MANTELLISAURUS Paul, 2007 MANTELLISAURUS ATHERFIELDENSIS (Hooley, 1925)
The posthumous work by Hooley (1925) based on a nearly complete skeleton recovered (in 1914) from broken blocks of shale following a cliff collapse near Atherfield Point, Isle of Wight, provided the first detailed anatomical description of any Wealden-aged Iguanodon-like ornithopod 100 years after the first Iguanodon teeth were described by Mantell. This paper founded a new species: Iguanodon atherfieldensis Hooley, 1925. The importance of this discovery and its description cannot be overemphasized, given the previous century of attempts to identify and name new species using material that was often inadequate and compounded by the startling failure to provide detailed descriptions when material was, in fact, available. Noteworthy, in the latter respect, is the remarkable fully articulated skeletal material collected between 1878 and 1881 from Bernissart in Belgium, which was described only superficially by Louis Dollo (Norman, 1980, 1986, 1987). What became increasingly obvious, with the benefit of hindsight, was that material (notably that collected from the Isle of Wight) described variously under the names Vectisaurus, Sphenospondylus, or Iguanodon mantelli – the latter name usually considered synonymous with the “Mantel-piece” collected from Maidstone in 1834 (Norman, 1993) – would eventually be referred to I. atherfieldensis (Norman, 1986, 1990, 2004).
2.9. Hypselospinus fittoni. Preliminary skeletal reconstruction based upon the holotypes of Iguanodon fittoni Lydekker, 1889, and I. hollingtoniensis Lydekker, 1889, supplemented by information from several additional referred partial skeletons (from Norman, in press).
Taxonomic Discussion
Recently, the taxon Iguanodon atherfieldensis has been subjected to revision. Paul (2007) proposed Mantellisaurus as a new generic name for I. atherfieldensis. The reasoning for this change relied on osteological differences originally regarded as sufficient to distinguish these forms as “osteological species” (Norman, 1986:327). Having proposed the generic name Mantellisaurus, Paul (2008) then extended his taxonomic revision of Wealden iguanodonts by creating an entirely new taxon, Dollodon bampingi (Fig. 2.2) for the gracile skeleton (RBINS R57 [formerly “IRSNB 1551”]; see Norman, 1986) and referred to, historically, as “Iguanodon mantelli” (e.g., Dollo, 1882; Casier, 1960). The first monograph on this specimen (Norman, 1986) referred it to Iguanodon atherfieldensis. The case for erecting the new binomial Dollodon bampingi was supported by a list of diagnostic characters derived from some simplistic outline drawings, some “fleshed-out” restorations of the heads of these animals and the interpretation of photographs of mounted specimens (named “technical restorations” by Paul, 2008:202). Norman (2012) evaluated the diagnostic characters proposed by Paul and demonstrated that none could be considered to be valid, and that on that basis alone, the new name should be considered a nomen dubium: a very similar conclusion was reached independently by McDonald (2012a).
Description
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