only a cursory description of Eotrachodon orientalis. Here, we thoroughly describe the anatomy of this taxon, including the osteology of the neurocranium and the axial and appendicular skeleton. We also provide additional information on the geological setting and occurrence of E. orientalis, a detailed comparative osteology with all other Appalachian hadrosauroids, and greater insights into the developmental stage of the holotype specimen. Material and Phylogenetic Framework Eotrachodon orientalis is known from a single individual (MSC 7949) that includes a well-preserved, nearly complete, disarticulated skull and mandible, as well as part of the vertebral column and appendicular skeleton. We estimated the body length of MSC 7949 at between 4 and 5.1 m. These values were derived from measurements of the length of the skull and available vertebral centra. To account for missing vertebrae, we multiplied the minimal central length within each cervical, dorsal and sacral series by the minimum and maximum number of vertebrae reported in hadrosaurids. Due to the variability of vertebral counts within various hadrosaurid genera, low and high counts were considered to provide size range estimates. These counts were as follows: 13 (Horner, Weishampel & Forster, 2004) and 18 (Godefroit, Bolotsky & Bolotsky, 2012), respectively, for the cervicals; 16 and 20, respectively, for the dorsals (Horner, Weishampel & Forster, 2004); and 8 (Prieto-Márquez, 2008) and 12 (Horner, Weishampel & Forster, 2004), respectively, for the sacrals. Given that the tail in hadrosaurids accounts for approximately 40% of the body length (e.g., Brachylophosaurus canadensis MOR 794; Corythosaurus casuarius AMNH 5338; Gryposaurus notabilis ROM 764), we added this percentage to our length estimate. Finally, following Taylor & Wedel (2013) we added 11% to the overall length estimate to account for the presence of intervertebral cartilage. We adhere to the recent phylogenetic hypothesis of hadrosauroid relationships presented by Prieto-Márquez, Erickson & Ebersole (2016; fig. 2) and, at a more inclusive level, to the phylogenetic framework of the Iguanodontia proposed by McDonald (2012). The definition of the Hadrosauridae adopted here is that of Prieto-Márquez (2010a), i.e., the last common ancestor of Hadrosaurus foulkii Leidy, 1858, Saurolophus osborni Brown, 1912, and Lambeosaurus lambei Parks, 1923, and all its descendants. We follow Sereno's (2005) definitions of the Hadrosauroidea and the Iguanodontia. Accordingly, the Hadrosauroidea is the most inclusive clade containing Parasaurolophus walkeri Parks, 1922; but not Iguanodon bernissartensis Boulenger in Beneden, 1881. The Iguanodontia is the most inclusive clade containing Parasaurolophus walkeri Parks, 1922; but not Hypsilophodon foxii Huxley, 1869 and Thescelosaurus neglectus Gilmore, 1913. Geological Setting The exposed Upper Cretaceous units in Alabama form a nearly time-continuous series ranging from the Cenomanian to the Cretaceous-Paleogene boundary (Kiernan, 2002; Ebersole & Dean, 2013). These units are exposed in 34 counties in Alabama and create an arc that spans across the center of the state and up towards its northwest corner (Fig. 1). MSC 7949 was discovered at locality AMg-1, a creek site located within the city limits of Montgomery in Montgomery County (more specific locality information for MSC 7949 is on file at McWane Science Center in Birmingham, Alabama). MSC 7949 was excavated in situ from the basal Mooreville Chalk, less than 24 cm above the conformable contact with the underlying Tombigbee Sand Member of the Eutaw Formation (Fig. 2). In Alabama, the contact between these two units is time-transgressive, ranging from the early Campanian in western Alabama to middle-to-late Santonian in the central part of the state. In the vicinity of locality AMg-1, the contact between these two units falls below the last occurrence of the planktonic foraminifer Dicarinella asymetrica Sigal, 1952 (Puckett, 1994; Mancini, Puckett & Tew, 1996; Puckett, 2005; Liu, 2007), the last occurrence of which defines the Santonian-Campanian boundary (Caron, 1985). Additional correlations of the planktonic foraminifers, ostracodes, and nanoplankton suggests the lower 32 m of the Mooreville Chalk in central Alabama falls within the upper Santonian (Puckett, 1994; Mancini, Puckett & Tew, 1996), placing the geologic age of MSC 7949 within this interval (Fig. 2), with an estimated age of 83–84 Ma. 10.7717/peerj.1872/fig-1Figure 1 Distribution of hadrosauroid type-specimens from Alabama (southeastern USA) with Upper Cretaceous surface geology and the approximate locations of fossil localities ADa-1 and AMg-1, where the holotypes of Lophorhothon atopus and Eotrachodon orientalis, respectively, were collected. 10.7717/peerj.1872/fig-2Figure 2 Upper Coniacian to Maastrichtian surface stratigraphy in Alabama and microfossil biostratigraphy. The approximate stratigraphic position of the holotypes of Lophorhothon atopus (FMNH P27383) and Eotrachodon orientalis (MSC 7949) are indicated. Striped areas represent unconformities. Biostratigraphic zones based on Caron (1985), Perch-Nielsen (1985) and Puckett (2005). The Mooreville Chalk in Alabama is comprised largely of clays made up of compact fossiliferous chalk and chalky marl. However, the lower 1.5 m of this unit, an unnamed member, consists of compact calcarenite and silt, scattered phosphatic pellets, quartz, glauconite, and abundant macrofossils (both vertebrate and invertebrate) and steinkerns (Raymond et al., 1988; Liu, 2007). This suggests the depositional setting of the Mooreville Chalk represents a calm, middle-shelf environment with dysoxic bottom conditions (Wylie & King, 1986). In Alabama, the Mooreville Chalk has long been known for its rich diversity of marine vertebrates; however this unit has also produced terrestrial forms (see Ebersole & King, 2011; Ikejiri et al., 2013). In addition to hadrosauroids, remains of tyrannosauroids, nodosaurids, and a dromaeosaurid have been found (Langston, 1960; Kiernan & Schwimmer, 2004; Ebersole & King, 2011). To date, all dinosaur material from the southeastern USA has been recovered within marine deposits, indicating that these specimens were fluvially transported prior to deposition (Schwimmer, 1997; Ebersole & King, 2011). Osteology of Eotrachodon Orientalis Eotrachodon orientalis is known from a single immature specimen tobrien@example.com. It consists of a well-preserved, nearly complete, partially articulated skull and mandible, and an incomplete postcranium. Cranial elements include both premaxillae, maxillae, jugals, partial right nasal, left lacrimal, left prefrontal, frontals, postorbitals, squamosals, left quadrate, parietal, partial braincase (supraoccipital, left opisthotic-exoccipital complex, prootics, basisphenoid, laterosphenoids, and parasphenoid), predentary, dentaries, surangulars, right angular, partial right hyoid, and various maxillary and dentary teeth. The postcraniumn is represented by the axis and several cervical, dorsal, sacral, and caudal vertebrae, partial left pubis, partial left ischium, partial right tibia, and left manual phalanges III-2 and V-2. The type specimen of Eotrachodon orientalis shows a relatively deep skull (Fig. 3), with a maximum depth along the quadrate/maximum length ratio of 0.54. The pre-orbital region of the skull accounts for 45% of the length of the skull. The dorsal surface of the rostrum is prominently curved rostroventrally. The orbit is elliptical, deeper than wide, with the long axis of the ellipse caudodorsally directed. The infratemporal fenestra is deeper and broader than the orbit, as in the hadrosauroid Tethyshadros insularis (Dalla Vecchia, 2009) and kritosaurin hadrosaurids (Prieto-Márquez, 2014). The supratemporal fenestra is subrectangular and its long axis is rostrocaudally oriented. 10.7717/peerj.1872/fig-3Figure 3 Assembled holotype skull of Eotrachodon orientalis, MSC 7949, in left lateral view. The mandible and nasal bone have been reversed to match the orientation of the other elements. Dark grey areas in the line drawing represent missing bones. Mount of the skull and mandible from Prieto-Márquez, Erickson & Ebersole (2016; figs. 1A and 1B), reprinted by permission of the Society of Vertebrate Paleontology, www.vertpaleo.org. Facial skeleton Premaxilla The premaxilla (Figs. 4 and 5; Table 1) forms the upper 'duck-bill' of hadrosauroid dinosaurs. The oral margin of the premaxilla of Eotrachodon orientalis is widely arcuate, as well as thin and reflected (Figs. 4D and 5C) as in saurolophin (Bell, 2011a) and kritosaurin (Prieto-Márquez, 2014) saurolophines. This reflected oral margin gradually becomes thinner distally along the lateral margin of the premaxilla. The premaxillary oral margin is moderately offset ventrally relative to the occlusal plane of the maxilla (Fig. 3), so that the distance between these premaxillary and maxillary areas is half of the mean depth of the dentary ramus. This offset is greater in saurolophid hadrosaurids, in which the distance between the oral margin of the premaxilla and the occlusal plane of the maxillary tooth row is greater than the mean depth of the dentary ramus (Prieto-Márquez, 2010a). 10.7717/peerj.1872/fig-4Figure 4 Left premaxilla of Eotrachodon orientalis (holotype MSC 7949). (A) Rostral view. (B) Dorsal view. (C) Rostral view of rostral layer of premaxillary denticles. (D) Lateral view. (E) Medioventral view. (F) Medioventral view of premaxillary denticles. (G) Ventral view. 10.7717/peerj.1872/fig-5Figure 5 Right premaxilla of Eotrachodon orientalis (holotype MSC 7949). (A) Dorsal view. (B) Rostral view. (C) Lateral view. (D) Rostral view of rostral layer of premaxillary denticles. (E) Ventral view of premaxillary denticles. (F) Ventral view. (G) Medioventral view. 10.7717/peerj.1872/table-1Table 1 Selected facial measurements (in mm) of the holotype specimen (MSC 7949) of Eotrachodon orientalis. Element Measurement Premaxilla (left), maximum length along lateral process 258 Premaxilla (left), maximum width from medial to lateral process 94 Premaxilla (right), maximum length along lateral process 237 Premaxilla (right), maximum width from medial to lateral process 88 Maxilla (left), maximum length 220 Maxilla (left), height from alveolar margin to highest point of dorsal process (incomplete) 87 Maxilla (right), maximum length 223 Maxilla (right), height from alveolar margin to highest point of dorsal process 93 Nasal (fragment), maximum length 54 Nasal (fragment), maximum width perpendicular to length 103 Lacrimal, maximum height along caudal margin including dorsal flange 73 Prefrontal, maximum length 74 Prefrontal, maximum width of the rostroventral flange 33 Jugal (left), length from apex of rostral process to caudal margin of quadratojugal flange 156 Jugal (left), height along the distal tip of the postorbital ramus perpendicular to the ventral margin 98 Jugal (right), length from apex of rostral process to caudal margin of quadratojugal flange 165 Jugal (right), height along the distal tip of the postorbital ramus perpendicular to the ventral margin 94 Postorbital (left), length from rostral end to distal tip of caudal ramus 100 Postorbital (right), length from rostral end to distal tip of caudal ramus 99 Quadrate, maximum length 193 Squamosal (left), length from caudal margin of postcotyloid process to rostral tip of postorbital process 89 Squamosal (right), length from caudal margin of postcotyloid process to rostral tip of postorbital process 86 Two rows, each containing three large denticles are present: one bordering external to the oral margin and the second set caudally on the ventral surface of the premaxilla (Figs. 4F and 5E). The external row consists of three sub-conical denticles that are rostroventrally directed. The external surfaces of these denticles show transverse grooves (Fig. 4C). The caudal denticles are mediolaterally compressed, ventrally directed, and larger than the rostral denticles. Most of the lateral surface of the premaxilla is excavated by an extensive elliptic circumnarial depression (Figs. 4D and 5C). As in all saurolophine hadrosaurids (Prieto-Márquez & Wagner, 2014), this excavation entirely surrounds the bony naris and is compartmentalized into subsidiary fossae. Specifically, the circumnarial depression of Eotrachodon orientalis consists of three fossae, as in species of Edmontosaurus Lambe, 1917 (Campione & Evans, 2011). Two thin ridges converge rostroventrally to divide the circumnarial depression of E. orientalis into dorsal, ventral, and accessory rostral fossae (Figs. 4D and 5C). There is no premaxillary foramen within the circumnarial depression of E. orientalis, although the area adjacent to the rostral margin of the apertura ossis nasi is deeply excavated. In saurolophines, a large foramen is present near that area of the premaxilla, rostroventral to the rostral margin of the apertura ossis nasi (e.g., Brachylophosaurus canadensis Sternberg, 1953 [CMN 8893], or Edmontosaurus annectens (Marsh, 1892, SM R4036). E. orientalis displays a large foramen on the rostral surface of the premaxilla dorsal to the external denticles (Fig. 4C), as in the basal hadrosauroid Bactrosaurus johnsoni Gilmore, 1933 (Prieto-Márquez, 2011a) and some saurolophine hadrosaurids like B. canadensis (see Prieto-Márquez, 2005) and Maiasaura peeblesorum Horner & Makela, 1979 (Horner, 1983). The medial process of the premaxilla is a thin rod-like structure that is rostrodorsally continuous with the rostromedial margin of the circumnarial depression (Figs. 4 and 5). The tapering distal end is missing in both premaxillae. The medial process contributes to form the dorsal margin of the apertura ossis nasi. The lateral surface of the process is occupied by a flat articular facet for reception of the rostral process of the nasal that would extend rostrally to the level of the rostrodorsal corner of the apertura ossis nasi. The lateral process of the premaxilla is a wedge-shaped blade that extends caudodorsally, adjacent to the caudoventral end of the circumnarial depression and the caudal end of the lateral margin of the main body of the premaxilla (Figs. 4 and 5). Proximally, the lateral process is abruptly deflected ventrally, forming a 165° angle with the long axis of circumnarial depression. Most of the medial side of the main body of the premaxilla, including the medial process, is occupied by an extensive and flat interpremaxillary articular surface (Figs. 4E and 5G). This surface is separated from the depressed ventral side of the premaxilla by a sharp and prominent ridge. The premaxilla gradually becomes narrower caudally until reaching the lateral process at its narrowest point, which corresponds to the post-oral constriction of the skull. Maxilla The maxilla (Figs. 6 and 7; Table 1) is an elongate and mediolaterally compressed element. The rostral half of the maxilla is about twice as deep as the caudal third. The palatal process is a deep flange that extends mediodorsally and curves rostroventrally, forming the rostrodorsal margin of the maxilla (Figs. 6B and 7B). Its medial surface bears a prominent ridge that extends throughout the length of the process and rises dorsomedially (Figs. 6C, 6D, 7E and 7F). This ridge is most prominent at its mid-length. The area comprised in between the medial surface of the palatal process and the dorsal surface of this ridge support the rostral extent of the vomer. Laterally, the base of the palatal process is continuous ventrally with the premaxillary shelf, a smooth and narrow surface that steeply slopes rostroventrally and lateroventrally and supports the caudal region of the main body of the premaxilla (Fig. 7B). Rostroventrally, the premaxillary shelf ends in a broad and blunt apex (the rostroventral process of authors; e.g., Prieto-Márquez et al., 2013). Lateroventral to the palatal process and the premaxillary shelf lays the large rostral foramen (Fig. 7B). This foramen opens rostrolaterally and is found at about mid-depth of the rostral region of the maxilla. This position is below that typically seen in hadrosaurids, where the foramen is located closer to the rostrodorsal margin of the maxilla (Prieto-Márquez, 2010a). 10.7717/peerj.1872/fig-6Figure 6 Left maxilla of Eotrachodon orientalis (holotype MSC 7949). (A) Dorsal view. (B) Lateral view. (C) Ventral view. (D) Medial view. 10.7717/peerj.1872/fig-7Figure 7 Right maxilla of Eotrachodon orientalis (holotype MSC 7949). (A) Dorsal view. (B) Lateral view. (C and D) Labial views of maxillary tooth crowns. (E) Medial view. (F) Ventral view. The lateral view of the maxilla is from Prieto-Márquez, Erickson & Ebersole (2016; fig. 1C), reprinted by permission of the Society of Vertebrate Paleontology, www.vertpaleo.org. The maxilla of Eotrachodon orientalis displays a steep premaxillary shelf. This shelf is angled at 45° with respect to the rostral region of the tooth row. The laterodorsal margin of the maxilla is sub-trapezoidal in lateral view and extensively exposed under the lacrimal (Figs. 6B and 7B). This sub-trapezoidal surface is dorsally bordered by a narrow groove where it articulated with the ventral margin of the lacrimal. The groove is rostrally continuous with the premaxillary shelf. Caudally, it extends onto the dorsal region of the dorsal process of the maxilla, where it becomes an elongate and recessed articular facet for the lacrimal (Fig. 7B). Both the steep premaxillary shelf and extensive laterodorsal margin are attributes shared with the kritosaurins Gryposaurus spp. (see Lambe, 1914; Gates & Sampson, 2007) and Kritosaurus horneri (see Prieto-Márquez, 2014). The dorsal process is located caudal to the mid-length of the maxilla (Figs. 6B and 7B). This process exhibits a caudally-skewed lateral profile, being twice as broad as it is tall. The dorsal half of the articular surface for the jugal extends onto the lateral surface of the dorsal process, below the articular facet for the lacrimal (Fig. 6B). The articular facet for the jugal is sub-triangular and is oriented laterally, as typically occurs in hadrosaurids (Prieto-Márquez & Wagner, 2009). Yet, this articular surface shares a prominent and caudally projected dorsal tubercle that ends in a sharp apex (Fig. 7B) with basal hadrosauroids (e.g., Levnesovia transoxiana Sues & Averianov, 2009). At the caudal end of the maxilla, the ectopterygoid shelf only accounts for 25% of the total length of the bone, as in Eolambia caroljonesa Kirkland, 1998 (see also McDonald et al., 2012). The shelf is caudoventrally tilted to form a 20° angle with the caudal segment of the tooth row, as in Levnesovia transoxiana (see Sues & Averianov, 2009). As in other hadrosaurids, this shelf is also steeply inclined lateroventrally and bordered laterally by a thick ridge that gradually becomes broader caudally (Fig. 6B). Mediodorsally, the ectopterygoid shelf is continuous with the palatine ridge (Figs. 6A and 6B). This ridge raises mediodorsally and displays an irregular scalloped dorsal margin for reception of the ventral margin of the palatine. A partially preserved finger-shaped pterygoid process projects caudally from the caudal extent of the palatine ridge (Fig. 6D). There are eight and seven foramina piercing the lateral surfaces of the left and right maxilla, respectively. On the left maxilla, the foramina form a rostrodorsally-oriented row ventral to the jugal facet, as in hadrosaurids (Prieto-Márquez, 2010a; Fig. 6B). However, on the right maxilla, the foramina form a long row that extends throughout most of the lateral surface of the bone (Fig. 7B), as in basal hadrosauroids like Bactrosaurus johnsoni (see Prieto-Márquez, 2011a; Fig. 7B) and Equijubus normani You et al., 2003, and basal iguanodontians like Mantellisaurus atherfieldensis Hooley, 1925 (e.g. NHM R11521). The medial surface of the maxilla is relatively flat (Figs. 6D and 7E). The arcuate row of alveolar foramina is positioned dorsal to the mid-depth of the maxilla, bounding dorsally the bony parapet that medially encloses the maxillary dental battery. The latter bears 32 tooth positions, a count that is within the minimum range recorded in the Hadrosauridae (Prieto-Márquez, 2010a). There is only one functional tooth exposed on the occlusal plane (Fig. 7F), as in basal iguanodontians (Norman, 2004). Nasal The nasal (Figs. 3 and 8; Table 1) consists of a thin plate that curves lateroventrally to form the caudodorsal region of the rostrum and the caudodorsal margin of the apertura ossis nasi. The only available nasal is missing most of its medial margin, part of the caudolateral wall of its main body, most of the ventral process, and the distal segment of the dorsal process (Fig. 8). Unlike Lophorhothon atopus (Langston, 1960), the dorsal surface of the main body of the nasal of Eotrachodon orientalis gently curves caudodorsally showing no elevation that would indicate the presence of a cranial crest. A wedge-shaped caudal process exists along the caudolateral margin of the nasal (Figs. 8A and 8B) that would probably insert in between the rostrolateral and medial margins of the frontal and prefrontal, respectively. 10.7717/peerj.1872/fig-8Figure 8 Partial right nasal of Eotrachodon orientalis (holotype MSC 7949). (A) Dorsal view. (B) Lateral view. (C) Lateral view of the caudal region of the circumnarial depression. (D) Medial view. (E) Ventral view. The lateral view of the nasal is from Prieto-Márquez, Erickson & Ebersole (2016; fig. 1F), reprinted by permission of the Society of Vertebrate Paleontology, www.vertpaleo.org. The ventral border of the dorsal process of the nasal forms the caudodorsal margin of the apertura ossis nasi. Dorsally and adjacent to this margin, the nasal is excavated by the circumnarial depression. In this area, the circumnarial depression is deeply incised ventral to the proximal region of the dorsal nasal process. Caudally, however, the depression becomes gradually shallower above the proximal region of the ventral process of the nasal (Figs. 8B and 8C). What remains of the dorsal process of the nasal is a mediolaterally compressed wedge-shaped structure that projects rostroventrally (Figs. 8B and 8D). This process overlapped the entire lateral surface of the medial process of the premaxilla, while both elements formed the dorsal margin of the apertura ossis nasi. As evidenced by the length of the articular facet for the nasal on the medial process of the premaxilla, the dorsal process of the nasal extended to above the rostral margin of the apertura ossis nasi. Prefrontal The prefrontal (Fig. 3; Table 1) forms the rostrodorsal margin of the orbit. The prefrontal of Eotrachodon orientalis is nearly completely preserved, missing the distal regions of the rostroventral flange and the caudomedial process (Figs. 9A–9C). The bone is crescent-shaped in lateral view. Caudally, the prefrontal is dorsoventrally compressed; however, as it curves ventrally, the prefrontal becomes mediolaterally compressed while expanding dorsoventrally to form a thin rostroventral flange (Figs. 9A and 9C). This flange overlaps the lateral surface of the dorsal flange of the lacrimal. A deep triangular notch, for reception of the lacrimal (Fig. 9A), occurs at the caudoventral corner of the flange. 10.7717/peerj.1872/fig-9Figure 9 Facial elements of Eotrachodon orientalis (holotype MSC 7949). (A–C) Left prefrontal in lateral, dorsal, and medial view, respectively. (D and E) Left lacrimal in lateral and medial view, respectively. The orbital margin of the prefrontal is carved with a series of short deep indentations that follow the rostrodorsal corner of the orbit and end abruptly near the proximal region of the rostroventral flange. The caudomedial region of the prefrontal is recessed and projected into a caudomedial process (Fig. 9B) that inserts into the rostrolateral margin of the frontal. A foramen is present on the laterodorsal surface of the prefrontal deepening caudolaterally (Fig. 9B). In contrast to the dorsoventrally convex lateral surface of the prefrontal, most of the medial side is deeply excavated, particularly throughout its caudodorsal extent (Fig. 9C). The ventral surface of the prefrontal is smooth and forms the roof of the rostral region of the orbit. The prefrontal is excluded from the circumnarial depression. Lacrimal As in non-lambeosaurine hadrosauroids, the lacrimal (Figs. 9D and 9E; Table 1) of Eotrachodon orientalis is a triangular rostrally elongate plate. The wedge-shaped rostral process forms about two-thirds of the lacrimal. Its medial surface is concave, particularly along the lacrimal canal that extends longitudinally at mid-depth of the rostral process (Fig. 9E). Both the rostrodorsal and ventral margins of the process show an extremely narrow ledge for articulation with the lateral process of the premaxilla, and the dorsal process of the maxilla and rostral process of the jugal, respectively. The ventral margin of the rostral process of the lacrimal displays a convex lateral profile rostral to the jugal notch (Fig. 9D), a condition also present in the saurolophines Gryposaurus spp. (Gates & Sampson, 2007), Kritosaurus horneri Hunt & Lucas, 1993 (Prieto-Márquez, 2014), Saurolophus spp. (Brown, 1912; Bell, 2011a; Bell, 2011b), and Prosaurolophus maximus Brown, 1916 (McGarrity, Campione & Evans, 2013). The jugal notch shows a triangular lateral profile and ventrally receives the caudodorsal region of the rostral process of the jugal (Fig. 9D). The notch is bounded caudally by the tetrahedral caudoventral corner of the lacrimal (Fig. 9E). The caudal margin of the lacrimal contributes to the rostroventral margin of the orbit. This margin is gently concave and represents the widest region of the lacrimal. It is pierced by the opening of the lacrimal canal (Fig. 9E). The caudodorsal region of the lacrimal consists of a medially deflected flange that extends dorsally to underlie the rostroventral flange of the prefrontal (Fig. 9D). The caudodorsal margin of this flange of the lacrimal is coarsely scalloped. Jugal The jugals (Figs. 10A–10D; Table 1) are nearly completely preserved, except for the distal ends of the rostral processes, postorbital rami, and quadratojugal flanges. Rostrally, the jugal is dorsoventrally expanded to form a triangular rostral process for articulation with the maxilla and lacrimal. The elongate apex of this process lays within its dorsal half, as in non-brachylophosaurin saurolophines (Gates et al., 2011; Fig. 10A). The caudodorsal margin of the rostral process forms a notched and relatively shallow lacrimal process (Fig. 10C) that minorly contributes to the rostroventral margin of the orbit. The caudoventral margin of the rostral process is also shallow and forms a broad flange that is medially recessed from the lateral surface and ventral margin of the rostral process (Figs. 10C and 10D). This flange represents the ventral extent of the medial articular surface of the process. The medial articular facet of the rostral process is roofed by a narrow shelf. This shelf extends medially from the orbital margin (Figs. 10B and 10D), a condition shared with basal hadrosauroids like Bactrosaurus johnsoni (see Prieto-Márquez, 2011b). However, unlike in basal hadrosauroids (e.g., Tanius sinensis Wiman, 1929), with the exception of Lophorhothon atopus Langston, 1960, the shelf of Eotrachodon orientalis is rostrodorsally rather than rostrally oriented (Figs. 10B and 10D). Caudally, the dorsal and ventral margins of the rostral process are continuous with the orbital margin and an extensive embayment that forms most of the ventral border of the jugal, respectively. 10.7717/peerj.1872/fig-10Figure 10 Facial elements of Eotrachodon orientalis (holotype MSC 7949). (A and B) Left jugal in lateral and medial view, respectively. (C and D) Right jugal in lateral and medial view, respectively. (E–H) Quadrate in lateral, rostral, medial, and caudal view, respectively. The lateral view of the right jugal is from Prieto-Márquez, Erickson & Ebersole (2016; fig. 1E), reprinted by permission of the Society of Vertebrate Paleontology, www.vertpaleo.org. The postorbital ramus rises dorsally and slightly caudally from about mid-length of the dorsal margin of the jugal (Fig. 10A). Dorsally, this ramus is rostrally excavated for reception of the jugal ramus of the postorbital. The postorbital ramus separates the ventral, and part of the caudal, margins of the orbit from the ventral, and part of the rostral, margins of the infratemporal fenestra. The caudal constriction that underlies the infratemporal fenestra is 85% as deep as the rostral constriction that lies beneath the orbit. The orbital and infratemporal margins are subequally wide. The caudoventral flange is greatly expanded, being 1.6 times deeper than the caudal constriction at its narrowest point. The quadratojugal flange forms the caudodorsal region of the jugal and displays an auricular lateral profile (Fig. 10C). There is a relatively wide and prominent concavity between the caudoventral and quadratojugal flanges. Quadrate The quadrate (Figs. 10E–10H; Table 1) is a columnar element linking the skull to the mandible at the caudal end of the skull. This bone is slightly curved caudally; specifically, the rostral margin of the dorsal segment of the quadrate forms an angle of 167° with the long axis of the quadratojugal notch (Fig. 10E). The proximal articular surface of the quadrate head is convex and mediolaterally compressed. Its caudal margin lacks a squamosal buttress. The medial and lateral surfaces of the quadrate shaft extend rostrally to form two flanges (Fig. 10F). The delicate wing-like pterygoid flange extends rostromedially and is missing most of its rostrodorsal margin (Fig. 10G). Proximally, its medial surface is deeply depressed for reception of the caudodorsal flange and the caudoventral process of the pterygoid. The lateral flange of the quadrate is limited to the dorsal segment of the bone and only extends a short distance beyond the rostrocaudal width of the shaft (Fig. 10E). The quadratojugal notch shows a wide arcuate lateral profile (Fig. 10E). Its dorsal margin forms a 26° angle with the caudal margin of the quadrate. The mid-point of the notch lays ventral to the mid-length of the quadrate. Both the dorsal and ventral margins of the notch are subequally long and display narrow facets for articulation with the caudal margin of the quadratojugal. The distal end of the quadrate is slightly expanded and consists of two condyles (Fig. 10F). The substantially smaller medial condyle is eroded away. As in Telmatosaurus transsylvanicus Nopcsa, 1900 (e.g., NHM R3386), Lophorothon atopus (e.g., FMNH P27383), and hadrosaurids (Prieto-Márquez, 2010a), the ventral articular surface of the quadrate that contacts the surangular is subtriangular in ventral view due to the rostrocaudal expansion of the lateral condyle. Squamosal The squamosal (Fig. 11; Table 1) forms the caudodorsal corner of the skull. The main body of this element consists of a thick, curved lamina that is deeply concave ventrally. Three processes radiate medially, rostrally, and ventrolaterally from the main body of the squamosal (Fig. 11D). The medial margin of the postorbital process and the rostral margin of the medial process caudally bound the supratemporal fenestra. 10.7717/peerj.1872/fig-11Figure 11 Squamosals of Eotrachodon orientalis (holotype MSC 7949). (A and B) Right squamosal in lateral and caudal view, respectively. (C–E) Left squamosal in lateral, ventromedial, and caudal view, respectively. The postorbital process is triangular, dorsoventrally compressed, and projects rostrally and slightly ventrally (Fig. 11C). Most of its dorsal surface is occupied by an extensive recessed facet that is overlapped by the caudal ramus of the postorbital, as the postorbital process inserts under the latter. The caudal margin of the articular facet for the postorbital process is Z-shaped for reception of the bifid distal margin of the postorbital caudal ramus (Fig. 11A). A sub-conical precotyloid process extends lateroventrally from beneath the lateral margin of the postorbital process (Fig. 11C). The precotyloid process is relatively short, being only 60% as long as the quadrate cotylus is wide. A small triangular depression exists in between the rostral margin of the proximal region of the precotyloid process and the lateral edge of the proximal region of the postorbital process. The caudal margin of the precotyloid process rostrally bounds the quadrate cotylus (Fig. 11C). The cotylus consists of a dorsoventrally-compressed but deep oval depression on the lateral surface of the main body of the squamosal that receives the head of the quadrate. The caudal wall of the cotylus is continuous caudally with the postcotyloid process (Figs. 11D and 11E). This process consists of a hook-like lamina that extends lateroventrally from the caudolateral corner of the squamosal. Its recessed caudal surface is rugose and contacts extensively the proximal region of the paroccipital process of the fused opisthotic-exoccipital. The medial process of the squamosal extends medially and slightly dorsally (Fig. 11B). Distally, this process curves rostrally to form the caudomedial corner of the supratemporal fenestra (Fig. 11D). The two squamosals probably met sagittally, excluding the parietal from the occiput. Postorbital The postorbital (Figs. 12E, 12F, 13A and 13C; Table 1) is a triradiate element that forms the caudodorsal margin of the orbit and the rostrodorsal margin of the infratemporal fenestra. Three rami extend rostrally, rostroventrally, and caudally from the central region of the postorbital. Near the proximal region of the squamosal ramus there is a gentle protuberance on the dorsal surface of the central body of the postorbital (Fig. 12E). 10.7717/peerj.1872/fig-12Figure 12 Partial skull roof and braincase of Eotrachodon orientalis (holotype MSC 7949). (A) Rostral view. (B) Interpretive line drawing of rostral view. (C) Caudal view. (D) Interpretive line drawing of caudal view. (E) Right lateral view. (F) Left lateral view. 10.7717/peerj.1872/fig-13Figure 13 Partial skull roof and braincase of Eotrachodon orientalis (holotype MSC 7949). (A) Dorsal view. (B) Interpretive line drawing of dorsal view. (C) Ventral view. (D) Interpretive line drawing of ventral view. The dorsal view is from Prieto-Márquez, Erickson & Ebersole (2016; fig. 1D), reprinted by permission of the Society of Vertebrate Paleontology, www.vertpaleo.org. The rostral ramus of the postorbital is triangular in dorsal view and shorter and wider than the other two rami (Fig. 13A). Like the prefrontal, the lateral margin of the rostral ramus is ornamented with a series of deep crenulations. Medially, this ramus interdigitates with the frontal throughout most of its length. At its caudal-most extent, however, it articulates with the parietal by means of a small wedge-shaped medial process. The intervening frontal at the orbital margin prevents the rostral ramus of the postorbital from meeting the prefrontal rostrally (Figs. 13A and 13B). The ventral ramus projects and wedges rostroventrally, forming about half of the caudal and rostral margins of the orbit and the infratemporal fenestra, respectively (Figs. 12E and 12F). The caudal surface of the ventral ramus shows a V-shaped excavation for reception of the postorbital ramus of the jugal (Fig. 13C). In contrast, the rostral surface of the ramus is smooth and concave. The ventromedial side of the central body of the postorbital, dorsal to the ventral ramus, displays a rounded depression that receives the lateral process of the laterosphenoid (Fig. 13C). The caudal ramus of the postorbital is dorsoventrally compressed. It extends caudally and slightly dorsally to overlap the postorbital process and as much as one-fourth of the quadrate cotylus of the squamosal. Specifically, the postorbital process of the squamosal inserts on a deep V-shaped excavation that occupies most of the ventral surface of the caudal ramus of the postorbital (Fig. 13C). The distal end of this ramus is bifid. Skull table and neurocranium Frontal The frontal (Figs. 12 and 13; Table 2) occupies an extensive area of the skull roof. The ectocranial surface of the frontal is 30% longer than wide. It is slightly convex medially, particularly caudomedially near the sagittal plane, around the contact with the parietal. In contrast, the ectocranial surface is gently concave laterally and rostrolaterally near the articulation with the postorbital and prefrontal, respectively. The frontal contributes substantially to the orbit, forming nearly one third of the width of the dorsal orbital margin. Specifically, a subrectangular process extends rostrolaterally from the orbital depression to intervene in between the prefrontal and the postorbital (Figs. 13A and 13C). 10.7717/peerj.1872/table-2Table 2 Selected neurocranial measurements (in mm) of the holotype specimen (MSC 7949) of Eotrachodon orientalis. Element Measurement Frontal (left), maximum length of ectocranial surface 81 Frontal (left), maximum width of ectocranial surface 63 Frontal (right), maximum length of ectocranial surface 81 Frontal (right), maximum width of ectocranial surface 63 Parietal, maximum length 67 Parietal, maximum width across rostrolateral processes 84 Basisphenoid-parasphenoid length 75 Length from rostral end of cultriform process of parasphenoid to distal end of left basipterygoid process of basisphenoid 84 Basisphenoid, maximum depth of contribution to spheno-occipital tubera 36 Basisphenoid, maximum width of contribution to spheno-occipital tubera 50 Basisphenoid, maximum width across basipterygoid processes 62 Prootic (left), maximum length from rostral margin to distal end of caudal process 65 Supraoccipital, maximum length 40 Supraoccipital, maximum width 66 Fused opithotic-exoccipital (left), maximum length from medial margin to distal tip of paroccipital process 120 The rostral margin of each frontal displays a broad rostrolaterally-oriented finger-like process (Figs. 13A and 13C). The rostral margin of each process is perpendicular to the sagittal edge of the frontal. The rostrolateral corner of this process is recessed and deeply excavated, bearing part of the articular region for the prefrontal and probably the nasal more medially. This articular region for the prefrontal extends further caudally along the rostrolateral margin of the frontal, forming a deep V-shaped concavity adjacent to the contribution of the bone to the orbital margin (Fig. 13A). The frontal becomes gradually shallower rostromedially near the sagittal plane of the skull, medial to the rostral process, where the bone becomes a thin lamina (Fig. 13A). This thinning of the rostromedial margin of the sagittal plane and the lack of articular facets in this area suggests that a fontanelle was present. A thin short peg-like process projects rostrally in-line with the interfrontal suture (Fig. 13A). The caudal margin of the frontal sutures extensively with the parietal and contributes to the formation of the rostral border of the supratemporal fenestra (Fig. 13A). Ventrally, the frontal exhibits wide and smoothly textured olfactory and orbital depressions that are located rostrally and laterally, respectively (Fig. 13C). A low ridge that is continuous with the rostrolateral corner of the rostral process constitutes the boundary between the olfactory and orbital depressions. The ventral surface of the rostral process is part of the olfactory depression. Caudally and caudolaterally, both depressions are contacted by the rostrodorsal and rostrolateral margins of the orbitosphenoid (Figs. 13C and 13D). Parietal The parietal (Fig. 13A; Table 2) is roughly hourglass-shaped in dorsal view, and is about 1.9 times as long as it is wide at its narrowest point. Rostrally, the parietal is greatly expanded owing to two large processes that curve and extend laterally. The distance between the distal end of these processes is 2.5 times the width of the tobrien@example.com. They contribute to the rostral margin and rostrodorsal wall of the supratemporal fenestra. A sharp ridge forms the rostrodorsal margin of the proximal region of each lateral process. The ridges are continuous medially with the sagittal crest of the parietal (Fig. 13A). The lateral processes articulate extensively rostrally with the caudal margin of the frontal, ventrally with the lateral process of the laterosphenoid, and laterally with the medial surface of the postorbital. Centered on the sagittal plane of the skull, the parietal displays a fan-shaped median process that forms a crenulated suture with the caudomedial margins of both frontals (Fig. 13A). The dorsal surface of this process is gently depressed. The main body of the parietal gradually becomes slightly narrower caudally. Its lateral surfaces are moderately concave longitudinally. Dorsally, the parietal shows a tall longitudinal crest that is steeply down-warped rostrally (Fig. 12F). The original morphology of the crest, however, appears to be distorted because of post-depositional dorsoventral crushing. Parietal crests are typically down-warped in lambeosaurine hadrosaurids (Horner, Weishampel & Forster, 2004). Unlike lambeosaurines, the elevation and down-warping of the parietal crest in Eotrachodon orientalis is not accompanied by deepening of the squamosal and elevation of the supratemporal bars (Figs. 12F and 13A). Thus, the parietal crest of E. orientalis was likely substantially elevated above the supratemporal bars (Figs. 12E and 12F). The ventral side of the parietal is deeply concave; the lateral margins of this concavity articulate, from rostral to more caudal, with the laterosphenoid, the prootic and the supraoccipital (Fig. 13C). The caudal end of the parietal shows three finger-like projections; at least the median of these projections meets the squamosals dorsally. Supraoccipital The supraoccipital (Fig. 14; Table 2) is a dorsoventrally compressed and relatively small element that occupies a median position in the occipital region of the neurocranium. This bone rests over the two fused opisthotic-exoccipitals, inset in between the proximal regions of the paroccipital processes of those elements (Fig. 14D). The dorsal surface of the supraoccipital is smooth, gently concave mediolaterally, and mediolaterally expanded caudally. The rostral median region of the supraoccipital is gently elevated rostrodorsally and bisected by a thin ridge that extends caudally to end at about mid-length of the bone (Fig. 14C). That rostral elevation, or ascending process, meets the parietal rostrally (Fig. 14B). Lateral from this process, the supraocciptal displays a rugose rostroventrally-sloping surface for articulation with the squamosal. The ventral side of the supraoccipital is deeply excavated longitudinally (Fig. 14E). Caudally, on the ventral side of the supraoccipital, the subtriangular articular surface for the fused opisthotic-exoccipital faces ventrally and slightly caudally (Fig. 14D). 10.7717/peerj.1872/fig-14Figure 14 Partial occipital region of the skull of Eotrachodon orientalis (holotype MSC 7949). (A) Rostral view. (B) Left lateral view. (C) Dorsal view. (D) Caudal view. (E) Ventral view. Fused opitshotic-exoccipial These elements (Fig. 14; Table 2) form most of the occiput and part of the caudodorsal wall of the braincase. The rostral region of this bony complex is mediolaterally compressed and meets the prootic dorsolaterally, the supraoccipital dorsomedially, the basisphenoid rostroventrally, and the basioccipital caudoventrally (Figs. 14A, 14B and 14D). Its lateral surface is dorsoventrally concave and continuous dorsally with the proximolateral surface of the paroccipital process. Ventrally, this region of the lateral wall of the braincase is mediolaterally expanded to form the ventral articular facet for the underlying basioccipital (Fig. 14D). The paroccipital process is a large horn-like structure that diverges caudolaterally from the caudodorsal corner of the occiput (Fig. 14D). Specifically, the thicker proximal region of this process extends caudodorsally to gradually curve and wedge lateroventrally along its distal segment. The dorsal margin of the paroccipital process forms a ridge that becomes progressively more prominent proximally. Prootic The prootic (Fig. 15; Table 2) is a rostrocaudally elongate and relatively small component of the dorsolateral wall of the braincase. The rostral half encloses nearly the entire foramen for the trigeminal nerve, except for the central portion of its ventral margin (Fig. 15A). The dorsal margin of the prootic bears a series of crenulations for articulation with the parietal (Fig. 15D). The ventral margin is slightly expanded mediolaterally, particularly caudal to the trigeminal foramen (Fig. 15C), and meets the basisphenoid. Rostrally, the prootic joins the laterosphenoid. 10.7717/peerj.1872/fig-15Figure 15 Prootics of Eotrachodon orientalis (holotype MSC 7949). (A–D) Left prootic in lateral, medial, ventral, and dorsal view, respectively. (E and F) Right prootic in lateral and medial view, respectively. The caudal half of the prootic consists of a wedge-shaped process that projects caudodorsally and is laterally deflected relative to the rostral half of the bone (Figs. 15A and 15D). The medial surface of this process shows a series of fine longitudinal ridges for articulation with the opisthotic-exoccipital complex (Fig. 15B). A relatively broad but shallow and faint ridge extends caudodorsally on the lateral surface of the prootic (Fig. 15A). This ridge originates caudal to the trigeminal foramen and extends further caudodorsally along the entire lateral surface of the caudal process. Fused parasphenoid-basisphenoid This bony complex (Fig. 16A; Table 2) occupies a median position in the rostroventral region of the braincase (Fig. 16). The main body of the basisphenoid is mediolaterally expanded and fused rostrally to the cultriform process of the parasphenoid. Its preserved proximal segment extends rostrally, and is slightly compressed mediolaterally and gently curved ventrally. While the ventral surface of the cultriform process is smooth, the dorsal surface shows a proximal deep triangular excavation, followed by a broad longitudinal sulcus (Fig. 16D). 10.7717/peerj.1872/fig-16Figure 16 Partial basisphenoid of Eotrachodon orientalis (holotype MSC 7949). (A) Left lateral view. (B) Caudal view. (C) Ventral and slightly rostral view. (D) Dorsal view. Caudally, the ventral surface of the cultriform process becomes gradually broader as each of its lateral margins diverges and becomes continuous with the ventral edge of each basipterygoid process (Fig. 16C). The basipterygoid process is a large horn-like structure that projects caudoventrally and laterally to meet the pterygoid bone of the palate. The right pterygoid process of MSC 7949 is only preserved proximally and shows a triangular cross-section; the left process is complete, but its distal end is fractured and medially displaced (Fig. 16B). The extensive ventral surface existing between the bases of the basipterygoid processes is concave and smooth. Its caudal margin shows a short peg-like median process, but unlike most hadrosaurids (Prieto-Márquez, 2010a) lacks a prominent ridge (Fig. 16C). The caudal region of the basisphenoid consists of the rostral portions of the paired spheno-occipital tubera (Fig. 16A). The tubera are separated ventrally by a deep indentation (Fig. 16B). The caudal articular surfaces of the spheno-occipital tubera are nearly vertically oriented and concave for reception of the basioccipital. The lateral margins of these surfaces are greatly expanded caudolaterally. A sub-elliptical lamina, the alar process, extends caudoventrally from the lateral surface of the proximal region of each spheno-occipital tuber (Fig. 16A). The alar process is relatively small, as in most hadrosauroids except brachylophosaurins (Prieto-Márquez, 2005). The neck-like basisphenoid constriction that occurs in between the basipterygoid processes and the spheno-occipital tubera is moderately developed. Specifically, the minimum width of this constricted area is 50% less than the maximum width of the basisphenoid across the spheno-occipital tubera. Laterosphenoid The laterosphenoid (Figs. 12C and 13C; Table 2) is a triradiate element that contributes to the laterodorsal wall of the braincase. The lateral surface of the laterosphenoid is gently inflexed rostrocaudally and mediolaterally compressed. Three processes extend from the central region of the laterosphenoid laterally, medially, and ventrally. The rostral border of the laterosphenoid meets the caudal margin of the orbitosphenoid (Figs. 12A and 13C). The caudal margin of the laterosphenoid is slightly expanded mediolaterally and articulates with the rostral border of the prootic. The laterosphenoid meets its counterpart by means of a short but rostrocaudally broad process that extends medially perpendicular to the lateral wall of the braincase (Figs. 12A and 13C). The large lateral process is perpendicular to the dorsal region of the lateral surface of the body of the laterosphenoid (Figs. 12A, 12C and 13C). The process becomes both substantially shallower and
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