Hypselospinus fittoni (NHMUK R, 1635)

Norman, David B., 2015, On the history, osteology, and systematic position of the Wealden (Hastings group) dinosaur Hypselospinus fittoni (Iguanodontia: Styracosterna), Zoological Journal of the Linnean Society 173 (1), pp. 92-189 : 145-146

HYPSELOSPINUS FITTONI

Figure 47 is a first attempt to develop a composite reconstruction of the skeleton of Hy. fittoni , based upon what is known of the type and referred material described above. Cranial material is unknown. The dentary is based upon NHMUK R1831 and R1834, the axial skeleton is based primarily upon NHMUK R33, R604, R1148, and R1834, the pectoral girdle and forelimb are based upon NHMUK R1831, R1834, and R604, and the pelvic girdle and hindlimb are based upon NHMUK R1635, R604, R811, R1834, and R1148.

Hypselospinus is a large-bodied ornithopod with a body length that probably ranged up to 7 or even 8 m (judged from the largest fragmentary referred skeleton so far recovered: NHMUK R1627). Its general build would best be described as ‘mesomorph’: for example, this taxon was not as robustly constructed as the sympatric contemporary B. dawsoni . The forelimb and manus are constructed for weight support and locomotion, so the quadrupedal pose was probably normal, if not obligatory (the precise proportions of forelimb: hindlimb are not known). This pose is also echoed in the evidence of a massive, and reinforced, pectoral girdle. The term ‘reinforced’, is perhaps exaggerated in this instance because of the hyperostosis (in appearance similar to the medical condition diffuse idiopathic skeletal hyperostosis − ‘DISH’) visible in NHMUK R1831; this latter associated skeleton exhibits excessive bone growth adjacent to articular surfaces, e.g. across the sternocoracoid plate, antebrachium, carpus, manus, and unguals of the pes.

The general pose and gait of this animal as reconstructed here is particularly influenced by the orientation of the pelvic girdle. This is shown tilted posteriorly (and this orientation also applies to the reconstruction of closely related taxa such as B. dawsoni ( Norman, 2011a: text-fig. 25), M. atherfieldensis ( Norman, 1980: fig. 83), and I. bernissartensis ( Norman, 1980: fig. 84). In each of these examples the ilium is notable for having a dorsal acetabular margin that is shallow and smoothly rounded (when the dorsal edge of the iliac blade is positioned horizontally – as it is in most illustrations). The pubic peduncle of the ilium is by contrast stout, triangular in cross-section, and bears a prominently lipped supra-acetabular crest. In addition, the pubic peduncle is sutured mediodorsally to the massive, ventrolaterally directed first sacral rib. It is clear from this structural arrangement that the primary weightbearing capacity of the entire pelvis is located on the pubic peduncle and the adjacent ‘keystones’ represent- ed by the first sacral ribs and sacral centrum, rather than the central section of the iliac acetabulum. In order to reflect these implied articular mechanics at the hip joint the ilium has to be rotated posterodorsally from the horizontal so that the pubic peduncle itself lies horizontally (in lateral view) and its supraacetabular crest is positioned so that it forms the dorsal margin of the acetabulum. Pelvic rotation affects the overall pose of the animal because of the way in which it alters the pattern of curvature along the vertebral column, especially insofar as it lowers the anterior caudal series.

There has, in recent decades, been a near universal tendency to adopt by default ‘high-tailed’ and dynamic silhouette-style reconstructions for ornithopod dinosaurs [starting with Peter Galton’s (1970) ‘ Anatosaurus in a hurry’]. These artistic renderings are attractive to the eye and chime with the dynamic interpretation of dinosaurs promoted most notably by Robert T. Bakker during the 1970s. Although some of these reconstructions (notably those for theropod dinosaurs), seem biologically plausible, it has been realised that the anatomy portrayed in some dinosaur images has been compromised. The ‘cocked’ wrists and ‘rotating’ shoulder blades depicted in Gregory Paul’s earlier reconstructions of large-bodied ornithopods such as Iguanodon ( Brett-Surman, 1997: fig. 24.6A) suggest the influence of Eadweard Muybridge’s stop-frame photographs of mammalian (horse) locomotion. The reorientation of the pelvis in Hypselospinus (and related ornithopods) has the visual effect of ‘cramping’ the pose and implied gait in these reconstructions because it removes some of the intrinsic dynamism of the pose of these dinosaurs.

Figures

Figure 47. Hypselospinus fittoni. A preliminary reconstruction of the skeleton based upon the type and referred material described in this article. NB: forelimb relative to hindlimb length, and proportions, are currently unknown. Scale bar = 1 m.