Iguanodon bernissartensis, BOULENGER, 1881

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

IGUANODON BERNISSARTENSIS BOULENGER, 1881 (IN BENEDEN, 1881) – ( NORMAN, 1980)

This upper Barremian–Lower Aptian ( Fig. 2 ) sympatric taxon is large (10+ m long) with a robustly construct- ed skeleton that is reminiscent of that of the Valanginian B. dawsoni .

Teeth and jaws

Individual dentary crowns ( Norman, 1980: fig. 19) lack the complex pattern of primary, secondary, and accessory ridges seen in Hypselospinus and are indistinguishable from those currently referred to Barilium . The lower jaw is deep, robust, and essentially straight [although some relatively uncrushed specimens (RBINS R56 (IRSNB 1680)) exhibit modest arching of the dentary ramus anteriorly]; this morphology contrasts with the more slender and arched dentary ramus morphology of the referred dentary of Hypselospinus (NHMUK R1834: Fig. 44 ). The coronoid process is also distinct in being tall and perpendicular to the long axis of the jaw in I. bernissartensis ( Norman, 1980: pls I−IV) by comparison with the shorter and more obtuseangled coronoid process in the referred specimen NHMUK R1831 ( Fig. 36 ). It should be noted that breakage and remedial reconstruction of NHMUK R1831 might account for some of the differences noted here.

Axial skeleton

The cervical and dorsal vertebral centra are generally similar in shape and proportions, but are substantially larger and do not exhibit the extreme eversion of their articular rims seen in Hypselospinus . The dorsal neural spines of I. bernissartensis are typically thick and tall (‘plank-like’: Norman, 1980: figs 34–40) compared with the very slender and elongate neural spines seen in some of the better-preserved dorsals of Hypselospinus . Caudals of I. bernissartensis also lack the tall, narrow neural spines that are characteristic of Hy. fittoni .

Appendicular skeleton

The robust shoulder girdle and forelimb of I. bernissartensis resembles that seen in Hypselospinus , except that in the former the proportions of the limb are overall more elongate. The former taxon has a deeply notched coracoid foramen (rather than a fully enclosed foramen) and a more elongate, curved, and conical (rather than laterally compressed) pollex ungual. The manus of I. bernissartensis is proportionally larger and it has more elongate metacarpals ( Norman, 1980: figs 52– 62). Both taxa share a tendency to ossify the connective tissue of the median sternal area between the coracoids and sternals in a manner reminiscent of secondary cartilage ossification (this was referred to as an intersternal ossification – Norman, 1980: 47). One example of Hypselospinus (NHMUK R1831 – Fig. 43 ) exhibits co-ossification of the sternals and this pathology may have involved the coracoids (fusion between the sternal bones has not been observed in any specimens referred to I. bernissartensis , although coracoid articulation against the intersternal ossification appears probable). The pelvis is structurally distinct: the ilium of I. bernissartensis is notable for its thick, robust preacetabular process; the thickened and rolled posterodorsal edge of the iliac blade, and the extremely elongate, tapering postacetabular ramus with its pronounced lateral ridge and very broad brevis fossa ( Norman, 1980: fig. 64). The pubis has a thick, but comparatively narrow, proximal prepubic process that expands abruptly distally; this is quite distinct in outline from what is known of the shape of the prepubic process of Hypselospinus . The hindlimb is similar in overall morphology in both taxa, although the femoral shaft appears to be less markedly angular-sided and less bowed along its length in I. bernissartensis .

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.

Figure 43. Hypselospinus cf. fittoni. NHMUK R1831 (R1835). Sternal apparatus. A, external (ventral); B, internal (dorsal). Abbreviations: apr, apron area of sternal blade; con, condylar area for attachment of dorsal rib cartilage; co.s, coracoid suture; ‘h’, sternal handles; iso, intersternal ossification; k, midline keeled structure traversed by strands of bony tissue; lf, lateral fingers of ossified tissue; st, sternal bone. Scale bar = 10 cm.

Figure 44. Hypselospinus cf. fittoni. NHMUK R1834. Dentary (left, partial). A, medial (lingual); B, lateral (labial); C, dorsal. Abbreviations: am, alveolar margin; alv, alveolar trough; dias, margin forming a diastema; ds, dentary symphysis; mgr, Meckelian groove; sa.s, surangular suture; sl, posterior ‘slot-and-lip’ portion of the dentary symphysis; vc, vascular channel. Scale bar = 10 cm.