Brighstoneus simmondsi, Lockwood & Martill & Maidment, 2021
Lockwood, Jeremy A. F., Martill, David M. & Maidment, Susannah C. R., 2021, A new hadrosauriform dinosaur from the Wessex Formation, Wealden Group (Early Cretaceous), of the Isle of Wight, southern England, Journal of Systematic Palaeontology 19 (12), pp. 847-888: 5-31
treatment provided by
gen. et sp. nov
Brighstoneus simmondsi gen. et sp. nov.
( Figs 3–22 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 View Figure 8 View Figure 9 View Figure 10 View Figure 11 View Figure 12 View Figure 13 View Figure 14 View Figure 15 View Figure 16 View Figure 17 View Figure 18 View Figure 19 View Figure 20 View Figure 21 View Figure 22 , 24 View Figure 24 )
Etymology. The specific name honours Mr Keith Simmonds who made the discovery of the specimen.
Holotype. MIWG 6344, a partial skeleton composed of the following elements: dorsal process of right premaxilla; both maxillae; both jugals; left palpebral; left nasal; both dentaries; predentary; one transitional dorsal and seven dorsal vertebrae; sacrum; six caudal vertebrae; dorsal ribs, nine from the left side and five from the right side; both ilia; right ischium; possible prepubic process; and the right femur. Some parts of the same individual (including two dorsal vertebrae and other fragments) remain in private ownership and are not described herein.
Diagnosis. Differs from all other iguanodontians by possessing the following autapomorphies and unique combination of characters (autapomorphies indicated with an asterisk): maxillary crowns possessing both a primary ridge and mesially placed accessory ridges on the lingual surface *; nasal expanded postnarially to produce a modest nasal bulla with convex lateral walls *. Acharacter combination of at least 28 dentary tooth positions in a dentary with one active crown and one replacement tooth for each position and non-parallel alveolar septa.
In addition, Brighstoneus can be distinguished from other Barremian–Aptian Wealden Group iguanodontians by possession of the following combination of features: ratio of precoronoid length of the dentary to minimum depth> 6.0; coronoid process projects at approximately 90 ° with respect to the dorsal margin of dentary; bilobed ‘heart-shaped’ ventral predentary process with prominent anterior denticles with concave mesial and distal edges; posteriorly positioned maxillary ascending process in lateral view with length of the anterior section approximately twice the length of the posterior section; prominent anterodorsal process present on maxilla; anterior (maxillary) process of jugal relatively long (60% of overall length) and tapers distally to form a triangular ending; ventral section of the posterior margin of the jugal (heel) projects posteriorly to form a spurlike feature; in dorsal view the jugal is straight; ventral border overlapped by maxillary process of premaxilla; anteroventral nasal process forms posteroventral margin of external narial opening; ventral surface of ischiadic peduncle of ilium parallel with the anteroventral margin of the postacetabular process; ischiadic peduncle of ilium has flat lateral wall with no pronounced posterolateral boss; ventral shelf at base of iliac preacetabular process weakly developed; dorsoventrally deep preacetabular process with little or no axial twist; deep and short iliac central plate with ratio of depth to length> 1.2.
Overall, MIWG 6344 is excellently preserved with little distortion or crushing (measurements are available in the Supplementary material). Many of the bones exhibit unusual eroded areas, which have left smooth, ‘scooped out’ regions extending down to and including the internal trabecular bone. Some of these areas are partially covered by the original sediment, suggesting these features formed prior to, or soon after, burial. It is unlikely that this damage resulted from prolonged subaerial exposure as the cortical surface is generally well preserved ( Behrensmeyer 1978). As many of these eroded features are associated with highly cartilaginous regions, it is possible that they are examples of invertebrate bioerosion, potentially by isopterans ( Britt et al. 2008, and references therein; Huchet et al. 2011), which are known from the Wealden Group ( Jarzembowski 1981). The term ‘eroded’ is used in the descriptive text to indicate these areas. There is also evidence, particularly on the ribs, of shallow circular pits varying between 2–4 mm in diameter, which might represent dermestid beetle activity ( Britt et al. 2008).
Only the dorsal (nasal) process of the right premaxilla is preserved, but without its posterior-most tip ( Fig. 3 View Figure 3 ). In lateral view, the fragment has a long mediolaterally thin process, which is gently curved, convex dorsally and gradually tapers as it extends posteriorly. The lateral surface is rounded and grooved in the posterior threefifths of its length for contact with the anterodorsal process of the nasal ( Fig. 3A View Figure 3 : this area is described as ‘bevelled’ in Mantellisaurus atherfieldensis rather than grooved: Norman 1986). Anteriorly the ventral margin curves ventrally forming a thin blade of bone, which is incomplete but would have contributed to the internarial septum. The medial surface of the dorsal process is flat for articulation with its antimere.
Both maxillae are present, with the left being the better preserved. The left maxilla ( Fig. 4 View Figure 4 ) is almost complete, although it has lost parts of the anterior ends of the anteroventral and anterodorsal processes. Between the anterior processes a fragment of bone has been glued into a position that is probably anatomically incorrect ( Fig. 4A, C View Figure 4 ). The ascending process is almost complete, having a thin fractured margin, but preserving the groove for reception of the maxillary process of the premaxilla. The maxilla has been subjected to minor transverse lateral compression that has slightly relocated the incomplete jugal process medially.
The maxilla is a robust bone but is relatively thinner in dorsal and ventral views than that of Mantellisaurus atherfieldensis (NHMUK PV R5764). Its general shape in lateral view is of an anteroposteriorly expanded triangle. The apex is directed dorsally as the ascending process but is situated in a markedly posterior position, and if complete the section anterior to this process would have been approximately twice the length of the posterior section. Aratio of ~ 2.0 or higher is also seen in Altirhinus kurzanovi (2.1: Norman 1998), Zhanghenglong yangchengensis (2.3: Xing et al. 2014), Protohadros byrdi (2.3: Head 1998), Proa valdearinnoensis (2.4: McDonald et al. 2012a) and Ouranosaurus nigeriensis (2.6: Taquet 1976). In contrast the ratio is 1.4 in Mantellisaurus atherfieldensis (NHMUK PV R5764). In lateral view the ventral edge of the maxilla is shallowly concave, the tooth row being almost straight for most of its length. There are 29 tooth positions: most of the alveoli are empty but two established and two emerging replacement crowns are present (see below). In ventral view the tooth row is straight for most of its length but posteriorly the last quarter curves laterally. The bone preservation of the lateral surface is better in the right maxilla, which has eight small nutrient foramina (1–3 mm in diameter) that open anterolaterally and form an anteroposteriorly orientated row in the ventral half of the maxilla. Anteriorly, the ventral surface of the maxilla forms an anteroventral process that, although incomplete, follows the curve of the tooth row and tapers anteriorly in lateral view. The first alveolus is situated at the base of this process, anterior to which the process is edentulous. Although the area dorsal to the anteroventral process has suffered some damage, a prominent anterodorsal process is present. Posteriorly the transverse cross-section of this process is triangular with a gently dorsoventrally concave lateral surface, a flat ventromedial surface and a dorsomedial surface that is roughened and has a longitudinal groove in its posterior section. Extending anteriorly, the process becomes increasingly compressed transversely to form a thin blade, whose end is missing. Possession of an anterodorsal process is considered ancestral for Archosauria ( Wagner & Lehman 2009) and has been employed as a character in iguanodontian phylogeny (e.g. Sues & Averianov 2009; Xing et al. 2014; McDonald et al. 2017). Its presence has been used to distinguish Saurolophinae from Lambeosaurinae ( Wagner & Lehman 2009), but it is also present in non-hadrosaurids such as Altirhinus kurzanovi ( Norman 1998), Bactrosaurus johnsoni ( Godefroit et al. 1998), Eolambia caroljonesa (CEUM 35492: McDonald et al. 2012b) and Gilmoreosaurus mongoliensis ( Prieto-Marquez & Norell 2010) , and is especially prominent in Protohadros byrdi ( Head 1998).
The anterior and posterior dorsal edges of the lateral wall of the maxilla meet to form the apex of the transversely compressed ascending process. The jugal process arises from the base of the posterior edge of the ascending process ( Fig. 4A, D View Figure 4 ). The trough between the jugal process and lateral body of the maxilla expands posteriorly to form a mediolaterally broad ectopterygoid shelf. When viewed dorsally the posterior maxilla is more ovoid (long axis anteroposterior) and the medial wall more convex than in Mantellisaurus atherfieldensis (NHMUK PV R5764). Below the apex of the ascending process and situated posteriorly is a depressed area, which would presumably have contributed to the antorbital fenestra and might have formed a small antorbital fossa ( Fig. 4A View Figure 4 ). This depressed area is also present in Iguanodon bernissartensis ( Norman 1980) , Mantellisaurus atherfieldensis (NHMUK PV R5764), Ouranosaurus nigeriensis ( Taquet 1976), Camptosaurus dispar ( Gilmore 1909) and Bolong yixianensis ( Wu & Godefroit 2012) . There is no evidence of an antorbital fenestra in Altirhinus kurzanovi ( Norman 1998) or Jinzhousaurus yangi ( Wang & Xu 2001; Barrett et al. 2009) and this feature is not described in Equijubus normani ( McDonald et al. 2014). Anteriorly, the lateral part of the dorsal surface is furrowed to form the premaxillary groove for contact with the premaxilla ( Fig. 4A, B View Figure 4 ). Medially the maxilla has a vertical surface that has an elongate rectangular outline with the ascending process rising above its dorsal edge. Above the tooth row is a concave arcade of circular special foramina (sensu Edmund 1957) or replacement foramina (sensu Jin et al. 2010), the curve of which is considerably deeper than the ventral edge ( Fig. 4C View Figure 4 ). Between the special foramina and the tooth row the bone is thin with a textured surface and forms the alveolar parapet. Dorsally there is a medial shelf that is damaged anteriorly but appears to have a rounded edge.
Only the left nasal is preserved ( Fig. 5 View Figure 5 ). It is undistorted but broken at both ends, so missing the posterior articulation for the frontal and prefrontal. The nasal is elongate anteroposteriorly, relatively broad and convexly curved mediolaterally so that with its antimere the external surface formed a vault over the nasal cavity.
Anteriorly the nasal is divided into two processes. The anterodorsal process is dorsoventrally deeper in lateral view than the anteroventral process. The anterodorsal section, which articulated with the dorsal process of the premaxilla, is incomplete but the area that contributed to the posterior margin of the external naris is intact. The base of the anteroventral process is present showing that the nasal also made a contribution to the posteroventral margin of the external naris. There is no anteroventral process in Iguanodon bernissartensis ( Norman 1980) , Mantellisaurus atherfieldensis ( Norman 1986) , Protohadros byrdi ( Head 1998), Bactrosaurus johnsoni ( Godefroit et al. 1998), Equijubus normani ( McDonald et al. 2014) or Gobihadros mongoliensis ( Tsogtbaatar et al. 2019) . In hadrosaurids an anteroventral process is usually present, for example in Maiasaura peeblesorum and Gryposaurus notabilis ( Prieto-Marquez & Norell 2010) and is particularly prominent in Brachylophosaurus canadensis (Prieto-Marquez 2001) and Edmontosaurus annectens ( Campione & Evans 2011) . Among non-hadrosaurid hadrosauriforms, an abbreviated anteroventral process is present in Altirhinus kurzanovi ( Norman 1998), with longer processes in Ouranosaurus nigeriensis ( Taquet 1976), Jinzhousaurus yangi ( Wang & Xu 2001; Barrett et al. 2009) and Bolong yixianensis ( Wu & Godefroit 2012) . The posterior margin of the external naris is almost straight and posteroventrally orientated. The ventral border of the lateral surface of the nasal has a shallowly bevelled surface ( Fig. 5C, E View Figure 5 ) and presumably underlapped the maxillary process of the premaxilla as a scarf joint, although anteriorly there is also a slight groove along the dorsal edge of this surface. The nasals of Gobihadros mongoliensis and Choyrodon barsboldi have a grooved ventrolateral surface ( Tsogtbaatar et al. 2019) and both appear to underlap the posterolateral process of the premaxilla; Bactrosaurus johnsoni ( Godefroit et al. 1998) and Altirhinus kurzanovi ( Norman 1998) are grooved along their ventral borders. The medial border of the dorsal surface is grooved anteriorly in Brighstoneus simmondsi , presumably where it articulated with the dorsal process of the premaxilla, but tapers to a thin blade posteriorly that most likely formed a simple butt joint with its antimere (see Weishampel 1984) as in Altirhinus kurzanovi ( Norman 1998) and Jinzhousaurus yangi ( Barrett et al. 2009) , although in Ouranosaurus nigeriensis the nasals are asymmetrical with one having a blade and the other a groove along the dorsal edge ( Taquet 1976).
When viewed laterally in life position the dorsal border of the nasal is sinusoidal and forms a distinct anterior convexity with the most dorsal part of the curve situated posterior to the posterior border of the external naris. The section of the lateral wall of the nasal, posterior to the naris, is slightly convex in ventral view. Collectively these features would have produced a rounded bulge and formed a distinct step between the anterior and posterior sections of the nasal ( Fig. 5E View Figure 5 ). This is less prominent but similar in shape and position to the nasal bulla of the iguanodontian Muttaburrasaurus langdoni ( Bartholomai & Molnar 1981) , although in Muttaburrasaurus the lateral walls are slightly concave. Ahigh arched region, dorsal to the posterior narial, is also seen in Altirhinus kurzanovi ( Norman 1998), Choyrodon barsboldi ( Gates et al. 2018) and in some hadrosaurids, for example Gryposaurus notabilis ( Lambe 1914) and Kritosaurus navajovius ( Brown 1910). Ouranosaurus nigeriensis also has a rounded protuberance, but this is situated more posteriorly on both nasals, just anterior to the suture for the frontals ( Taquet 1976), while Jinzhousaurus yangi has mediolaterally convex nasals with a sagittal depression between them ( Barrett et al. 2009), a feature also seen in Altirhinus kurzanovi ( Norman 1998) and Choyrodon barsboldi ( Gates et al. 2018). There is no evidence of a sagittal trough in Brighstoneus simmondsi . Temporal and geographical separation, together with the pattern of phylogenetic relationships and variety of morphologies in these examples, suggests that nasal ornamentation evolved independently on several occasions in iguanodontids. This feature also appears to be unique among the Wealden Group iguanodontids.
Both jugals are preserved and are almost complete ( Fig. 6 View Figure 6 ). The dorsal sections of the postorbital process and the process overlapping the quadratojugal are broken.
It is a triradiate bone with an anteriorly directed maxillary process, a central dorsally projecting postorbital process and a posterior dorsally projecting quadratojugal process. Both jugals are very straight and narrow in dorsal view ( Fig. 6E, F View Figure 6 ) compared to Mantellisaurus atherfieldensis (NHMUK PV R5764), which is laterally convex with a relatively wider orbital shelf.
The maxillary process in lateral view occupies 60% of the total length of the jugal (ratio of length anterior to midpoint of postorbital process/maximum length ¼ 0.60). This ratio varies considerably in iguanodontians, from 0.29 in Xuwulong yueluni ( You et al. 2011) and 0.33 in Iguanodon bernissartensis ( Norman 1980) to 0.56 in Mantellisaurus atherfieldensis (NHMUK PV R 5764) and 0.62 in Ouranosaurus nigeriensis ( Taquet 1976). However, in most hadrosauriforms including hadrosaurids this ratio ranges between 0.40 and 0.54. For example, Altirhinus kurszanovi (0.46: Norman 1998), RBINS R57 (0.52: Norman 1986), Equijubus normani (0.49: You et al. 2003), Eolambia caroljonesa (0.48: McDonald et al. 2012b), Brachylophosaurus canadensis (0.51: Prieto-Marquez 2001) and Gryposaurus latidens (0.48: Prieto-Marquez 2012). The maxillary process in lateral view is gently curved with a convex ventral margin and a concave dorsal margin. The process is quite gracile and lacks the dorsoventral expansion seen in many hadrosauriforms, such as those present in Probactrosaurus gobiensis ( Norman 2002); Eolambia caroljonesa ( McDonald et al. 2012b) and Gobihadros mongoliensis ( Tsogtbaatar et al. 2019) , instead the anterior dorsal margin slopes anteroventrally to join the ventral margin as a point. This slope forms a facet for articulation with the lacrimal. Amaxillary process with a pointed end, lacking dorsoventral expansion, is also seen in Altirhinus kurzanovi ( Norman 1998), Batyrosaurus rozhdestvenskyi ( Godefroit et al. 2012), Dakotadon lakotaensis ( Weishampel & Bjork 1989) and Choyrodon barsboldi ( Gates et al. 2018). The maxillary process of the jugal in Mantellisaurus atherfieldensis (NHMUK PV R 5764) is not dorsoventrally expanded but has a much blunter, less tapered anterior end, albeit with some damage ventrally. Medially the maxillary process is deeply excavated to form an anteroposteriorly elongate facet with a roughened surface for reception of the finger-like jugal process of the maxilla. The jugal is expanded medially at the posterior end of the maxillary facet to form a buttress ( Fig. 6E, F View Figure 6 ) for the distal end of the jugal process of the maxilla. Posterior to the maxillary facet is a posterodorsally orientated ridge that separates the facet from a shallow oval fossa for the ectopterygoid. Between the maxillary and postorbital processes, the dorsal surface is concave in lateral view and provided a shelf, forming the ventral margin of the orbit. The orbit is bounded posteriorly by the postorbital process, which is long, straight and ascends perpendicular to the main axis of the jugal body. Dorsally it develops an anterolaterally facing facet for suture with a reciprocal facet on the jugal process of the postorbital. Between the postorbital process and the quadratojugal process is a ‘U’-shaped embayment that forms the ventral border of the lateral temporal fenestra. On its medial surface, the posterior and ventral margins of the fenestra have a subtle, smooth and shallowly convex rolled edge ( Fig. 6B, C View Figure 6 ), which is not readily apparent in other hadrosauriforms, including Mantellisaurus atherfieldensis (NHMUK PV R 5764), Eolambia caroljonesa ( McDonald et al. 2012b) and Sirindhorna khoratensis ( Shibata et al. 2015).
The right jugal has a nutrient foramen ~ 1 mm in diameter, located medially below the ventral border of the fenestra and postorbital process, while on the left jugal two foramina are present (with diameters of 4 mm and 2 mm). The presence or absence of a relatively large foramen ventral to the postorbital process is a character used by McDonald et al. (2017: character 51), but the configuration in Brighstoneus simmondsi suggests that there may be considerable individual variation in this feature, perhaps calling into question its phylogenetic utility.
In lateral view, the ventral border of the jugal is sinuous, being concave ventral to the postorbital process and convex ventral to the maxillary and quadratojugal processes, the latter forming the ventral flange of the quadratojugal process ( Fig. 6 View Figure 6 ). The posterior border of the quadratojugal process is slightly concave in its ventral half, although the posteroventral corner of the ventral flange is developed posteriorly and forms an angular heel-like expansion, although damage makes the full extent of this unknown. Posterior expansion of the ventral flange is seen in Camptosaurus dispar ( Gilmore 1909), Equijubus normani ( McDonald et al. 2014: where it is spur-like), Zalmoxes robustus ( Weishampel et al. 2003) and Z. shqiperorum ( Godefroit et al. 2009), although in Zalmoxes spp. this is due to the presence of a posterior quadratojugal recess that is not present in other iguanodontians. On the medial surface of the posterior process a faint ridge extends anteroposteriorly, slightly ventral to the ventral margin of the lateral temporal fenestra. Above this is a roughened area that presumably denoted the area of overlap with the quadratojugal, although this is neither as deeply marked nor as well-defined as in Mantellisaurus atherfieldensis (NHMUK PV R 5764), Altirhinus kurzanovi ( Norman 1998), Probactrosaurus gobiensis ( Norman 2002) or Zalmoxes robustus ( Weishampel et al. 2003). Faint muscle scars are present on the medial surface of the heel area.
Only the left palpebral is preserved ( Fig. 7 View Figure 7 ). It is largely complete although the posterior tip is a recent fracture, so it is unknown whether an accessory palpebral was present. An accessory palpebral is known in Iguanodon bernissartensis ( Norman 1980) and Jinzhousaurus yangi ( Barrett et al. 2009) . Anteriorly the palpebral has an ovate and anteroposteriorly concave articular surface that faces anteromedially, presumably for articulation with the convex surface of the prefrontal. It extends posteriorly as a tapering process that is almost straight and dorsoventrally compressed in lateral view but broader and curved with concave internal and convex external surfaces in dorsal and ventral views. The ventral and medial surfaces are smooth, but the lateral and dorsal surfaces are rugose. The posterior rim of the articular surface is expanded medially, forming a process that stands proud of the main body of the palpebral ( Fig. 7A, C View Figure 7 ), such that when articulated the body of the palpebral would have been raised clear of the prefrontal and postorbital. Asimilar palpebral is found in RBINS R57 (formerly known as IRSNB 1551: Norman 1986), Ouranosaurus nigeriensis ( Taquet 1976), Altirhinus kurzanovi ( Norman 1998), Bactrosaurus johnsoni ( Godefroit et al. 1998), Bolong yixianensis ( Wu & Godefroit 2012) , Gobihadros mongoliensis ( Tsogtbaatar et al. 2019) and Xuwulong yueluni ( You et al. 2011). The body of the palpebral is more rod-like in Tethyshadros insularis ( Dalla Vecchia 2009) and it has been suggested that it became fused with the prefrontals in hadrosaurids ( Maryanska & Osmolska 1979).
The predentary is exceptionally well preserved but slightly compressed transversely ( Fig. 8 View Figure 8 ). In dorsal view it has a horseshoe-shaped outline, with the convex anterior section overlying the dentary symphysis, and almost parallel lateral processes extending posteriorly, which curve slightly medially in their distal portions. This is similar to the ‘U’-shape seen in all non-hadrosaurid hadrosauriforms except for Proa valdearinnoensis ( McDonald et al. 2012a), in which the sides diverge posteriorly and form a ‘V’-shaped angle of ~ 50 ° anteriorly. The ventral surface is bevelled and fits against a groove on the anterior dentaries. The lingual surface has a rough texture compared to the smooth ventral and lateral surfaces. There is no posteriorly projecting dorsomedial process extending in the sagittal plane from the posterodorsal surface of the ventral process, nor a dorsomedial process arising from the distal lateral processes. A dorsomedial process is known in Eotrachodon orientalis ( Prieto-Marquez et al. 2016), Plesiohadros djadokhtaensis ( Tsogtbaatar et al. 2014), Proa valdearinnoensis ( McDonald et al. 2012a), Iguanacolossus fortis ( McDonald et al. 2010a) and Bayannurosaurus perfectus ( Xu et al. 2018), and on the distal lateral processes in Gobihadros mongoliensis ( Tsogtbaatar et al. 2019) and Plesiohadros djadokhtaensis ( Tsogtbaatar et al. 2014). However, in Brighstoneus simmondsi a midline process arises from the ventral surface and projects posteriorly as a bilobed ‘heart-shaped’ structure, which has a similar morphology to the process in Proa valdearinnoensis ( McDonald et al. 2012a) and Bayannurosaurus perfectus ( Xu et al. 2018). It is likely that this would have enclosed the ventral surfaces of the dentaries, thereby bracing the symphysis. The lateral walls of the predentary curve convexly dorsoventrally, while anteriorly the surface is flatter and directed anterodorsally forming a slope of ~ 45 °. The entire occlusal margin is highly irregular, with five prominent denticles anteriorly, the central one situated sagittally. These denticles are pointed and prominent with distinct concave mesial and distal edges, giving the interdenticle spaces a circular appearance. More posteriorly, some of the irregularity probably represents much smaller denticles. Prominent anterior denticles are reported widely across Iguanodontia from Tenontosaurus dossi ( Winkler et al. 1997) to hadrosaurids, for example Eotrachodon orientalis ( Prieto-Marquez et al. 2016). Below the occlusal margin is a series of foramina. The largest of these foramina are situated anteriorly on either side of the median denticle and, although the area has some damage, there appear to be ventrally directed vascular grooves associated with them.
Both dentaries are preserved with little distortion ( Fig. 9 View Figure 9 ). The left dentary has a damaged and incomplete symphysis. In lateral view, the dentary forms an elongate rectangle that is ventrally deflected anteriorly. The dorsal edge of the dentary is straight apart from anteriorly where it initially curves convexly. The ventral edge in lateral view forms a gentle concave arc so that the depth of the dentary is lowest in the central section and greatest in the anterior and posterior sections. The posteriormost part of the ventral margin is missing in both dentaries.
Anteriorly the ventral deflection of the dentary twists so that its medial wall faces dorsally, while also curving medially to meet its antimere at the symphysis. When viewed laterally the anterior margin of the ventrally deflected section is slightly concave. In dorsal view, the medial margin of the symphysis and the lateral margin of the dentary diverge posteriorly. Ventral deflection of the dentary symphysis is common in iguanodontians. In early-diverging taxa such as Zalmoxes robustus ( Weishampel et al. 2003), Tenontosaurus tilletti ( Thomas 2015), Mochlodon vorosi (Osi¨ et al. 2012) and Dysalotosaurus lettowvorbecki (Hubner 2010) the dentary is almost straight, but the distal tip is deflected slightly. In Brighstoneus simmondsi there is a more pronounced ventral deflection of the anterior third of the body, the ventral border of which, in lateral view, forms an angle of 16 ° with the ventral border of the midsection. The range of deflection angles in hadrosauriforms varies, but Brighstoneus simmondsi has a similar angle to that found in Proa valdearinnoensis (18 °: McDonald et al. 2012a), Mantellisaurus atherfieldensis (20 °: NHMUK PV R5764: Fig. 10 View Figure 10 ), Batyrosaurus rozhdestvenskyi (15 °: Godefroit et al. 2012) and Choyrodon barsboldi (19 °: Gates et al. 2018). Some hadrosaurids have much greater deflections and several non-hadrosaurid taxa are not deflected at all, including Ouranosaurus nigeriensis ( Taquet 1976), Hypselospinus fittoni ( Norman 2015) and Kukufeldia tilgatensis ( McDonald et al. 2010b).
The tooth row includes 28 alveolar positions although a small pit at the anterior end of the row of the right dentary may signify a further diminutive alveolus. This represents the highest number of dentary alveolar positions recorded in any hadrosauriform taxon that possesses one active and one replacement crown per alveolar position and alveolar septa shaped by the teeth (Supplementary material, Table S2), in contrast to more deeply nested taxa with higher numbers of replacement teeth. Other non-hadrosaurid hadrosauriforms with similar alveolar counts first appeared in the Cenomanian with 30 in Eolambia caroljonesa ( Kirkland 1998; McDonald et al. 2012b) and 28 in Protohadros byrdi ( Head 1998), although both of these taxa possess more hadrosaurid-like dentaries with parallel-sided alveolar septa as well as more teeth in each tooth position (two active and two replacement teeth for each alveolar position in the former [ McDonald et al. 2012b] and at least three and possibly four teeth per tooth file in the latter [ Head 1998]). In dorsal view the alveolar row forms a slight sigmoid curve but is essentially straight for most of its length, except posteriorly where it curves laterally to meet the base of the coronoid process. The alveolar septa are not parallel and reflect tooth crown shape. This is the usual morphology in early-diverging hadrosauriforms such as Altirhinus kurzanovi ( Norman 1998), Fukuisaurus tetoriensis ( Kobayashi & Azuma 2003) , Proa valdearinnoensis ( McDonald et al. 2012a), Lanzhousaurus magnidens ( You et al. 2005) , Mantellisaurus atherfieldensis (NHMUK PV R5764) and Iguanodon bernissartensis ( Norman 1980) . In more derived hadrosauriforms such as Probactrosaurus gobiensis ( Norman 2002), Bactrosaurus johnsoni ( Godefroit et al. 1998) and all hadrosaurids the alveolar walls are parallel ( Horner et al. 2004). In Brighstoneus simmondsi the tooth sizes (estimated from the mesiodistal diameters of alveoli) are maximal in the mid-section of the tooth row and become progressively smaller at both ends. In dorsal view the alveolar row is situated medially, as in all iguanodontians, while laterally there is a buccal shelf that is widest posteriorly ( Fig. 9E, F View Figure 9 ). There appears to be a relative difference in the width of this shelf between the two dentaries, with the right side being bulkier, especially in its posterior half, although this has been exaggerated by an area of taphonomic crushing. The buccal shelf of Brighstoneus simmondsi tapers anteriorly but extends to the end of the tooth row and, as in Ouranosaurus nigeriensis ( Taquet 1976), Sirindhorna khoratensis ( Shibata et al. 2015) and Zhanghenglong yangchengensis ( Xing et al. 2014), is a relatively narrow structure. In most non-hadrosaurid hadrosauriforms the shelf has a relatively greater transverse width, especially posteriorly, including in Iguanodon bernissartensis ( Norman 1980) , Mantellisaurus atherfieldensis (NHMUK PV R5764), Proa valdearinnoensis ( McDonald et al. 2012a), Eolambia caroljonesa ( McDonald et al. 2012b) and Shuangmiaosaurus gilmorei ( You et al. 2003), with a markedly wider shelf in Bactrosaurus johnsoni ( Godefroit et al. 1998) and Probactrosaurus gobiensis ( Norman 2002). The shelf has a steep margin that forms the lateral wall of the dentary. This is predominantly dorsoventrally convex but becomes vertically flat anterior to the sixth tooth position. There are several large foramina along the dorsolateral surface. Anteriorly these are replaced by pairs of smaller, closely associated foramina that are orientated dorsoventrally, signifying bifurcation of the arteries prior to exiting the dentary. In both dentaries there are four nutrient foramina arranged along the inflected section of the dentary as it descends to the symphysis. In dorsal view, the middle section of the lateral wall is slightly convex.
Anteriorly the tooth row is preceded by a diastema of approximately three tooth positions in length, as also seen in Choyrodon barsboldi ( Gates et al. 2018), Mantellisaurus atherfieldensis (NHMUK PV R5764) and Bolong yixianensis ( Wu & Godefroit 2012) . There is no diastema in Dakotadon lakatoensis ( Weishampel & Bjork 1989) or Fukuisaurus tetoriensis ( Kobayashi & Azuma 2003) and only a short diastema in Iguanodon bernissartensis ( Norman 1998) . The edentulous rim almost immediately begins to descend ventrally leaving only a small horizontal element, before forming a moderately long convex arc that broadens transversely to form an articular surface for the predentary.
On the medial wall of the dentary below the tooth row, an anteroposteriorly orientated line of special (replacement) foramina is present, forming a ventrally convex arcade, although the dorsal margins of the foramina and the alveolar parapet are largely incomplete in both dentaries. Those foramina that remain intact are elliptical in outline with their long axes extending anteroposteriorly. The Meckelian canal is exposed ventrally and becomes broader posteriorly, eventually merging with the adductor fossa. The adductor fossa is undistorted and narrower than in Mantellisaurus atherfieldensis (NHMUK PV R5764). The Meckelian canal is visible on the right dentary as little more than a groove in the anterior third, which ends just anterior to the first alveolar position. Anterior to this is a flat surface that extends for a few millimetres separating the Meckelian canal from the groove for the predentary. The alveolar parapet and all the teeth are missing from the left dentary, although the alveolar sockets are perfectly preserved. The first four alveolar sockets are round and small. There is a replacement tooth socket placed medial to the alveolus for the functional dentary crown.
The medial surface of the coronoid process is divided into anterodorsal and posteroventral areas by a low ridge, which extends diagonally across the surface in an anteroventral direction to meet the most posterior alveolar socket at the midpoint of the base of the coronoid process. The posteroventral area is heavily striated with the striae lying parallel to the ridge. This contrasts with Mantellisaurus atherfieldensis (NHMUK PV R5764), where there is a much more pronounced ridge but weaker striae. There is no buccal platform in Brighstoneus simmondsi separating the base of the coronoid process from the tooth row and the posterior alveoli excavate the base of the coronoid process. The absence of a buccal platform is plesiomorphic in iguanodontians and is common in hadrosauriforms prior to the Aptian, for example Mantellisaurus atherfieldensis (NHMUK PV R5764), Iguanodon bernissartensis ( Norman 1980) , Hypselospinus fittoni ( Norman 2015) , Hippodraco scutodens ( McDonald et al. 2010a) and Lanzhousaurus magnidens ( You et al. 2005) , with the possible exception of Tenontosaurus tilletti ( Thomas 2015). By the Albian its presence in iguanodontians becomes almost universal.
Maxillary dentition. The left maxilla preserves three crowns, one broken, one emergent and one in position six that is nearly complete and leaf-shaped with no wear facet ( Fig. 11B, H View Figure 11 ). The lingual side of this crown is not as heavily coated with enamel as the labial surface. The labial surface of the crown has a strong and straight primary ridge, which extends from the base of the crown to its apex and is set distally, dividing the crown at its widest point into a distal third and a mesial twothirds. Aprimary ridge, set distally to this degree, is also seen in Bayannurosaurus perfectus ( Xu et al. 2018) and Altirhinus kurzanovi ( Norman 1998). As the apex is set so far distally the crown is asymmetrical with a more gently convex curve mesially, which bears more marginal denticles (14 mesially, although the base is damaged, and the total count was probably nearer 20, vs nine on the distal margin). The marginal denticles are wedge-shaped and distally larger, more robust and obliquely positioned, whereas mesially they are set more vertically. The apical margins are mammillated, with up to three small, spherical mammillae arranged labiolingually on the marginal denticles approaching the apical region of the crown, reducing to one or two at the apex. An emergent crown on the left maxilla has three or four mammillae, reducing to two at the apex, implying that the numbers are variable within the dentition. The mammillae are not as pronounced as on the dentary crowns (see below).
Accessory ridges are present on the labial surface, situated mesial to the primary ridge. In the complete crown there are six accessory ridges: the distal four extend basally but the mesial two terminate very rapidly. Five accessory ridges are visible on the emergent crown in the left maxilla. Other crowns are more damaged but appear consistent with this morphology.
The complete crown on the left maxilla is also visible lingually. There is a primary ridge on this surface, which is straight, extends up to the alveolar margin and is distally offset, mirroring the position of the primary ridge on the labial surface. This lingual ridge is much less prominent but appears to be a distinct feature rather than a change in slope between the lingual surface and the facet for the adjacent replacement crown. Under low power magnification some faint accessory ridges are visible, all located mesial to the primary ridge ( Fig. 11H View Figure 11 ). Aprimary ridge on the lingual side of a maxillary crown has only been described in Koshisaurus katsuyama ( Shibata & Azuma 2015) , a contemporaneous basal hadrosauriform from the Kitidani Formation in Japan (Barremian–early Aptian), where the character was considered autapomorphic. No accessory ridges were observed on the lingual surface of the maxillary crowns of Koshisaurus katsuyama and to our knowledge are not reported in other iguanodontians. Among noniguanodontians, in the basal ornithopod Hypsilophodon foxii , numerous faint longitudinal ridges are seen on the lingual surfaces of the maxillary teeth ( Galton 1974). However, an isolated tooth referred to Hypsilophodon foxii shows stronger ridges on the lingual and labial maxillary tooth surfaces, although this feature is not described in the text ( Galton 2009, fig. 3N, O). Within Iguanodontia , we regard the presence of primary and accessory ridges on the lingual surface of the maxillary crown as an autapomorphy of Brighstoneus simmondsi . This character is only properly observable on the one maxillary crown of Brighstoneus simmondsi , although a lingually placed primary ridge also appears to be present in some of the cross-sections of broken crowns ( Fig. 11C, E, F, G View Figure 11 ), being perhaps more prominent anteriorly. Aprimary ridge is also seen on the lingual surface of a maxillary crown from another partial skeleton from the Wessex Formation of the Isle of Wight (IWCMS 2001.445), which is possibly referable to Brighstoneus simmondsi , but that is currently unprepared.
Dentary dentition. Several functional and emerging dentary crowns are present in the right dentary, of which three are sufficiently intact to allow a description. They are heavily enamelled and ornamented on their lingual surfaces. All are virtually complete, lack an occlusal wear facet and possess a marked primary ridge that is placed slightly distal to the midline, marking the apex of the tooth, thus making the crown slightly asymmetrical ( Fig. 12 View Figure 12 ). Adistally placed primary apical ridge occurs in non-hadrosaurid hadrosauriforms such as Iguanodon bernissartensis ( Norman 1980) , Equijubus normani ( You et al. 2003) and Telmatosaurus transsylvanicus ( Weishampel et al. 1993) but it is centrally positioned in most hadrosaurids such as Edmontosaurus annectens ( Prieto-Marquez & Norell 2010) and Lambeosaurus lambei ( Prieto-Marquez & Norell 2010) and may be either central or mesially positioned in earlier diverging ornithopods such as in Hypsilophodon foxii ( Galton 1974) , Dryosaurus altus ( Galton 1983) and Tenontosaurus tilletti ( Thomas 2015). In one crown with a slightly worn surface a distinct secondary ridge is present, which extends to the occlusal surface and lies mesial to the primary ridge, separated from it by a shallowly mesiodistally concave groove ( Fig. 12C View Figure 12 ). Ashort accessory ridge is also present mesially. In the other two crowns, secondary and accessory ridges are not clearly defined. The crowns are mesiodistally expanded with denticulate margins, the denticles being wedgeshaped and largest in the mid-height region. The marginal denticles are approximately the same size on both surfaces but mesially they become more vertically orientated towards the apex. The occlusal surfaces of the denticles generally support three or more small spherical mammillae arranged labiolingually.
Dorsal vertebrae. Eight dorsal vertebrae are preserved ( Fig. 13 View Figure 13 ) that have been labelled A–H based on their estimated sequence in the axial skeleton, which has been established largely on centrum shape, the orientation of transverse processes and the positions of the parapophyses. Vertebrae Aand Brepresent anterior dorsals; C, Dand Eprobably represent middle dorsals, possibly dorsals 8, 9 and 10; Fis an early posterior dorsal; and Gand Hare placed posteriorly due to the relative height of the centra, although both are distorted by compaction. The neurocentral sutures of the dorsal series are fused but remain clearly visible, at least where the surface is better preserved in vertebrae C–E.
Dorsal vertebra A. This vertebra is opisthocoelous, the centrum having a shallowly concave posterior articular surface and a moderately convex anterior articular surface ( Fig. 14A–C View Figure 14 ). Although these features are often found in iguanodontian cervicals, other characteristics of dorsal A are more consistent with a transitional dorsal vertebra than a posterior cervical. The relatively large parapophysis is shallowly concave and slightly ovoid with its long axis orientated posterodorsally, which suggests a relatively substantial rib attachment. The neurocentral suture is not visible but the parapophysis appears to be situated entirely on the neural arch. The centrum articular surfaces are broader than high (ratio of maximum width/height of anterior articular surface ¼ 1.32) whereas in those iguanodontians where this feature is preserved and visible in the first dorsal vertebra, the centra are usually more laterally compressed as, for example, in Equijubus normani (0.82: McDonald et al. 2014), Magnapaulia laticaudus (0.98: Prieto-Marquez et al. 2012), Ouranosaurus nigeriensis (1.04: Bertozzo et al. 2017), Zhanghenglong yangchengensis (1.16: Xing et al. 2014), Uteodon aphanoecetes (1.19: Carpenter & Wilson 2008) and Eolambia caroljonesa (1.24: McDonald et al. 2012b). In lateral view, the centrum of vertebra Ais only slightly anteroposteriorly longer than dorsoventrally high (length/height ratio ¼ 1.04), although this may have been exaggerated by crushing. Asimilar ratio is seen in the first dorsal vertebra of Eolambia caroljonesa (1.06: McDonald et al. 2012b), while Iguanodon bernissartensis is taller than long (0.96: Norman 1980) and others are considerably longer than high, for example Probactrosaurus gobiensis (1.24: Norman 2002), Ouranosaurus nigeriensis (1.29: Bertozzo et al. 2017), Mantellisaurus atherfieldensis (1.32: Norman 1986), Equijubus normani (1.46: McDonald et al. 2014) and Uteodon aphanoecetes (1.48: Carpenter & Wilson 2008). In ventral view, the lateral walls of the centrum are concave and despite some damage a definite keel is present on the ventral surface. Dorsal to the parapophyses the transverse processes extend slightly posterodorsolaterally although distally the diapophyses are missing. Seated on the dorsal surfaces of the transverse processes, close to their origin, the prezyapophyses have large, flat dorsomedially facing facets that are ovoid in shape, with their long axes extending anteroposteriorly. The postzygapophyses are missing. The neural spine curves slightly posterodorsally into a hook-like shape, but has lost its apex. This morphology is commonly seen in the first transitional dorsal vertebrae of hadrosauriforms, although the spine of Brighstoneus simmondsi is considerably taller than most of this clade and exceeds the height of its centrum. The ratio of neural spine height above the postzygapophyseal facet to height of centrum for Brighstoneus simmondsi is 1.14, compared to Iguanodon bernissartensis (0.76: Norman 1980), Mantellisaurus atherfieldensis (0.55: Norman 1986), Probactrosaurus gobiensis (0.79: Norman 2002) and Zhanghenglong yangchengensis (0.66: Xing et al. 2014). An exception is Ouranosaurus nigeriensis ( Bertozzo et al. 2017), which is known for its exceptionally tall dorsal neural spines and has a ratio of 1.57.
Dorsal vertebra B. The anterior articular surface of the centrum is essentially flat but has a very slight convexity in lateral view although this is probably due to distortion ( Fig. 14D–F View Figure 14 ). The posterior articular surface is slightly concave. The centrum has been crushed anteroposteriorly on the left side and its ventral surface has been distorted although it appears that a keel was present. The lateral surfaces of the centrum are concave anteroposteriorly, convex dorsoventrally and continuous dorsally with the neural arch. The parapophysis is more circular, more dorsally placed on the neural arch and larger than in dorsal A. The transverse processes have eroded lateral ends and are angled dorsolaterally. They are broadly triangular in cross-section with an angular ventral surface and a slightly anteroposteriorly convex dorsal surface. The neural spine curves posterodorsally and is relatively short although still taller and more expanded anteroposteriorly than in dorsal A. These characters indicate that this vertebra was close to the transition between the cervical and dorsal vertebrae, possibly the second or third dorsal. The neural canal is narrow and dorsoventrally compressed. The articular facets of the prezygapophyses face dorsomedially. The neural spine appears to be intact, but its dorsal end is obscured by matrix. It is inclined slightly posterodorsally with a gently convex anterior margin in lateral view. Just posterior to the anterior margin a faint groove lies parallel to it. The neural spine to centrum height ratio is 1.7. The postzygapophyseal facets face ventrolaterally and extend considerably beyond the posterior articular surface of the centrum. Afragment of extraneous bone has become attached to the posterior neural arch, obscuring detail. Dorsals Aand Bappear to represent transitional dorsals. Dorsal Ahas a relatively tall neural arch and both have relatively tall neural spines, making them closer in appearance to transitional dorsals of Iguanodon bernissartensis ( Norman 1980) and Ouranosaurus nigeriensis ( Bertozzo et al. 2017) than to those of Mantellisaurus atherfieldensis ( Norman 1986) .
Dorsal vertebrae C–F. Vertebrae C–F ( Figs 13 View Figure 13 , 15 View Figure 15 ) appear to be middle dorsals. The centra are amphiplatyan although the anterior and posterior articular surfaces exhibit minimal concavity, which is slightly more pronounced posteriorly. The articular surfaces of the centra are all a little taller than wide and the posterior surfaces are all slightly larger than the anterior surfaces. Dorsal F has evidence of a notochordal boss in the centre of each articular surface, which is more pronounced anteriorly. Among ornithopods this has also been seen in some posterior dorsal vertebrae of Iguanodon bernissartensis ( Norman 1980) and Zalmoxes robustus ( Weishampel et al. 2003). The centra in lateral view are mostly taller than long (length/height ratios: C ¼ 0.8; D ¼ 1.1; E ¼ 0.9; F ¼ 0.9). Dorsal Chas a midline ventral prominence anteriorly, but taphonomic damage makes it unclear if this extended posteriorly as a keel. Dorsals D, Eand Fhave faint ventral keels. The parapophyses migrate up the neural arch onto the transverse processes along the series. The neural spines of dorsals C–H have all been affected by possible bioerosion at the apex of the spine. Dorsals Cand Ehave suffered pre-mortem fracture damage, with Cshowing a displaced fracture, presumably splinted in place by a partially intact ossified tendon meshwork ( Fig. 15A, B, C, E View Figure 15 ), and vertebra Eshowing callus formation undergoing remodelling ( Fig. 15F View Figure 15 ); therefore, the true height of the spines is unknown. However, measurements on the available material show that the neural spines were at least three times as long as the centrum height and possibly much longer (neural spine to centrum height ratios: C ¼ 2.8; D ¼ 3.4; E ¼ 3.0; F ¼ 3.3). In most iguanodontians the ratio of neural spine to centrum height is <3.0 and in earlier diverging taxa, <2.0. Taxa where the ratio exceeds three include Hypselospinus fittoni (3.7: Norman 2010), RBINS R57 (3.1: Norman 1986), Ouranosaurus nigeriensis (5.1: Bertozzo et al. 2017), Bactrosaurus johnsoni (3.7: Gilmore 1933) and Morelladon beltrani (4.0: Gasulla et al. 2015).
Dorsal vertebrae Gand H. The articular surfaces of the centrum are damaged in dorsal G, but it appears to be amphiplatyan ( Fig. 13 View Figure 13 ). Ventrally the centrum is smoothly convex and does not have a keel. Dorsal His badly crushed ventrally with the suggestion of a ventral keel. Unplaceable neural spine fragments are also preserved, two showing notable features. One dorsal fragment appears to show callus formation and a displaced fracture ( Fig. 15G View Figure 15 ), which is similar to that on dorsal C, while a second appears to have theropod (or possibly crocodilian) bite marks on its surface ( Fig. 15H View Figure 15 ).
Dorsal ribs. Fourteen partial and complete dorsal ribs are preserved, nine from the left side and five from the right. As far as can be ascertained, there appears to be little difference between those of Brighstoneus simmondsi and those of Iguanodon bernissartenesis ( Norman 1980) and Mantellisaurus atherfieldensis ( Norman 1986) . It is not possible to accurately place the ribs in specific positions but there are examples from the anterior, middle and posterior members of the dorsal series ( Fig. 16 View Figure 16 ) and general descriptions are given of these.
The capitulum or head of the rib bears a medially facing articular facet, which is supported by a medially directed process. The dorsal surface of this process is either flat or forms a gentle ridge and is quite rugose. Laterally, the tuberculum is circular or ovoid, with a posteromedially facing articulation on a slightly raised pedestal. From here the shaft of the rib extends ventrally, and in the anterior and middle part of the dorsal series forms an angle of ~ 90 ° with the capitulum. Immediately below the tuberculum the cross-section of the rib shaft is sub-triangular with anteromedially, posteromedially and laterally facing surfaces. In anterior view a rounded ridge extends ventrally from the base of the tuberculum and crosses the shaft diagonally to the anterior leading margin of the blade of the rib, below which the cross-section is ovoid with the long axis directed anteroposteriorly. Posteriorly, below the tuberculum a shallow costal groove develops laterally that extends ventrally and is bounded at the lateral margin by a relatively thin ridge. The ridge and groove disappear in the middle third of the rib shaft, although the groove reappears in the ventral third in one of the complete specimens. This groove was presumably for a neurovascular bundle, with the ridge perhaps providing attachment for intercostal musculature and additional protection in the proximal region. The anterior dorsal ribs appear to terminate in a point, but the middle dorsal series increase in length and have a blunt spatulate end, which suggests that they might have articulated with the sternal cartilaginous ribs ( Norman 1980).
The posterior dorsal ribs become gradually shorter and the angle between the shaft and the capitulum-bearing process becomes more oblique. The tuberculum, which is ovoid or circular in the anterior and middle series, is smaller and becomes more elongate posteriorly. The tuberculum and capitulum also move closer to each other although a small gap remains between them ( Fig. 16D View Figure 16 ) for the passage of a vertebral artery ( Romer 1956). In one posterior specimen ( Fig. 16D View Figure 16 ) it appears that the transverse process and tuberculum have started to fuse.
Sacral vertebrae. The sacrum of Brighstoneus simmondsi ( Fig. 17 View Figure 17 ) consists of six co-ossified vertebrae although the anterior part of the first vertebra is missing. It is likely that it originally had a sacral count of seven co-ossified vertebrae, as the possession of a sacrodorsal (SD) vertebra is present in all non-hadrosaurid hadrosauriform taxa with intact sacra. Asacral count of 6 þ SD is seen in Barilium dawsoni ( Norman 2011), Mantellisaurus atherfieldensis ( Norman 1986) , Gobihadros mongoliensis ( Tsogtbaatar et al. 2019) , Bactrosaurus johnsoni ( Godefroit et al. 1998) and Ouranosaurus nigeriensis ( Bertozzo et al. 2017). A count of 5 þ SD is present in Morelladon beltrani ( Gasulla et al. 2015), Equijubus normani ( You et al. 2003) and probably Cedroestes crichtoni ( Gilpin et al. 2007), although it is difficult to determine in disarticulated material, such as that of Morelladon beltrani and Equijubus normani, if a sixth true sacral vertebra failed to fuse until late in ontogeny and did not preserve. S1 is distorted, eroded on its right lateral surface and is obscured by a sacral rib on the left. The posterior half flares out to meet a similar anterior expansion on S2. Due to distortion, it is difficult to interpret the incomplete ventral surface of S1 but a keel was probably present. When viewed ventrally S2 expands anteriorly to meet S1 and is firmly fused to S3 posteriorly, which in turn is fused to S4, so that S2–4 form a rod of broadly uniform width. The ventral surfaces of S2–4 are flat with no central keel and are bounded laterally on each side by a shallow rounded ridge, although this does not form a true sulcus. S5 is expanded a little posteriorly. Its ventral surface has been transversely crushed but probably lacked a keel. S6 is much more robust and flares out considerably posteriorly, where the margin of the centrum is heavily striated. Its posterior articular surface is ovoid with the long axis orientated dorsoventrally ( Fig. 17B View Figure 17 ). The vertebra has been anteroposteriorly crushed, which has affected the posterior articular surface. The ventral surface is smooth and transversely convex without a keel. Morelladon beltrani ( Gasulla et al. 2015) has keeled ventral surfaces on S1–4 (anterior half only in S2–4) and a flat S5; Mantellisaurus atherfieldensis , based on referred material, is described by Norman (1986) as having keeled surfaces on S1–3 and a flat ventral surface or sulcus on S4–5. Hooley’ s (1925) description of the holotype differs, however, giving the ventral surface of S1 as being transversely flat. S2 is not described, presumably due to damage, S3 and S4 have a wide sulcus ending at S5, and there is a slight keel on S6. Iguanodon bernissartensis (RBINS R55) has a ventral keel on S1–3, a sulcus on S4–6 and flat surface on S7 ( Norman 1980). The siting of ventral keels and sulci shows intraspecific variation in some taxa, for example in Bactrosaurus johnsoni ( Godefroit et al. 1998), and caution is required interpreting their phylogenetic significance. The sacral neural spines are robust and tall, being approximately 360 mm long and directed posterodorsally at an angle of approximately 45 ° in lateral view, presumably due to taphonomic distortion. The neural spines have been eroded dorsally where an anteroposterior groove has formed, therefore the heights are a minimum value. The anterior surface is wider than the posterior surface and grooved so that the sacral neural spines would have locked against each other. In lateral view, as the neural spines extend dorsally, they also expand slightly anteroposteriorly and become gently curved, concave anteriorly. Fragments of the dorsal section of three neural spines were recovered, which also have grooves anteriorly and probably represent the missing three anterior sacral neural spines. In the reconstruction ( Fig. 17A View Figure 17 ) it appears that the anteroposterior width of the spines in lateral view diminishes posteriorly. Sacral ribs are present but damaged and distorted. They appear to be firmly fused and form a sacricostal yoke for attachment to the ilium.
Caudal vertebrae. Six caudal vertebrae are preserved ( Fig. 18 View Figure 18 ), four from the proximal series (possessing caudal ribs) and two from the middle series (possessing chevron facets but no caudal ribs). They have been labelled A–F in presumed anterior to posterior order although this does not imply an articulation sequence.
Proximal caudal series. Caudal A ( Fig. 18A, B View Figure 18 ) has a centrum with a very rounded anterior articular surface with a slight bulge on the dorsal half (also noted by Norman  in the first caudal of RBINS R57). The anterior articular surface is slightly wider than tall and is essentially flat but minimally concave with an everted margin. The posterior articular surface also has an everted margin but is more heart-shaped and a little more concave than the anterior articular surface. There is a boss situated dorsal to the centre of the posterior surface that might represent a notochordal remnant. The lateral walls of the centrum are deeply striated close to its anterior and posterior margins, especially anteriorly. There is no keel or chevron facet ventrally, the latter indicating that caudal Ais probably the first caudal. The bases of the caudal ribs are present but badly damaged and the zygopophyses are angled steeply dorsally, with the facets of the prezygapophyses diverging approximately 20 ° from the sagittal plane. The neural spine is tall and angled slightly posterodorsally. Ashallow groove extends dorsoventrally just posterior to the anterior margin in the ventral half of the spine. Near the dorsal extremity the spine expands transversely, while the dorsal surface itself has been eroded to expose a cavity, which is interpreted as pathological. The loss of the dorsal surface means the true length of the spine is unknown, but it is consistent with the sacral neural spine height and probably almost complete. The ratio of the incomplete neural spine height to centrum height is 3.9, which is greater than the ratio seen in the first caudal of Ouranosaurus nigeriensis (3.4: Bertozzo et al. 2017) and an anterior caudal in Hypselospinus fittoni (3.2: Norman 2015), both of which have long dorsal neural spines. No caudals are preserved in the ‘sailbacked’ iguanodontian Morelladon beltrani ( Gasulla et al. 2015). By contrast, the first caudal in Iguanodon bernissartensis is 1.8 ( Norman 1980). The Mantellisaurus atherfieldensis holotype (NHMUK PV R5764) has no surviving caudal neural spines and RBINS R57 has been heavily restored, although the ratio in the latter, based on Norman (1986, fig. 39), is approximately 2.6. The lack of keel in the first caudal is shared with Barilium dawsoni ( Norman 2011), Iguanodon bernissartensis ( Norman 1980) and Mantellisaurus atherfieldensis ( Norman 1986) , but differs from Ouranosaurus nigeriensis, which has a keel on the first and second caudals, and a faint keel on the third ( Bertozzo et al. 2017). Caudal B ( Fig. 18C, D View Figure 18 ) is slightly more platycoelous than caudal A. It is otherwise similar except that it has a posterior chevron facet but lacks an anterior facet, suggesting a second caudal position. As with caudal Athere is no ventral keel. The articular surfaces of the centrum are of almost equal height and maximum width, and both are heart-shaped. The neural spine is also eroded dorsally and expanded transversely, with the latter again interpreted as pathological. The incomplete neural spine to centrum height ratio is 3.6. The caudal ribs, which are incomplete, extend laterally and are almost horizontal. In anterior view they describe a gentle curve that is concave ventrally. The posterior centrum and caudal ribs of caudal C ( Fig. 18E, F View Figure 18 ) are crushed. It is essentially amphiplatyan but both articular surfaces are minimally concave. The articular surfaces are both heart-shaped but are slightly taller than wide. Ventrally it has anterior and posterior chevron facets, but no definite ventral keel, although crushing damage makes this uncertain. The neural spine is shorter than in caudals Aand B (neural spine to centrum height ratio 3.4) and also angled more posterodorsally. As with caudals Aand B the apex of the neural spine has been eroded into a shallow anteroposterior groove, therefore the true length is unknown, but the presumed pathological transverse expansion in caudals Aand Bis not apparent. The rate at which the height of the neural spines decreases in more posterior anterior caudal vertebrae varies between taxa. In Ouranosaurus nigeriensis the height of the neural spines decreases rapidly after the third caudal ( Bertozzo et al. 2017), but they reduce more gradually in RBINS R57 ( Norman 1986) and Iguanodon bernissartensis ( Norman 1980) . Caudal D ( Fig. 18G–K View Figure 18 ) has similar morphology to caudal C but lacks its neural spine. The size of the centrum is a little smaller than in caudals A–C so it was presumably more posteriorly positioned. Ventrally the posterior chevron facet forms a ‘B’-shaped outline, the straight margin being posterior, while the anterior facet is ‘D’-shaped, the straight margin being anterior. Broad ridges connect the two facets laterally leaving a shallow anteroposteriorly directed sulcus in between ( Fig. 18K View Figure 18 ).
Middle caudal series. Two vertebrae in good condition are preserved from the middle part of the caudal series. Both are of similar size, with amphiplatyan centra, although caudal E ( Fig. 18L, M View Figure 18 ) has a slight concavity posteriorly. The articular surfaces of the centrum are sub-circular, being a little taller than wide. There is a very weakly developed longitudinal ridge extending anteroposteriorly along the midline of the centrum in lateral view and some irregularity at the base of the neural arch that could indicate a vestigial caudal rib. This would mark caudal E as transitional between the anterior and middle caudal series. Anterior and posterior chevron facets are present, the anterior facet being semicircular and the posterior having a double-headed ‘B’- shaped outline. Abroad ridge extends anteroposteriorly between each of the two posterior facet heads and the anterior facet, producing ashallow midline sulcus. The neural spine is angled posteriorly at approximately 45 ° and gradually expands anteroposteriorly as it extends dorsally (in contrast to the anterior series, which have almost parallel margins). The incomplete neural spine has a spine to centrum height ratio of 1.3, which is much lower than in the anterior series. Caudal F ( Fig. 18O, P View Figure 18 ) is of similar size and shape to caudal Ebut has no trace of a caudal rib and a stronger lateral ridge on the centrum, which gives the articular surfaces a distinctive hexagonal shape, which can be observed in many iguanodontians ( Barrett & Bonsor 2021).
Both ilia are preserved ( Fig. 19 View Figure 19 ). The right has suffered considerable erosive damage with most of the ventral border including both peduncles missing. Furthermore, some of the medial surface and the posterior postacetabular process has been eroded, as has the tip of the preacetabular process. However, what is preserved of this bone appears to be free from major distortion, apart from some possible slight transverse compression. The left ilium is more complete and forms the basis for much of this description. Its preacetabular process has been crushed transversely in its most distal section and the very tip of the preacetabular process is also missing. The posterodorsal section of the postacetabular process has been lost to post-mortem fracture and erosion, and the distal end of the pubic peduncle is damaged by erosion.
The preacetabular process is transversely compressed along its length, forming a robust and uniformly deep strap-like structure. At the base of the left process, the facet for the transverse process of the first true sacral vertebra is bounded ventrally by a ridge, although its prominence may have been enhanced somewhat by crushing. Proximally the cross-section of the preacetabular process has the outline of a dorsoventrally expanded capsule, which is shallowly concave on its lateral surface ( Fig. 19B View Figure 19 ). This contrasts with Iguanodon bernissartensis ( Norman 1980) and Mantellisaurus atherfieldensis (NHMUK PV R5764), where the proximal process is sub-triangular in cross-section with an almost horizontal ventral shelf. Norman (1986) has also commented on this character difference in two Barremian-aged ilia, which he referred to Mantellisaurus atherfieldensis , noting a well-developed shelf in NHMUK PV R5347 from the Wessex Formation of the Isle of Wight, but a weak one in NHMUK PV R6462 from Weald Clay Formation of Surrey. The preacetabular process of Mantellisaurus atherfieldensis (NHMUK PV R5764) is also less straplike in lateral view, being slightly waisted and more gracile in its midsection. In dorsal view, the thickness of the dorsal surface of the process tapers very slightly as it extends distally. The dorsal surface is mediolaterally rounded but unequally, so that the dorsolateral surface is flatter and forms a bevelled surface. Also, in dorsal view the outline of the left process is gently sinusoidal, being convex laterally in its proximal half and concave laterally in its distal section as it curves slightly outwards. On the right side the process curves gently laterally but the proximal convexity is much less evident. There is no evidence of axial twisting in either of the preacetabular processes. Axial twisting, when present, typically causes the lateral surface to turn dorsolaterally as it extends distally and is seen in Mantellisaurus atherfieldensis (NHMUK PV R5764), Barilium dawsoni ( Norman 2011), Iguanodon bernissartensis ( Norman 1980) , Ouranosaurus nigeriensis ( Bertozzo et al. 2017) and Bactrosaurus johnsoni ( Godefroit et al. 1998), but is absent in some iguanodontians, for example Camptosaurus dispar ( McDonald 2011), Hypselospinus fittoni ( Norman 2015) and Magnapaulia laticaudus ( Prieto-Marquez et al. 2012). In lateral view, the preacetabular process is moderately decurved, the dorsal margin forming a gentle convex arc extending anteroventrally, and the long axis of the process forming an angle of 20 ° with the dorsal margin of the iliac central plate. This value is typical of many basal iguanodontians such as Uteodon aphanoecetes (19 °: Carpenter & Wilson 2008), Osmakasaurus depressus (19 °: McDonald 2011), Mantellisaurus atherfieldenis (21 °: NHMUK PV R5764) and Ouranosaurus nigeriensis (17 °: Taquet 1976). Hadrosaurids tend to have greater angles, as in Edmontosaurus annectens (48 °) and Magnapaulia laticaudus (40 °: Prieto-Marquez et al. 2012); however, variation in this character can differ significantly between individuals of the same taxon, for example in Gilmoreosaurus mongoliensis (holotype, AMNH 6551, 23 ° [ McDonald et al. 2010c], lectotype AMNH FARB 30735, 39 °, referred ilium AMNH FARB 30736, 40 ° [ Prieto-Marquez & Norell 2010]) and Iguanodon bernissartensis (holotype RBINS R51, 51 ° [ Norman 1980], referred ilium RBINS R352, 2 °) (for a full discussion see Verdu et al. 2017a). The ventral margin of the preacetabular process is concave in lateral view but forms an obtuse angle at the anterior end, which continues horizontally as the base of a boot-like structure, although this is incomplete anteriorly. Abootshaped distal preacetabular process is commonly seen in many iguanodontians but is most marked in Iguanacolossus fortis ( McDonald et al. 2010a), Eolambia caroljonesa ( McDonald et al. 2012b), Ouranosaurus nigeriensis ( Bertozzo et al. 2017), Proa valdearinnoensis ( McDonald et al. 2012a) and Mantellisaurus atherfieldensis (NHMUK PV R5764). Some have a more tabulate ending such as Tenontosaurus tilletti ( Forster 1990), Camptosaurus dispar ( McDonald 2011), Bactrosaurus johnsoni ( Godefroit et al. 1998) and Edmontosaurus annectens ( Brett-Surman & Wagner 2007) . On the medial surface of the preacetabular process, a prominent ridge ( Fig. 19A, B, D View Figure 19 ) is continuous with the ridge bordering the ventral margin of the first sacral facet. It extends distally along the medial wall of the preacetabular process, dividing it into a larger dorsal section which faces medially and a smaller ventral section that faces medioventrally, forming an angle of approximately 20 ° with the vertical dorsal section in anterior view. The ventral section corresponds to the horizontal ventral shelf in Mantellisaurus atherfieldensis (NHMUK PV R5764). The ridge gradually decreases in prominence so that anteriorly the preacetabular process is represented by a vertical thin structure with flat medial and lateral walls. On the proximal half above the prominent ridge of the better-preserved right preacetabular process the dorsal section is further divided by a fainter secondary ridge ( Fig. 19A View Figure 19 ) that lies parallel to the prominent ridge and is separated from it by a broad but shallow groove, perhaps acting as a vascular channel. The position of the main ridge is variable between taxa, forming the ventral angle to the ventral shelf in Mantellisaurus atherfieldensis (NHMUK PV R5764), positioned onethird of the way up in Brighstoneus simmondsi (MIWG 6344) and close to the dorsal margin in Morelladon beltrani ( Gasulla et al. 2015).
The body of the ilium in Brighstoneus simmondsi is relatively short, with its maximum dimensions forming a vertical rectangular plate with its long axis extending dorsoventrally, with a ratio of depth to length>1.2 (ratio measured following methodology in Prieto-Marquez & Norell , character 234). This is also the case in earlier-diverging ornithopods such as Zalmoxes shqiperorum ( Godefroit et al. 2009), Tenontosaurus tilletti ( Ostrom 1970) and Hypsilophodon foxii ( Galton 1974) , but also in the more derived Jinzhousaurus yangi ( Wang & Xu 2001; Barrett et al. 2009); others have the long axis oriented anteroposteriorly, for example Iguanodon bernissartensis ( Norman 1980) , Gilmoreosaurus mongoliensis ( Prieto-Marquez & Norell 2010) and Dryosaurus altus ( Gilmore 1925), while others such as Barilium dawsoni ( Norman 2011) and Mantellisaurus atherfieldensis ( Norman 1986) are almost square. The dorsal margin of the ilium is transversely rounded, moderately rugose and expands in thickness as it extends posteriorly. The lateral wall of the ilium, including the proximal preacetabular process and the postacetabular process, is concave dorsoventrally. In lateral view the anterior margin of the iliac body is deeply concave, producing an embayment extending from the base of the preacetabular process as a thin ridge of bone, which continues as the anteromedial angle of the pubic peduncle.
The pubic peduncle has been eroded distally and its length cannot be reliably estimated. The peduncle is triangular in cross-section with medially, posterolaterally and anteriorly facing surfaces. The anterior surface is smooth and flat, while the medial surface is heavily striated with the striae orientated anteroventrally in line with the long axis of the peduncle. The medial surface is depressed to form a shallow fossa at the base of the peduncle. The posterolateral side of the peduncle forms the smooth surface of the anterior section of the acetabular component, which curves around to meet the ischiadic peduncle. The smooth acetabular surface ends where it meets the ischiadic peduncle. This peduncle is transversely expanded, being broadest posteriorly. The ventral surface is heavily rugose. Laterally the wall is smooth and forms a dorsoventrally deep rim. The ischiadic peduncle is oriented posterolaterally and as it is broadest posteriorly this creates a relatively modest lateral protrusion or step, which then curves medially to continue as the postacetabular process. However, the lateral wall of the ischiadic peduncle is essentially flat and lacks a distinct posterolateral boss. Aboss causing a distinct step between the anterior and posterior sections of the lateral wall of the ischiadic peduncle (‘posteroventral protuberance’ and ‘ischial tuberosity’ of some authors) is seen in some non-hadrosaurid hadrosauriforms such as Mantellisaurus atherfieldensis (NHMUK PV R5764), Altirhinus kurzanovi ( Norman 1998, fig. 32), Choyrodon barsboldi ( Gates et al. 2018), Hypselospinus fittoni ( Norman 2015) , Gilmoreosaurus mongoliensis (Prieto-Marquez & Norell 2010) and all hadrosaurids, although in the latter clade the articular surface is also divided by a groove into anterior and posterior segments (Prieto-Marquez & Norell 2010). Aboss is absent in Morelladon beltrani ( Gasulla et al. 2015), Proa valdearinnoenesis ( McDonald et al. 2012a), Iguanodon bernissartensis ( Norman 1980) and Ouranosaurus nigeriensis ( Taquet 1976).
The ventral margin of the ischiadic peduncle in lateral view is inclined posterodorsally compared to the dorsal margin of the iliac blade and is almost parallel to the angle of the ventral margin of the postacetabular process. This is also seen in Gilmoreosaurus mongoliensis ( McDonald et al. 2010a) , Eolambia caroljonesa ( McDonald et al 2012b), Bactrosaurus johnsoni ( Godefroit et al. 1998) and Ouranosaurus nigeriensis ( Taquet 1976), and appears to become increasingly common in more derived hadrosauriform taxa including hadrosaurids (Supplementary material, Fig. S8 View Figure 8 ). In earlier-diverging taxa, the articular surface of the ischiadic peduncle is more horizontal and closer to being parallel with the dorsal margin of the iliac blade, for example in Hypsilophodon foxii ( Galton 1974) , Zalmoxes robustus ( Weishampel et al. 2003), Rhabdodon sp. ( Chanthasit 2010), Camptosaurus dispar ( Gilmore 1909), Mantellisaurus atherfieldensis (NHMUK PV R5764) and Iguanodon bernissartensis ( Norman 1980) .
In the lateral view Brighstoneus simmondsi has virtually no notch or embayment between the posterior ischiadic peduncle and the postacetabular process. The ventral surface of the postacetabular process is incomplete posteriorly, but what remains suggests there was only a modestly developed brevis shelf. The postacetabular process is angled medially to the main body of the ilium ( Fig. 19F View Figure 19 ) and whereas the lateral wall is flat above the acetabulum it is dorsoventrally concave in the postacetabular process. In lateral view, a very gently everted dorsal rim begins at the posterior end of the preacetabular process. This continues posteriorly, eventually expanding dorsoventrally and slightly transversely at the level of the junction between the ischiadic peduncle and the postacetabular process to form a lateral (supraacetabular) process, which slightly overhangs the lateral wall in similar fashion to that seen in many other iguanodontians such as Iguanodon bernissartensis ( Norman 1980) , Ouranosaurus nigeriensis ( Taquet 1976) and Mantellisaurus atherfieldensis (NHMUK PV R5764) but it is not as everted as it is in Altirhinus kurzanovi or hadrosaurids ( Norman 1998). In lateral view the dorsal margin is almost horizontal anteriorly, but slightly convex over the acetabulum and slightly concave posteriorly.
Abroad ridge is seen when the ilium is viewed medially, extending anteroposteriorly across the surface of the iliac blade. Above this ridge the surface of the ilium is slightly concave dorsoventrally and angled so that it faces dorsomedially and overhangs the lateral wall. Its surface is weakly striated with the striations orientated dorsoventrally in the midsection and posterodorsally in the posterior section. There are four crescentic facets ventral to this ridge that are buttressed dorsally by the ridge. These become shallower but much broader posteriorly and would have articulated with the transverse processes of the first four true sacral vertebrae. Posterior to these facets the ridge is heavily rugose. Ventral to the facets the surface of the bone is smooth except for the area just above the ischiadic peduncle, which is striated. The area ventral to the ridge is also heavily striated where it extends across the postacetabular process.
Only the dorsal half of the right ischium is preserved ( Fig. 20 View Figure 20 ). As with other parts of this individual there are deep erosions into the articular surfaces, which affect the pubic and iliac peduncles and the obturator process. The shaft of the proximal ischium has a triangular cross-section, with lateral, anteromedial and posteromedial surfaces. In lateral view, the surface of the distal section of the surviving shaft is almost flat. However, as it extends dorsally a wide, shallow central groove develops bounded by two rounded ridges. As the shaft reaches the head the groove on the lateral surface flattens out and expands into a shallowly concave area that forms the ventral margin of the acetabulum. Astout posterodorsally directed iliac peduncle is present posteriorly. This is transversely compressed and probably formed an elliptical articular surface, although an unknown amount of bone has been lost to erosion. There are no deep striae present for ligamentous attachment, indicating that the process was originally much longer. Anteriorly the pubic peduncle is well developed: this is also transversely compressed especially ventrally, and in lateral view it flares out distally. Again, the distal articular surface has been lost and there is little evidence of rugosity. Ventrally the pubic peduncle tapers to a narrow blade, which curves concavely to form an anterior embayment before expanding anteriorly to form the obturator process. This process curves laterally to produce a concavity that probably would have supported the postpubic rod ( Norman 1986). The ventral margin continues distally as a keel, which curves diagonally from anterior to posterior before merging with the shaft. The ischium of Brighstoneus simmondsi is too incomplete to draw many useful comparisons. The Mantellisaurus atherfieldensis holotype includes only fragments of the ischium ( Hooley 1925) but that of Brighstoneus simmondsi differs from Iguanodon bernissartensis ( Norman 1980) in having a relatively more robust shaft and a more deeply curved anterior embayment between the pubic peduncle and obturator process. In this respect it more closely resembles the ischium of Ouranosaurus nigeriensis ( Taquet 1976).
A thin bone fragment might represent the distal end of the prepubic process. It shares similarities in size and shape with the distal end of the prepubic blade of Altirhinus kurzanovi ( Norman 1998) and a small Iguanodon bernissartensis ( Norman 1980) but is not compatible with the paddle-shaped processes of Mantellisaurus atherfieldensis ( Norman 1986) or Ouranosaurus nigeriensis ( Bertozzo et al. 2017). The orientation of the fragment is unknown, but assuming the position in Fig. 21A View Figure 21 , the ventral margin is approximately 3 mm thick and cortical bone is visible along its length. The thickness of the specimen in the central region is approximately 9 mm, thinning to 2 mm dorsally. The dorsal margin is obscured by a layer of pyrite but given the thinness of the bone was probably close to the margin. The curved margin (on the left in Fig. 21A View Figure 21 ) has been eroded, so its shape cannot be defined with certainty but it is compatible with a distal iguanodontian pubic blade. The right margin is curved laterally and has a pathologic appearance perhaps due to hypertrophic callus formation from chronic non-union in a displaced fracture.
Only the right femur is preserved ( Fig. 22 View Figure 22 ). There are deeply excavated areas proximally that have affected the femoral head, greater trochanter and the dorsal surface of the anterior trochanter, while much of the distal end of the femur, including the condyles, is missing. Other than these eroded areas the bone surface is generally well preserved although is there crushing to the diaphysis posteriorly.
The femur is slender in anterior and posterior views and essentially straight except distally where it curves medially to create a concave medial margin. In medial view the anterior diaphysis describes a gently convex curve.
The head of the femur is directed medially but damage to the dorsal surface means that there is little anatomical information, although the presence of a saddle-shaped surface and an anteroposteriorly elongate greater trochanter seem likely. The anterior trochanter has some erosion dorsally but forms a mediolaterally compressed flange that is separated from the greater trochanter by a definite cleft (although this is filled with matrix). The lateral surface of the anterior trochanter has dorsoventral striations while its anterior margin is also roughened and mediolaterally wider than the posterior margin. The anterior margin continues ventrally as a rounded ridge that extends diagonally down the shaft of the femur towards the medial condyle, gradually becoming smoother and less prominent.
The fourth trochanter is transversely compressed and forms an elongate trapezium ( Fig. 22B View Figure 22 ). Although the femur is incomplete, the fourth trochanter was almost certainly situated predominantly in the dorsal half of the femoral shaft, but it also extended onto the ventral half. The ventral border is angled posterodorsally and is continuous with the concave curve of the posterior surface of the distal femur, while the posterior border for most of its length is parallel with the axis of the mid-diaphysis before curving convexly to join the shaft of the proximal femur. Asimilarly shaped fourth trochanter is also seen in NHMUK PV R1148, the holotype of ‘ Iguanodon hollingtoniensis ’ (currently regarded as a synonym of Hypselospinus fittoni: Norman 2015 ), NHMUK PV R2503, the femur of ‘ Iguanodon seelyi ’ ( Hulke 1882), in adult ( Verdu et al. 2017b) and perinate ( Verdu et al. 2015) Iguanodon galvensis, and in RBINS R347 a ‘sub-adult’ specimen of Iguanodon bernissartensis ( Verdu et al. 2017a) . NHMUK PV R2503 has also been referred to Iguanodon bernissartensis ( McDonald 2012a) and these differences probably represent intraspecific variation from the more usual triangular configuration. The shaft of the femur is shallowly excavated just medial to the base of the fourth trochanter and is heavily scarred.
The ventral articular surface of the femur has been lost but part of its mediolateral expansion is preserved. Anteriorly, the intercondylar extensor groove is narrow and extends distally becoming deeper, ‘U’-shaped in mediolateral cross-section and slightly wider transversely. Posteriorly the intercondylar flexor groove is broader and relatively shallower. Both grooves are slightly ventromedially directed.
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