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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 and the handle of the hatchet tends to be more curved; and associated with these in some individuals there is also an unusual, somewhat irregular mass of bony material located in the center of the chest and referred to as an “intersternal ossification” (Norman, 1980:fig. 56). The humerus is very robust and nearly straight rather than strongly sigmoid (obscured by crushing), and has a massively thickened deltopectoral crest. The forearm bones are equally massive and parallel, with very little gap between the shafts of the two bones, which supports the contention that this dinosaur used its forelimbs for walking and body-weight support. The carpals and metacarpal I are fused into a large block (Norman, 1980:fig. 59); the metacarpal has a roller-like articular surface for articulation of the large, conical, and slightly curved pollex. Unlike the condition seen in Hastings Group taxa (B. dawsoni and H. fittoni), the pollex does not become fused to the carpometacarpal block in mature specimens and remained freely mobile at its base. A small, flattened phalanx is occasionally seen lodged in the base of the pollex ungual. The central metacarpals are more massive, and in proportion shorter, than those of Mantellisaurus (Fig. 2.15).

The pelvis (Fig. 2.22) is distinct from that seen in Mantellisaurus (Fig. 2.16). The ilium has a very robust, thick, preacetabular process, which is supported by an enlarged medial ridge (note the shape of its cross section in silhouette; Fig. 2.22). The main part of the iliac blade is vertical, but the upper edge is thick and posteriorly it becomes more so – so that it forms a somewhat everted and curved ledge that overhangs the ischiadic peduncle; there is no abrupt inflection along the upper margin of the postacetabular process that characterizes the ilium of Mantellisaurus. The dorsal margin of the posterior ilium is elongate and pointed in profile and its ventral surface forms a broad, shallowly vaulted brevis fossa (br.f) bounded laterally by a prominent ridge (Fig. 2.22). The ischiadic peduncle does not exhibit the prominent lateral and stepped expansion typical of all other Wealden iguanodontians. The prepubic process forms an elongate anterior blade that is transversely thick (Fig. 2.22, silhouette), but dorsoventrally narrow along much of its length, before expanding distally; this is distinct from the thinner and deeper blade that is typical of Mantellisaurus (Fig. 2.16). The pubic shaft forms a tapering rod that is much shorter than the shaft of the ischium. The ischium has a shaft that is elongate and stout, with a generally rounded, rather than angular, cross section; the shaft is curved along its length (J-shaped) and ends in a prominent, anteriorly expanded “boot.” This structure is distinct from the narrower, angular-sided, and far straighter ischial shaft with a small boot that characterizes Mantellisaurus.

In the hindlimb the femur is large and very stout compared to that of Mantellisaurus; the fourth trochanter is very elongate and forms a very thick crested blade along the postero-interior edge of the mid-shaft. The shin or lower half of the limb is similar in overall shape in the two taxa, but the difference in robustness is notable; this is especially so in the massive construction of the pes. The first metatarsal of the foot in I. bernissartensis forms a small, oblique, flattened spatula-shaped splint bone that is distinct from the thin, pencil-like, metatarsal that lies parallel to the shaft of metatarsal II in one articulated example of Mantellisaurus from the Isle of Wight.