Mantellisaurus atherfieldensis, (HOOLEY, 1925)

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 : 150-151

MANTELLISAURUS ATHERFIELDENSIS ( HOOLEY, 1925) – ( NORMAN, 1986)

This is an upper Barremian−Lower Aptian sympatric Wealden taxon ( Fig. 2 ). It has a more gracile morphology than Hy. fittoni . Osteologically mature skeletons of M. atherfieldensis appear to range between 6 and 7 m in body length.

Teeth and jaws

Individual dentary crowns are smaller and lack the complexity of ridge patterning when compared with that of Hypselospinus ( Norman, 1986: figs 19, 21). The dentary ramus is slender and arched anteriorly and the coronoid process rises perpendicular to the long axis of the jaw, rather than at an obtuse angle, as appears to be the case in NHMUK R1831 ( Fig. 36 ).

Axial skeleton

The dorsal column of M. atherfieldensis reveals centra that are smaller and more gently waisted than those in Hy. fittoni ; they do not show the pronounced thickening noted on the articular rims of the centra, the oblique inclination of the centra, or extreme slenderness and elongation of dorsal and caudal neural spines ( Norman, 1986: figs 29–32).

Appendicular skeleton

The pectoral girdle is more lightly built in M. atherfieldensis than in Hy. fittoni . In the former taxon, the scapula has a narrow proximal portion and a flared distal blade. The coracoid has a completely enclosed coracoid foramen ( Norman, 1986, 2011b: text-fig. 27.43). The proportions of the sternals also differ: there is a broader and more elongate posteromedial extension to the sternal ‘blade’ in Hy. fittoni compared with that in M. atherfieldensis ( Norman, 1986: fig. 45) as well as a shorter, somewhat flattened, and more robust ‘handle’. The forelimb is slender and lightly built in M. atherfieldensis , reflected in the shorter, sinuously shafted humerus and the slender, bowed radius, the partial co-ossification of the carpals, and the comparatively short, conical pollex ungual. The metacarpals are also comparatively slender and elongate ( Norman, 1986: figs 50, 51; 2011b: text-fig. 27.44). The pelves are distinct ( Norman, 2011b: text-fig. 27.10): the pubis of M. atherfieldensis has a very thin and dorsoventrally expanded prepubic process; the shaft of the ischium is essentially straight, angular-sided (with a slight curvature apparently present in some specimens), and narrow with a small, distal, anteriorly expanded ‘boot’. The ilium resembles (in simple outline shape) that of Hy. fittoni . However, in detail ( Norman, 1986: fig. 54) the blade is lower and the preacetabular process is narrower proximally and develops an expanded medial ridge, which is very different when compared with that seen in Hy. fittoni . The postacetabular process develops a much less extensive and more posteriorly positioned brevis fossa. The hindlimb of M. atherfieldensis is less robust than that of Hy. fittoni ; the femur ( Norman, 2011a: text-figs 27.11, 27.46) has a less angular-sided shaft, the anterior trochanter is positioned more laterally and is narrow and blade-like, and the fourth trochanter is more proximally positioned and proportionally smaller than that seen in Hy. fittoni . The more distal portions of the limb and pes differ only in their comparative gracility.

Figures

Figure 2. Stratigraphy of the Wealden of southern England. Stratigraphical chart based upon Batten (2011: textfig. 2.1) with the approximate stratigraphical distribution of the principal large-bodied ornithopod taxa indicated using solid vertical bars. Abbreviations: Fm, Formation; GC Fm, Grinstead Clay Formation; Lower Grnsd, Lower Greensand; L.T.W. Sand Fm, Lower Tunbridge Wells Sand Formation; U.T.W. Sand Fm, Upper Tunbridge Wells Sand Formation; B., Barilium; H., Hypselospinus; I., Iguanodon; M., Mantellisaurus.

Figure 36. Hypselospinus cf. fittoni. NHMUK R1831. Dentary right with teeth preserved in situ. A, medial; B, lateral; C, dorsal views. Abbreviations: am, alveolar margin; br, badly broken portion of the dentary; cp, coronoid process; ds, dentary symphysis; m, matrix; mgr, Meckelian groove; pr, anterior lateral process of the dentary; sf, sutural facet; sl, ‘slot-and-lip’ portion of the dentary symphysis; tf, tooth fragments in alveolar bone; vc, vascular channel. Scale bar = 10 cm.