The aim of this research was to uncover the following: (1) to disentangle the history of the discovery of Ouranosaurus skeletons with a particular focus on the Venice specimen; (2) to describe the latter and compare its osteology with Taquet (1976) ; (3) to perform the first osteohistological analysis on O. nigeriensis by sampling a variety of bones to determine the ontogenetic stage of the Venice specimen; (4) to establish whether the bones of the nearly complete mounted skeleton can be reliably referred to a single individual or whether it is composed of several individuals.

Since 1975, a nearly complete mounted skeleton of O. nigeriensis has been exhibited at the Museo di Storia Naturale (Natural History Museum) of Venice, Italy. Apparently, the Venice specimen is not referred to by Taquet (1976) or in any other scientific papers dealing with Ouranosaurus. Therefore, this skeleton can be considered as undescribed.

Ouranosaurus nigeriensis and Spinosaurus aegyptiacus are iconic African dinosaurs because of their common possession of hypertrophic neural spines. O. nigeriensis comes from the upper part of the El Rhaz Formation at the Gadoufaoua locality of the Sahara Desert, located 145 km east of Agadez, Niger ( Taquet, 1976 ). The El Rhaz Formation of Niger has yielded a rich dinosaur fauna including theropods ( Suchomimus, Cristatosaurus , Kryptops and Eocarcharia ), sauropods ( Nigersaurus ) and the ornithopods Ouranosaurus and Lurdusaurus ( LeLoeuff et al., 2012 ). It was considered to be Aptian by Taquet (1976) and Aptian–Albian by Sereno et al. (1999) , but LeLoeuff et al. (2012) have proposed a Barremian age.

Bone surface texture, degree of fusion of the elements and obliteration of the sutures in skulls and vertebrae are the most common approaches to assess the ontogenetic stage of fossil tetrapods (e.g., Bennett, 1993 ; Brochu, 1996 ; Werning, 2012 ). However, histological analysis remains the most reliable methodology for establishing osteological maturity or immaturity and for estimating the absolute age of an individual (e.g., Erickson et al., 2004 ; Chinsamy, 2005 ; Erickson, 2005 ). The left humerus, right femur, right tibia, neural spine of dorsal vertebra 14 and right dorsal rib 15 were selected for osteohistological analysis. Core samples were taken from the long bones following the method described by Stein & Sander (2009) , using an electric drill press Timbertech Kebo01 and a cylindrical diamond drill bit (16 mm in diameter, 80 mm in height and with a 2 mm-thick wall). Samples were taken from the diaphysis of the long bones. Only areas lacking evident superficial erosion and surface cracks were selected. The proximal shaft of the rib was cut transversely. That area was selected because it is considered to preserve the most complete growth record ( Erickson, 2005 ). The neural spine was cross-sectioned at three different levels: at the base, in the middle, and in the apical region. Samples were then mounted on glass slides, polished to a thickness of ∼70 µm and analyzed with Leica DMLP and Nikon Optiphot2-pol microscopes. The type of microstructure, the density and type of vascular canals, the amount of remodeling, the number of Lines of Arrested Growth (LAGs) and the presence or absence of an External Fundamental System (EFS) are the proxies used in this study to evaluate the ontogenetic stage of the sampled skeletal elements. The definition of the type of arrangement of the vascular canals was based on the orientation of their main axis. LAGs were identified and counted when an arrest in bone deposition was visible at different magnifications and when the interruption was continuous along the slide. When two or more LAGs were tightly spaced in the inner cortex, these were considered as annuli and counted as a single year.

Caudal vertebrae with pleurapophyses (often reported as caudal ribs or transverse processes in the literature) are considered as proximal caudals; those lacking pleurapophyses but with haemapophyses are middle caudals; distal caudals lack pleurapophyses and hemapophyses. The cervical-dorsal transition in the vertebral column was identified following Norman (1986) . The height of the centrum was measured at the caudal (posterior) articular facet, and the height of the neural spine was measured as the straight line from the mid-point of the spine in correspondence with the dorsal margin of the postzygapophysis to the apex of the spine (see Supplemental Information 1 ).

This specimen is mounted on a metal frame and in order to photograph and describe it, all of the bones were dismounted from the frame, with the exception of the sacrum, which is fixed to the frame. Pictures of every original bone in its cranial (anterior), caudal (posterior), dorsal, ventral, lateral, and medial views were taken, using a Canon EOS 600D camera with 100 ISO sensitivity and a Tamron 17–50 mm (F/2.8) lens at focal distance 50 mm. The photographs are stored in the archive of the MSNVE, which is accessible to researchers by contacting the responsible people for Research and Scientific Divulgation of the Museum. A 200 mm-long caliper with measurement error of 0.01 mm and a 100 cm-long metric string (measurement error of 0.1 cm) were used to measure the bones. A table with all the measurements is included in the Supplemental Information 1-2 . In order to identify the reconstructed parts, we took pictures under UV-light using a Wood Lamp (SKU 51029, emitting ultraviolet light at 4 W).

The focus of this paper is the Venice specimen of O. nigeriensis (MSNVE 3714; Fig. 1 ). It appears to be a nearly complete skeleton mounted in a bipedal posture. The specimen was donated to the MSNVE by the Italian entrepreneur and philanthropist Giancarlo Ligabue (founder of the Centro Studi e Ricerche Ligabue, Venice), who passed away in 2015. According to the available information, the specimen underwent two distinct restoration phases. The first preparation of the bones used in the mount was done by French preparators at the Muséum National d’Histoire Naturelle of Paris before 1975 when the skeleton was mounted in Venice. The specimen was restored, casted and remounted by an Italian private firm in 1999–2000. No reports or any kind of documentation exist about the restoration of the bones. A list of the original material does not exist. Ronan Allain, MNHN, kindly made available to us a copy of the field map of the in situ specimen, which was drawn by Philip Taquet and is stored at MNHN.

Each bone in the two sheets is identified by a number in order to identify the elements and reassemble the skeleton once in the laboratory ( Taquet, 1975 ); those numbers are not reported in Fig. 2 , but are discussed in the Supplemental Information 3 .

The presence of a total of three pubes and possibly three ilia and scapulae, as well as a duplication of segments of the caudal vertebral column, indicates that the two sheets refer to two distinct skeletons.

The second sheet is the map of a set of bones, which is not clearly identifiable in Taquet ( 1976 , pl. IX, fig. 2; also Taquet (1998) , fig. 12). The relative location with respect to sheet 1 is unknown (see Supplemental Information 3 ). The morphology of the pelvic elements indicates that the partial skeleton belongs to a relatively large ornithopod; the shape of the pubes and the tall neural spines suggest that it belongs to O. nigeriensis .

Part of sheet 1 with the writing “]—Airfield—1970—(specimen Venice Museum pro parte)” was omitted. Some original handwritten notes have been translated into English and typewritten in dark gray. The author of the map marked with zig-zag lines some elements that he supposed to have wrongly drawn; those zig-zag lines are also in dark gray. When it is not possible to establish if the original identifications are correct, the names of the bones are in dark gray; black abbreviations are our identifications of the mapped bones or confirmed original identifications. “Near ulna” and “fragment of ulna” are a handwritten notes that refer to collected elements numbered 96 and 97, which were not drawn on the map. Abbreviations: ca, calcaneum; ch, chevron; co, coracoid; dv, dorsal vertebra; fe, femur; fi, fibula; h, humerus; il, ilium; mc, metacarpal; mt, metatarsal; ph, manus phalanx; pph, pedal phalanx; pu, pubis; ra, radius; sv, sacral vertebra; sc, scapula; st, sternal plate; ti, tibia; u, ulna. When the elements are reported as left in the original map, they are in brackets. See Supplemental Information 3 for further details.

The field map sent to us by R Allain is divided into two sheets. The first sheet contains the writing “Ouranosaurus nig[eriensis]—Airfield—1970 —(specimen Venice Museum pro parte)” (in French) and the field map of the partially articulated paratype skeleton that is pictured in Taquet ( 1976 , pl. IX, fig. 2; and also Taquet (1998) , fig. 12) and was found at the margin of the landing strip built by the CEA in 1970. Furthermore, the map reports “GDF 381 today in Venice” ( Fig. 2 ). The word “pro parte” (=for part) means that not all of the mapped bones were used in the mount of the Venice specimen or that the latter contains elements from other sources. Therefore, the correspondence of the bones reported in the map and those occurring in the mounted skeleton is checked below and the implications are discussed.

Philippe Taquet (P Taquet, pers. comm., 2012) confirmed that the Venice specimen is the paratype with the missing bones being casts of the holotype. He also told us that he mapped the paratype bones in the field and that the map is still kept at the MNHN. Ronan Allain sent us a copy of that field map, which confirms that the Venice specimen contains the paratype material.

In the formal description of the new species, Ouranosaurus nigeriensis , Taquet ( 1976 , p. 58) mentions only the holotype (GDF 300), the paratype (GDF 381- MNHN) and the referred material (a large coracoid and a femur, indicated with the field acronyms GDF 301 and GDF 302, respectively). As stated above, no mention is made of the Ouranosaurus material supposedly collected by the 1973 Italian-French expedition in Taquet (1976) . Despite being reported as a practically complete skeleton missing just the skull (p. 58), only the elements of the paratype that are not preserved in the holotype were described by Taquet (1976) . A description of the whole paratype was never published. The holotype was returned to Niger after study ( Taquet, 1976 ) and it is on exhibition at the MNBH in Niamey ( Taquet, 1976 , pl. IX, fig. 1). No further reference to the paratype, as well as the referred specimens GDF 301 and GDF 302, was made in the literature. According to A. McDonald (A McDonald, pers. comm., 2011) and Currie & Padian ( 1997 , p. 369), the MNHM has only a plaster copy of the holotype.

In 1971, Giancarlo Ligabue and Cino Boccazzi knew about the Gadoufaoua locality while travelling across the Sahara desert ( Ligabue et al., 1972 ). Ligabue and the CNR financially supported the first Italian expedition (February 3rd–22nd, 1972; at the same time as the fifth French expedition), which was actually a prospecting expedition in order to establish the basis for a future expedition ( Ligabue et al., 1972 ; Boccardi & Bottazzi, 1978 ). This expedition occurred the following year (November 4th–December 11th, 1973) and was an Italian-French expedition led by Giancarlo Ligabue and Philippe Taquet ( Ligabue & Rossi-Osmida, 1975 ). A field report and a list of the excavated material was published in Ligabue & Rossi-Osmida (1975) . The list included “1 [sic] Ouranosaurus nigeriensis ” (p. 80). According to Rossi-Osmida (2005) , all of the fossils collected during the Italian expedition were brought to the MNHM where they were prepared, restored and casted. In a letter dated August 27th 1974, Giancarlo Ligabue communicated to the Municipality of Venice his desire to donate a complete skeleton of an iguanodontian dinosaur and “other fossils found during the field campaign in the Sahara desert...undertaken in the years 1972/73”. The donation was accepted by the Consiglio Comunale (town council) of Venice on December 30th, 1974 (documentation is available at the MSNVE). In 1975, that skeleton, i.e., MSNVE 3714, was mounted in a room of the MSNVE and exhibited to the public along with the other specimens (including a complete skull of the crocodyliform Sarcosuchus imperator ). Since that date, the skeleton has been on exhibition for the public in the museum.

During the fifth expedition (January 5th–February 25th, 1972), the Ouranosaurus skeleton found in 1970 (i.e., the paratype) was excavated and brought to Paris ( Taquet, 1976 , p. 15 and 60). Apparently, this is the third and last ornithopod skeleton from Gadoufaoua excavated and brought to France by French expeditions, together with the holotypes of O. nigeriensis and L. arenatus found in 1965 and collected in 1966.

During the fourth expedition (January 5th–March 23rd, 1970), a nearly complete O. nigeriensis skeleton lacking its skull, but in better state of articulation than GDF 300, was discovered 4 km south of the “niveau des Innocents” at the margin of the landing strip built by the CEA, (p. 58). This skeleton also received the field number GDF 381 (see Taquet, 1976 , pl. IX, fig. 2). So, the field number GDF 381 was erroneously used three times to indicate three different specimens found in different years. This nearly complete skeleton without skull was later indicated as the paratype of O. nigeriensis and reported by Taquet (1976) as GDF 381-MNHN on p. 58 and as GDF 381 throughout the text.

During the second expedition (February 25th–April 7th, 1966), GDF 300 (a nearly complete but disarticulated and scattered skeleton) and GDF 381 (“a skeleton two thirds complete”, p. 54) were collected. The following year, those specimens were carried to Paris for preparation and study. GDF 300 became later the holotype of Ouranosaurus nigeriensis ( Taquet, 1976 , p. 57). The other specimen (GDF 381), which was found 100 m from GDF 300 and is referred to as the “ Iguanodontidé trapu (ponderous Iguanodontid)” by Taquet ( 1976 , p. 54; see also p. 14 and 53), subsequently became the holotype of Lurdusaurus arenatus (see Taquet & Russell, 1999 ; P Taquet, pers. comm., 2012). However, the holotype skeleton of L. arenatus received the new number MNHN GDF 1700 once in Paris, while the previous field number GDF 381 remained associated with an isolated right coracoid that was referred to the same species ( Taquet & Russell, 1999 , p. 3).

Between 1965 and 1972, five French palaeontological expeditions searched for dinosaurs in the Gadoufaoua area of the Sahara desert in Niger ( Taquet, 1976 ). The first expedition took place in January–February 1965, resulting in the discovery of eight iguanodontian specimens at the site “niveau des Innocents”, located east of the Emechedoui wells. Two further iguanodontian skeletons, labelled GDF 300 and GDF 381, were found 7 km south-east of Elrhaz in the Camp des deux arbres locality.

“Phalangeal formula [of the manus] 1-3-3-3-3 or 4”—This is just a hypothetical statement because no complete manus of O. nigeriensis is preserved. Nevertheless, it is the plesiomorphic formula for ankylopollexians and cannot be considered diagnostic of O. nigeriensis.

“Phalangeal formula [of the pes] 0-3-4-5-0”—This is just a hypothesis because no complete foot of O. nigeriensis is preserved. Nevertheless, it is the formula of all hadrosauroids with a preserved foot ( Norman, 2004 ; Horner, Weishampel & Forster, 2004 ; Dalla Vecchia, 2009 ) and Dryosaurus ( Norman, 2004 ), so it would not be diagnostic of O. nigeriensis.

“Tridactyl foot”—Actually, no complete foot of O. nigeriensis is preserved, so the presence of three toes is just assumed. Nevertheless, all styracosternans that have a preserved foot have three toes, so it would not be diagnostic of O. nigeriensis.

“Ascending process of the astragalus placed posteriorly instead of anteriorly”—Actually, Taquet ( 1976 , p. 148) says that the astragalus of the holotype of O. nigeriensis has a posterior ascending process that is more developed than the anterior ascending process. This is a primitive feature occurring in Dysalotosaurus lettowvorbecki , as noticed by Taquet (1976) himself, and Eousdryosaurus nanohallucis (see Escaso et al., 2014 ). Tenontosaurus tilletti has no ascending processes at all ( Forster, 1990 ), while most iguanodontians have a well-developed anterior ascending process ( Norman, 2004 ). However, the astragalus of the Venice specimen differs from that of the holotype (see below), thus this feature cannot be considered a diagnostic feature.

“Shallow post-acetabular notch”—The notch is shallow also in Fukuisaurus tetorensis (see Carpenter & Ishida, 2010 , fig. 2.7), NHMUK R3741 and NHMUK R9296 ( Carpenter & Ishida, 2010 , fig. 2.10a and b; the first referred to M. atherfieldensis by Norman, 2015 ) and in P. gobiensis (see Norman, 2002 , fig. 27). The notch is not shallow in MSNVE 3714, suggesting a certain degree of individual variability (see below).

“Convex dorsal margin of the ilium with a hint of an anti-trochanter”—In the Venice specimen the dorsal margin is actually straight, while it is convex in the holotype. Both conditions occur in a sample of I. bernissartensis , suggesting that the curvature of the dorsal margin is intraspecifically variable ( Verdú et al., 2017 ). A “discrete bulbous boss present posterodorsal to the ischiadic peduncle” ( Norman, 2015 , character 90, p. 188) occurs also in P. valdearinnoensis, B. yixianensis, B. johnsoni and G. mongoliensis , as well as in Cedrorestes crichtoni (see McDonald et al., 2010c , fig. 18c).

“Preacetabular process accounting for half the total length of the ilium”—The preacetabular process of O. nigeriensis is 47–50% the total length of the ilium. This is also the case of Planicoxa venenica (see DiCroce & Carpenter, 2001 , fig. 13.5) and probably also of Iguanacolossus fortis (see McDonald et al., 2010c , fig.14). The preacetabular process is 47% of the total length in D. lettowvorbecki (see Galton, 1981 , fig. 11I) and E. caroljonesa (see McDonald et al., 2012a , fig. 31).

“Straight pubic rod, much shorter than ischium and with widened distal extremity”—The “pubic rod” (=posterior pubic ramus or pubis s.s.) is straight and shorter than the ischium in many styracosternans ( Norman, 2015 , character 94). However, the distal extremity is usually pointed ( Norman, 2015 ), while it is slightly expanded and bulbous in O. nigeriensis . Taquet (1976) did not notice the peculiar morphology of the obturator opening of the pubis, which is unlike that of the other styracosternans (see below).

“Very deep and very developed prepubic blade”—Compared to the prepubic portion of the pubes of I. bernissartensis and M. atherfieldensis , the pubes of O. nigeriensis have a shorter and deeper neck. However, the prepubic blade is not much deeper than that of M. atherfieldensis and it is similar in depth and development to those of Lanzousaurus magnidens (see You, Ji & Li, 2005 , fig. 3), Xuwulong yueluni (see You, Li & Liu, 2011 , fig. 2), P. gobiensis (see Norman, 2002 , fig. 28) and B. johnsoni (see Godefroit et al., 1998 , fig. 32).

“Long and straight ischium with a foot-like distal expansion”—The ischium of O. nigeriensis is not proportionally longer than that of I. bernissartensis (see Norman, 1980 ) or M. atherfieldensis (see Norman, 1986 ). The shaft in the Venice specimen is straight like that of the ischia of A . kurzanovi , Jinzhousaurus yangi and the hadrosaurid Shantungosaurus and Edmontosaurus , while it is slightly bowed in the holotype as in many other styracosternans (see Norman, 2015 , character 95). A foot-like expansion at the end of the shaft of the ischium occurs in many styracosternans as well as in rhabdodontids and Camptosaurus dispar (see Norman, 2015 , character 97).

“Extremely long neural spines of the dorsal vertebrae”—This is clearly a potentially diagnostic feature, but needs to be quantified because Hypacrosaurus altispinus , Barbsoldia sicinskii , Morelladon beltrani and GPIT 1802/1-7 (Iguanodontia indet.; Pereda-Suberbiola et al., 2011 ) also have tall neural spines on their dorsal vertebrae. The neural spines of the dorsal vertebrae of O. nigeriensis reach up to seven times the height of the centrum, while those of the other iguanodontians are shorter reaching less than five times the height of the centrum (see below). Furthermore, the neural spines of the sacral and proximal caudal vertebrae are also tall and altogether, they form a back ‘sail’, which has a sinusoidal outline unlike that of similar structures in other iguanodontians (see below). Neural spines of dorsal vertebrae flare apically in lateral view and the tallest spines have a paddle-like outline (“petal-shape” according to Pereda-Suberbiola et al., 2011 , p. 557), with a basal neck and an expansion toward the apex, while those of other iguanodontians with tall neural spines have parallel or only slightly divergent margins ( Maryanska & Osmólska, 1981 ; Pereda-Suberbiola et al., 2011 ; Gasulla et al., 2015 ).

“Relatively short tail”—This apparent shortness is due to the fact that the caudal segment of the vertebral column is incomplete in both the holotype and the paratype. Consequently it is not a diagnostic feature.

“Dentary that is deep anteriorly and low posteriorly”— E. caroljonesa (see McDonald et al., 2012a , figs. 3A–3B) and P. byrdi (see Head, 1998 , figs. 11 and 14) also have a dentary that is deeper anteriorly than posteriorly. However, O. nigeriensis is unique in having a perfectly straight ventral margin of the dentary, i.e., there is no ventral deflection of the rostral end; the anterior increase of the dentary depth (up to the beginning of the tooth row) is due to the anterior divergence of the dorsal margin (see Taquet, 1976 , figs. 29a, b and d).

“Circular orbit with the same height as the lower temporal fenestra”—This is not due to a larger size of the orbit but to a comparatively small size of the lower temporal fenestra. In nearly all other styracosternans, the lower temporal fenestra is higher than the orbit (e.g., I. bernissartensis, M. atherfieldensis , Maiasaura peeblesorum and Prosaurolophus maximus ), an exception being Parasaurolophus walkeri (see Horner, Weishampel & Forster, 2004 , fig. 20.6B). However, the extent of the lower temporal fenestra is just hypothesized in many styracosternan species because skulls are incomplete, disarticulated or deformed by compression. Thus, the validity of the relative size of the two skull openings as a diagnostic feature needs to be confirmed.

“Small antorbital fenestra”—The presence of a small antorbital fenestra is a primitive feature within the iguanodontians occurring for example in Tenontosaurus tilletti , Dysalotosaurus lettowvorbecki and Camptosaurus dispar (see Norman, 2004 ) and Hippodraco scutodens (see McDonald et al., 2010c ). According to Norman (2015) , O. nigeriensis shares with I. bernissartensis and M. atherfieldensis the antorbital fenestra perimeter, which forms a posteromedially directed canal when viewed laterally.

“Low maxilla”—The maxilla of O. nigeriensis is not lower than those of many other styracosternans (e.g., M. atherfieldensis , E. caroljonesa and P. byrdi ; see Gasulla et al., 2014 , fig. 3). The maxilla appears to be low because of the elongation of the rostral part of the skull.

“Short predentary bone, wider than long”—Many other styracosternans have predentaries that are wider than long (e.g., E. caroljonesa and all those listed in Prieto-Márquez, 2010 , http://www.morphbank.net/Show/?id=461224 , excluding Gryposaurus monumentensis ). However, O. nigeriensis is the only iguanodontian to have a predentary maximum mediolateral width/maximum rostrocaudal length along the lateral process ratio (character 22 in Prieto-Márquez (2010) ; see http://www.morphbank.net/Show/?id=461224 ) that is higher than 2 (it is 2.35 based on measurements taken on Taquet, 1976 , fig. 28).

The external narial openings of O. nigeriensis are comparatively small and placed nearly at mid-rostrum. I. bernissartensis, M. atherfieldensis, Protohadros byrdi and B. johnsoni also have comparatively small external narial openings, which are in a slightly more anterior position in the rostrum than O. nigeriensis . However, this apparent posterior displacement of the openings in O. nigeriensis is only due to its more elongated rostrum.

“External nares widely visible in dorsal view” and “orifice of convergence between the nasal ducts very back placed”— Taquet ( 1976 , see fig. 16) considers as “external nares” the circumnarial depression (sensu Prieto-Márquez & Wagner, 2014 ) and as “orifice of confluence of the nares” the narial openings (“apertura ossis nasi” of Prieto-Márquez & Wagner, 2014 , fig. 37.7). The circumnarial depressions are widely visible in dorsal view in other styracosternans where the skull can be observed in such a view, for example in M. atherfieldensis (see Norman, 1986 , fig. 4) and in hadrosaurids ( Horner, Weishampel & Forster, 2004 ). So, this is not an apomorphy of O. nigeriensis .

“Extremely long, straight and anteriorly expanded premaxillae, which separate posteriorly the nasals from the maxillae”—This is the condition observed also in M. atherfieldensis and in all hadrosauroids. The elongation of the premaxillae depends upon the elongation of the rostral part of the skull.

“Long and thin snout ending in a duck bill”—Proportionally, the snout of O. nigeriensis (the snout being the rostral part of the skull) is just slightly longer than that of M. atherfieldensis (rostrum/total skull length ratio is 0.66 and 0.60 respectively). “Duck bill” is a rather vague definition that is not adequately explained in Taquet (1976) . The rostral expansion of the snout in dorsal view expressed as W/w (W = maximum skull width in dorsal view; w = maximum width of the snout) is 1.63 in O. nigeriensis (based on Taquet, 1976 , fig. 10b) and 1.69 in M. atherfieldensis (based on Norman, 1986 , fig. 4). However, the snout of O. nigeriensis is also quite flattened dorsoventrally unlike that of M. atherfieldensis and other non-hadrosaurid styracosternans. Furthermore, the anterolateral margin of the narial fossa above the occlusal edge of the premaxilla is reflected dorsally to form a distinct rim like in some hadrosaurines ( Norman, 2002 ; Norman, 2015 ).

“Very long skull, narrow and relatively low, which maximum height occurs at the level of the nasal bulges”—This is a vague statement about elongation that should be supported by measurement ratios. The hadrosaurid E. annectens has a comparatively more elongate skull (length/height ratio is up to 3.4, while it is 3.2 in O. nigeriensis ) and the basal hadrosauroids M. atherfieldensis and Tethyshadros insularis also have elongated skulls (ratios 2.6 and 2.57, respectively), although comparatively less than O. nigeriensis (see Dalla Vecchia, 2009 ). O. nigeriensis does indeed have the most elongated skull (length/height ratio >3) among the non-hadrosaurid styracosternans. The skull of O. nigeriensis is actually broader than the skull of M. atherfieldensis (compare them in dorsal view in Norman, 2004 , fig. 19.4).

“Bipedal”—According to Norman (1980) , the hind limb/forelimb length ratio and the index of forelimb proportions (radius/humerus length × metacarpal III/humerus length) provide information about quadrupedalism or bipedalism in a dinosaur. In MSNVE 3714, the hind limb/forelimb length ratio is 1.89, which is close to the values in M. atherfieldensis , Edmontosaurus annectens , and Lambeosaurus lambei (supposed to be bipedal) and it is unlike that of adult I. bernissartensis (which is supposed to be quadrupedal by Norman, 1980 ). The index of forelimb proportions is 0.14 in MSNVE 3714, which is closer to the values of I. bernissartensis (see Norman, 1980 ). Maidment & Barrett (2014) criticized the reliability of those ratios, identifying some osteological features that are correlated with quadrupedalism and that occur in O. nigeriensis : hoof-like unguals, straight femur that is longer than tibia, prominent and not-pendant fourth trochanter and pes/hind limb length ratio of 0.14. Bipedalism, facultative bipedalism or quadrupedalism would not be apomorphic for Ouranosaurus , in any case.

“Medium-sized iguanodontid (7 metres long)”—This cannot be accepted as a diagnostic feature. O. nigeriensis is not considered an iguanodontid (i.e., a member of the Family Iguanodontidae) anymore (see Sereno, 1986 ; Norman, 2004 ; McDonald et al., 2012b ; Norman, 2015 ) and the actual length of a complete adult skeleton of this dinosaur taxon is unknown (see below). The ontogenetic stage of the holotype was not reliably established (see below); the paratype is an immature individual and is only slightly smaller than the holotype (see below). If the boundary between medium-sized and large-sized ornithopods is placed at 8 metres in length ( Norman, 2015 , p. 178), the holotype would probably approach it, if considering the complete tail in its body length estimate. That is the estimated body length of other styracosternans (e.g., Hypselospinus fittoni [7–8 m]; Dakotadon (= Iguanodon ) lakotaensis [∼8 m]; Altirhinus kurzanovi [∼8 m]; and Eolambia caroljonesa [∼7–8 m]; Norman, 2015 ), so it cannot be apomorphic for O. nigeriensis . It would be the same even considering the present length (i.e., without the distal part of the tail) of the two known skeletons of O. nigeriensis because the body length of M. atherfieldensis and B. johnsoni is estimated at 6–7 m ( Norman, 2015 ).

The original diagnosis by Taquet ( 1976 , p. 60) is actually a summary of the overall anatomy of the species, not a list of apomorphies or an apomorphic combination of characters. At the time the diagnosis was written (over 40 years ago), only a few taxa were available for comparison (see Taquet, 1976 for a list of those taxa). Therefore, that diagnosis needed to be emended. Below is a detailed analysis of the purported diagnostic features of O. nigeriensis listed in the original diagnosis by Taquet (1976) , in order to support their rejection or acceptance.

O. nigeriensis is also characterized by the following combination of characters that is apomorphic within the non-hadrosaurid styracosternans: elongate skull (length/height ratio =3.2) with laterally expanded and dorsoventrally flattened terminal part of the rostrum (“duck bill”) and oral margin of the premaxilla reflected dorsally to form a distinct rim (similar to some hadrosaurines); long ‘diastema’ in the dentary (as in Protohadros byrdi and hadrosaurids); tiny hand (humerus/metacarpal III length ratio >4 (similar to Uteodon aphanocetes and one specimen of Iguanodon bernissartensis ) with spreading metacarpals.

Emended diagnosis : Styracosternan dinosaur with the following autapomorphies: thickened, paired domes on nasals, so that nasals extend further dorsally than frontals; maximum mediolateral width of the predentary over twice maximum rostrocaudal length along the lateral process; dorsoventral expansion of the anterior part of the dentary caused by the anterior divergence of the dorsal margin (the ventral margin is straight horizontal and the rostral end of the bone is not ventrally deflected); extremely tall neural spines in dorsal, sacral and proximal caudal vertebrae (up to seven times the height of the centrum in the middle dorsal vertebrae) forming a dorsal ‘sail’ with a sinusoidal outline (lower peak in the sacral segment); petaloid and flat brevis shelf in the ilium (without brevis fossa); U -shaped obturator gutter of ischium, much deeper than long; obturator opening of pubis bordered by the ischial peduncle and a ventromedial blade-like process starting from the basal part of the ischial peduncle (the opening is nearly encircled by the peduncle and process in medial view, while it appears as an obturator gutter in lateral view); distal extremity of the posterior ramus of pubis (pubis s. s.) slightly expanded and bulbous.

Horizon and Locality : Level GAD 5, upper part of the Elrhaz Formation, Tégama Series, Aptian, Aptian-Albian, or possibly Barremian, Early Cretaceous. All specimens are from the Gadoufaoua area of Niger. The holotype comes from the Camp des deux arbres locality, 7 km south east of Elrhaz, 16°42′ lat. N. 9°20′ long. E. The paratype was found 4 km south of the niveau des Innocents locality, along the eastern border of the airfield, 16°26′ lat. N, 09°08′ long. E. The exact locality for GDF 301 and GDF 302 was not reported in Taquet (1976) .

Holotype : GDF 300, a nearly complete skeleton, lacking the left maxilla, the right lacrimal, the right quadratojugal, the stapes, the articulars, dorsal vertebra 1 and probably another dorsal or two, the centrum of caudal vertebra 1 and caudals 25–26 and 30–31, most of the distal elements of the tail and some distal chevrons, one left metacarpal and most of the manus phalanges, both femora (only the distal condylar end of one of them was found), the left tibia, the left astragalus and calcaneum, the left metatarsals, and eight pedal phalanges. The skeletal elements in situ were scattered on a 15 m 2 surface. The specimen is on exhibition at the Musée National Boubou-Hama in Niamey, Niger.

Description of MSNVE 3714 and Comparison with the Holotype

MSNVE 3714 is a partial skeleton; the missing elements were replaced by plaster copies, with the exception of the hyoid apparatus, the atlas and the cervical ribs, which are missing. Most of the original bones have also been partly reconstructed and restored (Fig. 3). Elements of the right side are more weathered then those of the left side because the paratype skeleton was exposed on the right side (as it is shown by the map; Fig. 2).

Figure 3: MSNVE 3714, Ouranosaurus nigeriensis, original and reconstructed parts in right (A) and left (B) views. The reconstructed parts are in red.

In this section, only the skeletal elements of MSNVE 3714 that add new information with respect to the description of the osteology of O. nigeriensis by Taquet (1976) are described and compared with those preserved in the holotype.