A styracosternan iguanodontian with the following autapomorphies: dorsal neural spines are at least 4.3 times centrum height, extremely tall and deep vertical recess between postzygapophyses and proximal portion of the neural spines in mid-posterior dorsal vertebrae (2.6 times spine anteroposterior length), dorsosacral rib distally fused to the sacral yoke, midline ventral keel restricted to the anterior half of the centrum in sacrals 2 and 3, slightly pronounced and broad ventral keel restricted to the anterior half of the centrum in sacral 4, sacrodorsal and first sacral rib facets closely spaced and located dorsal to the remaining sacral rib facets near the dorsal margin of the ilium, medial ridge on the postacetabular process slopes posterodorsally to meet the point in which the dorsal margin of the ilium slopes posteroventrally forming a medially expanded and shallowly concave dorsal platform at posterior portion of the postacetabular process, and distal end of the ischium with a D cross-section due to lateral expansion.

Description and comparisons

Dentition. Fourteen teeth were recovered, but except CMP-MS-03-89 all the elements are poorly preserved and consist of roots and portions of the crown bases that do not allow a detailed description. CMP-MS-03-89 (Fig 2) is a right dentary tooth that preserves the proximal portion of the root and the basal half of the crown. The general aspect of this tooth is similar to that seen in ankylopollexian ornithopods. The crown is heavily worn and the marginal denticles on the mesial and distal margins are not preserved. The crown is labiolingually narrow and mesiodistally expanded. The lingual surface of the crown is enamelled and bears a prominent distally offset primary ridge. Two narrow, subparallel accessory ridges are located mesial to the primary ridge (Fig 2A). The labial surface of the crown possesses an almost vertical, slightly concave wear facet (Fig 2B).

Dorsal ribs. The proximal ends of two dorsal ribs (Fig 8) together with several fragments of dorsal rib shafts are preserved. Both proximal fragments show similar features but are distinct in size and are probably representatives of the anterior dorsal series. A long and transversely compressed neck with subparallel margins separates the capitulum and the tuberculum. In both ribs, the capitulum is thickened, rugose and suboval in outline. The tuberculum is suboval and forms a very short and thickened articular process. In both ribs the tuberculum is located dorsal to the base of the capitulum. The preserved shaft of the rib is subtriangular in cross-section with the apex pointing anterolaterally. There is a well-marked ridge originating at the base of the tuberculum that extends distally along the anterolateral surface of the shaft (Fig 8A).

Sacrum. The sacrum consists of six firmly co-ossified sacral vertebrae (Fig 9). The centrum of the first preserved vertebra is similar to those of the sacrodorsal in Mantellisaurus [19] and it is also placed anteriorly to the anterior border of the sacral yoke. However, it bears a small posteriorly directed sacrodorsal rib that contact with the sacral yoke. This first vertebra can be regarded as a sacrodorsal, so the sacrum is composed of five true sacral vertebrae (Fig 9B and 9C). The sacrum is slightly convex dorsally along its length (Fig 9C). The centrum of the sacrodorsal vertebra is large and massive with anterior and posterior articular facets that are expanded laterally and compressed dorsoventrally. The profile of the anterior facet of the centrum is elliptical and slightly indented dorsally for the neural canal. The margins of the anterior articular facet are rugose, thickened and everted. The anterior articular facet is flat showing a slight central convexity. The ventral surface of this sacrodorsal centrum is flat and subquadrangular (Fig 9B). Dorsally, the neural arch is located in the middle of the centrum. The prezygapophyses are large, ventromedially inclined and slightly overhang the anterior facet margins (Fig 9A). The short and horizontal transverse processes are located immediately posterior to the prezygapophyses, and are triangular in section with the apex facing ventrally. The base of the incomplete neural spine is anteroposteriorly short. The dorsosacral rib is borne on the ventral surface of the transverse process and the lateral surface of the neural arch. The shaft of the dorsosacral rib is thin anteroposteriorly. The distal end of the sacrodorsal rib is fused to the sacrocostal yoke. A small and circular foramen, related to the lateral nerves [18], is located in the posterior part of the neural canal wall. Sacrals 1 to 3 have spool-like centra with modestly keeled ventral surfaces (Fig 9B). However, the keel is restricted to the anterior half of the centra in sacral vertebrae 2 and 3. In these sacral vertebrae, the neural arches and sacral ribs are clearly positioned in the intercentrum position. Because of that, the neural arch of the sacral 1 is very close to that of the sacrodorsal vertebra, what it is also evident by the closely associated facets from the sacrodorsal and first sacral ribs on the medial surfaces of the ilia. The transverse processes of the sacrals 1–3 are sub-triangular in section with the apex facing ventrally, horizontally directed, and slightly longer than those of the sacrodorsal vertebra. The bases of the neural spines of the sacrals 1–3 are fused to each other. The ribs of sacrals 1–3 are robust and ventrolaterally fused to form the sacral yoke (Fig 9B). Large foramina for lateral nerves [18] are present between the neural arches. The centra of sacrals 4 and 5 are transversely broad compared with those of sacrals 1–3. The ventral surfaces of the centra of sacrals 4–5 are subquadrangular, and sacral 4 preserves an anteroposteriorly short and transversely broad modest keel located anteriorly. In contrast, in sacral 5 the ventral surface is un-keeled and gently convex (Fig 9B). As in foregoing sacrals (s1–s3), the neural arch of sacral 4 is positioned in the intercentrum position. In sacral 5, the posterior facet of the centrum is elliptical, bears slightly expanded margins, and its surface is concave. The neural arch is positioned in the middle of the centrum. The transverse processes are short, whereas the sacral ribs are well-developed to form a large anteroventrally inclined platform in lateral view. Both postzygapophyses are preserved and they slightly overhang the posterior centrum facet margins. The postzygapophyses are small, almost vertical and have oval articular facets. As in Ouranosaurus [27], the sacrum of Morelladon is characterized by the presence of six co-ossified sacral vertebrae. In Barilium dawsoni [21] and Mantellisaurus atherfieldensis [4,19] the sacrum comprises seven co-ossified sacral vertebrae, a condition also probably present in Hypselospinus fittoni (NHMUK R811) [6] and Lurdusaurus arenatus [25]. Iguanodon bernissartensis [4,18] can be distinguished from the aforementioned taxa by the presence of eight co-ossified sacral vertebrae [4]. The sacrum of Morelladon differs from that of Ouranosaurus by having a sacrodorsal with a flat ventral surface and modestly keeled ventral surfaces in the first to fourth true sacral vertebrae. As in Ouranosaurus the ventral surface of the last sacral vertebra is transversely convex. The sacrum of Morelladon can be distinguished from that of Barilium dawsoni [21] and Mantellisaurus atherfieldensis (NHMUK R5764) [19] by the absence of a midline keel in the ventral surface in the last sacral vertebral centrum. It can be also distinguished from that of Iguanodon bernissartensis (IRSNB 1722) [18] by the absence of a flattened ventral surface in the last sacral vertebral centrum. As in Hypselospinus fittoni (NHMUK R811) [6] the centrum of the first three true sacral vertebrae (s1 to s3) are spool-like and modestly keeled ventrally. However in Hypselospinus fittoni the centra are mildly centrally constricted [6] whereas in Morelladon the centra of the equivalent vertebrae are more markedly constricted (Fig 9B) and the keels of sacral 2 and 3 are restricted to the anterior half of the centra. In Mantellisaurus atherfieldensis, centra of sacrals 2 and 3 are markedly constricted at midlength as is seen in Morelladon but keeled along the entire centrum length (NHMUK R11521).