2.16. Mantellisaurus. Attempted reconstruction of the articulated pelvic bones in left lateral view. The curvature of the ischial shaft is exaggerated; the shaft is typically straighter, more angular-sided proximally, and has a smaller distal “boot” (from Norman, 1977, 2011b). Abbreviations: ac, acetabulum; boss, faceted dorsal margin of the iliac blade; br.f, brevis fossa; il, ilium; is, ischium; obt, obturator process; obt.f, obturator foramen; prp, preacetabular process of the ilium; pu, pubis; sac, supra-acetabular crest; step, beveled external surface of the ischiadic peduncle.
2.17. Mantellisaurus atherfieldensis, holotype of I. atherfieldensis Hooley, 1925, NHMUK R5764. The principal bones of the hindlimb. (A, B) femur in anterior (dorsal) and medial views, respectively; (C, D) the crus in anterior (dorsal) and medial views, respectively. Abbreviations: 4t, fourth trochanter; aig, anterior intercondylar groove; ast, astragalus; at, anterior (lesser) trochanter; cal, calcaneum; cn, cnemial crest; fi, fibula; gt, greater trochanter; h, femoral articular head; ti, tibia (same as Norman, 2011b:fig.27.46).
Taxonomic Discussion
Despite some lingering concerns about the status of Iguanodon seelyi, the iconic taxon I. bernissartensis has remained largely unaffected by recent taxonomic revisions. However, an entirely unnecessary confusion concerns a new taxonomic combination, Dollodon seelyi (Fig. 2.2). Carpenter and Ishida (2010:148) consider the ilia of RBINS R57 that were renamed Dollodon bampingi by Paul (2008) to be “practically indistinguishable from that of Iguanodon seelyi Hulke, 1882 and therefore the species seelyi has priority over bampingi.” However, the two ilia are distinguishable. Dollodon is an invalid taxon (see above, D. bampingi) and referring another specimen to this genus by creating a new species without reference to diagnostic characters is logically and taxonomically unacceptable (McDonald, 2012a).
Description
Mature specimens of I. bernissartensis attained a body length in the range 10–13 m (I. seelyi being at the top end of the range) and, by virtue of their size and robustness, these remains are readily distinguished from those of the contemporaneous taxon Mantellisaurus. Subadult remains overlap the size range of Mantellisaurus (Norman, 1980), but can generally be distinguished anatomically without too much difficulty.
2.18. Mantellisaurus atherfieldensis, holotype of I. atherfieldensis Hooley, 1925, NHMUK R5764. (A) the right and (B) left pes elements as preserved in dorsal view. Abbreviations: dt, distal tarsal; mt, metatarsal; ung, ungual phalanx.
Craniodental Anatomy The skull of I. bernissartensis (Fig. 2.20) has been described in detail (Norman, 1980) and is distinctive in both its proportions and osteology. Compared to that of Mantellisaurus (Fig. 2.10) the skull is taller and less elongate rostrally. The lower jaw is deep and robust, parallel sided, and is less arched along its ventral margin. The other notable feature of the skull of I. bernissartensis compared to that of M. atherfieldensis is the double palpebral. Apart from their generally larger size, maxillary and dentary teeth are very similar in appearance to those described for Mantellisaurus.
Vertebrae The large size of the cervical and dorsal vertebrae (Fig. 2.21) are distinctive, and the spines of the dorsals are not as tall, relative to centrum height, as in Mantellisaurus; the form of the dorsal centra also shows an exaggerated change in shape from anterior to posterior along the series. Anterior dorsals tend to have comparatively narrow, tall centra, whereas posterior dorsals have centra that are broad and short with everted articular margins; the posterior articular surfaces (initially flat with a slight depression at the center) also become increasingly concave nearer the sacrum. The sacrum typically comprises eight fused vertebrae (including the sacrodorsal) and examples with nine fused caudals (by incorporation of the first caudal) are known (Norman, 1980), and there is typically a broad ventral sulcus on the posterior sacrals. Caudal vertebrae form slightly taller than broad subrectangular bodies anteriorly, with prominent horizontal caudal ribs and large chevron facets. More posteriorly in the caudal series, the loss of the caudal ribs and diminution of size often result in a change of centrum morphology from hexagonal cylinders to elongate, round cylinders. Ossified tendons are present as a latticelike array along the sides of the neural spines of the entire dorsal series, across the sacrum, and along the anterior third of the tail; they may also form what appear to be collapsed bundles lying in the longitudinal recess formed between the base of the neural spines and the adjacent transverse processes, either as preservational artifact or reflecting their involvement in different parts of the epaxial musculature.
2.19. Mantellisaurus. Skeletal reconstruction based primarily upon the articulated, albeit crushed and distorted, skeleton from Bernissart, RBINS R57 (formerly IRSNB 1551 [after Norman, 1986]).
2.20. Iguanodon bernissartensis Boulenger, 1881. Skull reconstruction, in lateral view, based on several of the Bernissart specimens (from Norman, 1980:fig. 2).
2.21. Iguanodon bernissartensis. Anterior dorsal series based upon an articulated series of vertebrae preserved in the Conservatoire Collections of the RBINS [individual “S”]. Abbreviations: 1–8, serial arrangement of dorsals; n.sp, neural spine; pa, parapophysis; poz, posterior zygapophysis (after Norman, 1980).
Girdles and Limbs Size and robustness are key features that distinguish these elements from those of the contemporary Mantellisaurus. The scapula tends to be generally less curved along its length and less expanded towards the distal end of the blade than in Mantellisaurus, although there is variation in both curvature and distal expansion among the individuals collected at Bernissart (pers. obs., 2009). The coracoid has a well-developed coracoid notch (co.n) rather than the discrete coracoid foramen seen in the much smaller Mantellisaurus. The sternal bones are comparatively very large