Читать книгу Hadrosaurs - David A. Eberth - Страница 34

Mantellisaurus atherfieldensis attained a probable adult body length of about 7 m. The type material (NHMUK R5764) represents a disarticulated partial skull and skeleton collected from the Isle of Wight, that is ontogenetically immature and has an estimated body length of approximately 5.5 m; the referred skeleton from Bernissart (RBINS R57) shows some residual features associated with immaturity, and is approximately 6.5 m long; and the length of the “Mantel-piece” individual (Norman, 1993) from Maidstone (NHMUK OR3741) is estimated (based on femoral length) at probably a little in excess of 7 m. Some material collected recently from the Isle of Wight exhibits very interesting anatomical variation (Martill and Naish, 2001:MIWG 6344).

Craniodental Anatomy The skull (Fig. 2.10) of this species is known in considerable detail (Norman, 1986). The lower jaw is elongate and its lower margin is gently arched towards its anterior tip; the coronoid process is comparatively short, vertical and slightly expanded anteriorly at its apex. The posterior end of the lower jaw is marked by a large surangular with a distinct surangular foramen and the angular is visible in lateral aspect. Dentary teeth are comparatively simple in construction with primary and secondary ridges alone on the lingual enameled surface resembling the pattern seen in examples of B. dawsoni. Maxillary teeth have narrower crowns than dentary teeth and have an extremely prominent distally offset primary ridge.

2.10. Mantellisaurus atherfieldensis. Skull restoration based upon the “Chase skull,” NHMUK R11521.

2.11. Mantellisaurus atherfieldensis. Anterior dorsal vertebrae, reconstruction in lateral view based upon examination of the original material of RBINS R57 and NHMUK R5764 (the holotype of I. atherfieldensis). Abbreviations: d.1–d.8, dorsals numbered in sequence; dia, diapophysis; n.sp, neural spine; pa, parapophysis (after Norman, 1986:fig. 29B).

2.12. Mantellisaurus atherfieldensis, holotype of I. atherfieldensis Hooley, 1925, NHMUK R5764. Articulated sequence of mid-dorsal vertebrae as preserved (same as Norman, 2011b:fig. 27.42B).

Vertebrae Cervical vertebrae exhibit the following characteristics: strongly opisthocoelous; low cylinders with ventral keels and a mid-height ridge that is expanded near the anterior condylar margin to form a parapophysis; neural arch develops a small midline spine lateral to which are prominent, stout diapophyses for the attachment of ribs; prezygapophyses are widely spaced and do not project beyond the articular margin of the centrum, whereas the postzygapophyses are long, arched, and divergent (and overlap the succeeding centrum). The general form of cervical vertebrae is seen in the first dorsal vertebra reconstructed in Figure 2.11.

Mid-dorsal vertebrae develop elongate spines in the articulated skeleton RBINS R57 (Fig. 2.11), but preservation is usually not nearly so good in Wealden specimens: all are broken in the holotype skeleton (Fig. 2.12). Ossified tendons are distributed in the form of a layered lattice across the taller neural spines. The centra are spool shaped and bear a modest ventral keel. The articular faces, which bear remnant opisthocoely across the cervicodorsal transition, have predominately amphiplatyan faces. Posterior dorsals develop centra that are broader and deeper than anterior members of the series, and also become slightly opisthocoelous in the region adjacent to the sacrum.

2.13. The “Saull Sacrum” illustrated in ventral view, NHMUK OR37685. Specimen referred to Mantellisaurus cf. atherfieldensis. Scanned from the original lithograph in Owen (1855:pl. 3). This specimen was one of the key specimens that Richard Owen used in order to diagnose his new “sub-order” Dinosauria (Owen, 1842).

Sacral Vertebrae One specimen (Fig. 2.13) comprises a nearly complete sacrum (lacking the sixth true sacral) with portions of an attached ilium (NHMUK OR37685), which is attributable to this species. The sacrum comprises seven fused vertebrae in mature specimens (fusion is incomplete in immature individuals) and involves the incorporation of a posterior dorsal with a free (non-sacralized) rib. There is a narrow keel present, unlike I. bernissartensis, which exhibits a broad, longitudinal midline sulcus.

Girdles and Limbs The pectoral girdle and forelimb bones differ little from those described for previous taxa (and the contemporary I. bernissartensis) except that they tend to be smaller and less robust. The blade of the scapula tends to have a narrower shaft and the blade flares distally to a greater extent than in I. bernissartensis. The coracoid also exhibits a discrete foramen (cf) externally, which is different from the coracoid “notch” seen in I. bernissartensis.

The sternal bone (Fig. 2.14C) has the classic “styracosternan” hatchet-like shape. The humerus (Fig. 2.14A) is sinuous. The ulna and radius (Fig. 2.14B) are comparatively slender and bowed, thus suggesting the possibility of some axial rotation between these elements. The wrist and hand are worth mentioning because they are distinctive (Fig. 2.15). The carpals are sutured together, but they are neither as massive nor as tenaciously bound by ossified ligaments as is the case in previous examples (above; Norman, 2011a, in press) or in I. bernissartensis (below; Norman, 1980). The first metacarpal is fused to the carpals and forms an oblique, roller-like structure for articulation with the base of the pollex; the latter is relatively diminutive and, unlike the Barilium and Hypselospinus, is genuinely conical rather than transversely compressed or truncated. In its general shape the pollex of M. atherfieldensis echoes, on a smaller scale, the conical pollex of I. bernissartensis. The central bones of the hand (metacarpals II–IV) are slender and more elongate than those known in either Hypselospinus or Iguanodon.

The ilium (Fig. 2.16) has a long, slender, preacetabular process (prp) that is buttressed by a curved medial ridge. The main body of the iliac blade is vertical, but the dorsal edge is thickened and everted so that it overhangs the lateral surface. Farther posteriorly, the dorsal edge thickens and becomes more everted, forming a beveled structure (boss) posterodorsal to the ischiadic peduncle. The dorsal edge of the postacetabular process beyond the iliac boss is inflected downward before terminating in a short transverse bar. Beneath this bar there is a narrow, vaulted brevis fossa (br.f). In overall shape the ilium resembles that of Hypselospinus fittoni from the Valanginian of the Weald Sub-basin; however, the preacetabular process is more slender and transversely thicker, whereas the equivalent portion of H. fittoni is more strongly compressed laterally and considerably deeper; the central portion of the iliac blade is shallower than in H. fittoni; and the postacetabular process differs also in having a far less pronounced brevis fossa than in H. fittoni and, as a direct consequence, the posterior bar is also much narrower.

The pubis (Fig. 2.16) has a thin, deep, prepubic process that expands distally, whereas the pubic shaft is narrow and short; there is a massive iliac peduncle, and beneath this a broad cup-shaped depression forms the anterior part of the acetabulum. The proximal part of the pubic shaft has a finger-like dorsal process that nearly encircles the obturator foramen (obt.f); its posterior surface forms a flattened vertical surface for attachment of the adjacent part of the ischium and, when articulated, the obturator foramen is completely enclosed. The shaft of the ischium is long, slender, and only slightly arched along its length (the arching is perhaps exaggerated in Figure 2.16, and the distal boot is too large) and has a modest anterodistal expansion.

2.14. Mantellisaurus atherfieldensis, holotype of I. atherfieldensis Hooley, 1925, NHMUK R5764. (A) humerus, right in dorsal view; (B) radius and ulna, right lateral view; (C) right sternal bone in ventral view. Abbreviations: h, articular head of the humerus; ra, radius; ul, ulna (same as Norman, 2011b:fig. 27.43C–E).

The hindlimb (Fig. 2.17) is not particularly distinctive, as is true of most similar-sized iguanodonts. The femur (Fig. 2.17A, B) has a shaft that is more slender, less angular-sided, and less curved along its length than that seen in B. dawsoni and H. fittoni from the Weald Sub-basin (any remaining curvature of the shaft is present only below the fourth trochanter [4t]); and the anterior trochanter (at) is narrower, less robust, more laterally compressed, and more closely appressed to the lateral surface of the greater trochanter, when compared to the latter taxa. The lower leg elements (Fig. 2.16C, D) are not distinctive, except insofar as they are more slender and lightly built than in the contemporaneous taxon I. bernissartensis, and the proximal tarsals are firmly attached (but not fused) to the crus (Fig. 2.17: ast, cal).

The pes (Fig. 2.18A, B) is slender and functionally three toed. Neither the holotype (NHMUK R5764) nor the referred specimen (RBINS R57) have metatarsal I preserved. A well-preserved and articulated pes that is commensurate and that is the same stratigraphic age has been referred to M. atherfieldensis (Norman, 1986; NHMUK R1829) exhibits a narrow, splint-like metatarsal I. The sympatric contemporary I. bernissartensis has a small, laterally compressed metatarsal I (Norman, 1980).

2.15. Mantellisaurus atherfieldensis, holotype of I. atherfieldensis Hooley, 1925, NHMUK R5764. The associated elements of the right manus in dorsal view. Abbreviation: mc, metacarpal (from Norman, 1977).