Hypselospinus fittoni, (LYDEKKER, 1889)

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 : 102-107

HYPSELOSPINUS FITTONI ( LYDEKKER, 1889)

FIGURES 3–5 , 7–10

*1889 Iguanodon Fittoni Lydekker : 354

v*1889 Iguanodon hollingtoniensis : 355

v 1890 Iguanodon Fittoni Lydekker : 38, fig. 1C

v 1890 Iguanodon hollingtoniensis : 40, figs 1E, 2

v*2010 Hypselospinus fittoni ( Lydekker, 1889) ; Norman, figs 5–9

v.2010 Wadhurstia fittoni ( Lydekker, 1889) ; Carpenter and Ishida, fig. 2.31

v.2012 Huxleysaurus hollingtoniensis ( Lydekker, 1889) ; Paul

v.2012 Huxleysaurus fittoni ( Lydekker, 1889) ; Paul

v.2012 Darwinsaurus evolutionis ( Lydekker, 1889) ; Paul, fig. 1B, b

Holotype

NHMUK R1635 ( Figs 3–5 , 7–10 ): incomplete left ilium, partial sacrum, midcaudal centrum, the eroded proximal end of an ischium (very dubious association). In addition, three isolated teeth (one stump of a dentary crown and two worn and rootless maxillary crowns) have the same registered number and may well have been part of the original accession.

Referred material

NHMUK R1148 (incorporating material registered as R1629 and R1632), R604, R604a (including bones registered as R811, R811a, R811b), NHMUK R33, R966, R1636 (ilium only), R1831 (incorporating specimens registered separately as R1832, R1833, and R1835), R1834, R4743 (scapula). N.B. NHMUK R1627 (a fragmentary skeleton collected from the village of Brede – see Fig. 1 ) is tentatively associated with the hypodigm of Hy. fittoni , pending further study. The specimens registered as NHMUK R2848 (an isolated femur and an associated scapula-coracoid), which were referred to B. dawsoni ( Norman, 2011a) may eventually prove to be referable to Hy. fittoni .

Stratigraphical horizon, age, and type locality

Lower Cretaceous, Hastings Group, Wadhurst Clay Formation ( Fig. 2 ). Age: Valanginian: 139−137 Mya ( Allen & Wimbledon, 1991; Gradstein, Ogg & Smith, 2004; Rawson, 2006). Type locality: Shornden Quarry, Hastings ( Fig. 1 ): originally an open-cast quarry site, the area where this quarry was located was landscaped and part converted into a reservoir in Alexandra Park during the late 19 th century, East Sussex, UK ( Brooks, 2011; Norman, 2011a).

Diagnosis

Asterisks* signify apomorphies. Other characters listed below form a unique combination of characters that are apomorphic when considered together, even though they may occur sporadically within a plexus of morphologically similar ornithopods.

Holotype diagnosis

Ilium ( Figs 3 , 9 ). Preacetabular process (prp) with medial and lateral surfaces that are vertical, laterally compressed and shows little evidence of long-axis torsion*; ventral edge of the proximal portion of the preacetabular process thicker than dorsal edge, and its dorsal edge is narrow and flat-topped*; low-relief, curved, medial ridge on the medioventral surface of the preacetabular process associated with a shallow, irregular facet marking the area for attachment of the distal end of the first sacral rib*; central portion of iliac blade above the acetabulum is flat and stands more or less vertically (rather than having its lateral surface concave vertically and leaning outward so that it faces ventrolaterally); straight, narrow, transversely compressed, and flat-topped dorsal edge to the central portion of the iliac blade*; postacetabular process with an inflection point dorsally, after which the dorsal margin slopes posteroventrally before terminating at a transversely expanded and thick bar*; medial deflection of the ventral half of the postacetabular process creates an elongate, broad, low-arched, brevis fossa; brevis fossa bordered laterally by a thick horizontal ridge; the postacetabular process displays sacral rib facets that track the ventral margin of the postacetabular blade and rise obliquely toward the posterior tip, and merge with the dorsally positioned ‘transverse process’ facets that run horizontally along the midsection of the iliac blade.

Vertebrae ( Fig. 7 ). Ventral surfaces of posterior sacral centra are keeled; anterior-middle caudal subcylindrical with a transversely convex ventral surface.

Supplementary diagnostic characters based

on the hypodigm

Cranial

Dentary ramus elongate and gently arched anteriorly; diastema comparatively short: two to three crown widths; coronoid process short and orientated at an oblique angle to the long axis of the dentary (N.B. This latter feature may be the consequence of breakage and subsequent restoration of the original specimen).

Dental

Dentary crowns are large, shield-shaped, and thickly enamelled on the lingual surface; marginal denticles on the mesial and distal margins of the crown form curved, oblique ledges that are mammillated; welldefined primary ridge offset distally on the lingual surface; the mesial sector of the crown has a low, broad mound that runs parallel to the primary ridge and is traversed by numerous, irregular, and strand-like accessory (tertiary) ridges*.

Axial

Dorsal and anterior caudal vertebrae have narrow, very elongate and obliquely inclined neural spines; the bases of anterior caudal neural spines are flanked by buttresses on either side of median anterior and posteri- or ridges*; dorsal centra have unusually thickened articular rims*; midcaudal vertebrae exhibit a ventral midline sulcus*.

Appendicular

Sternal plates have a broad, apron-like posterior edge to the ‘blade’*; the ‘handle’ portion of the sternal plate is robust and dorsoventrally flattened; calcification of the intersternal cartilage (leading to co-ossification of the sternal bones) occurred in ontogenetically mature specimens*; pollex ungual large, pointed, triangular in lateral profile, laterally compressed (rather than conical) and curved slightly palmwards along its length*; pollex claw grooves present; pubic shaft has a circular cross-section; lateral surface of the proximal end of the ischial shaft (adjacent to the obturator process) forms an elongate flattened facet*; ischial shaft comparatively short, stout, J-shaped and terminates in an anteriorly expanded ischial boot.

The holotype of Hypselospinus fittoni

( Lydekker, 1889)

Dentition

Three partial teeth are included in the material registered as NHMUK R1635 ( Fig. 5C ). One appears to be a very heavily worn (shed and subsequently eroded) dentary tooth stump ( Fig. 5C 1 ) whereas the other two are functional (worn) maxillary crowns of differing size. Figure 5C 2 appears to be a shed left crown in a state of advanced wear. The smaller right maxillary crown ( Fig. 5C 3 ) probably comes from either the mesial or distal ends of the maxillary ‘magazine’ (where teeth are normally smaller than those positioned nearer to the centre of the array). The maxillary crowns offer little morphological information beyond a similarity to that seen in ankylopollexians generally ( Norman, 1986: fig. 22): crowns are narrow and lozengeshaped; very prominent distally offset primary ridge (p); mesial sector of the labially enamelled face marked by a small number of narrow, subparallel accessory (tertiary) ridges (r); transversely thickened mesial and distal edges to the crown; and longitudinally channelled roots (ch).

Axial skeleton

Sacrum: The sacral material comprises the eroded remains of three fused posterior sacral centra including portions of their sacral ribs ( Fig. 7A–C ). The specimen is iron-stained, poorly consolidated, and appears not to be heavily permineralized. The most posterior sacral centrum has a smooth, shallow, rounded, and concave posterior articular face; the main body of the centrum is spool-like, being mildly contracted around its midlength whereas its ventral surface is pinched transversely to form a smoothly rounded ventral keel. The keel, in lateral view, appears to be slightly arched. The base of the sacral rib is fused at midheight on the centrum alone, rather than having its base encroaching on the sutured articulation with the preceding centrum (as seen in more anterior sacral ribs). The neural arch is similarly confined to the dorsal surface of the centrum and the sacral rib is fused to the lateral wall of the neural arch as well as the centrum. These features (smooth posterior articular surface, positioning of the neural arch, and sacral rib relative to the centrum) confirm that this was the last in the sacral series. The penultimate sacral is badly eroded but similarly spool-shaped and it is clear that the sacral rib was more anteriorly positioned so that its base was fused across the junction between its own vertebra and that of the preceding vertebra; the base of the neural arch also overlaps the dorsal edge of the preceding centrum. The preceding vertebra displays the spoolshape of the centrum, a keel, and the eroded portions of the sacral rib and neural arch (which are similarly intervertebrally positioned). The three fused sacrals diminish progressively in overall dimensions anteriorly.

Although difficult to interpret, this sacral block differs from a specimen attributed to B. dawsoni (NHMUK R3789 – Norman, 2011a) in the following characters: substantially smaller size, positioning of the last sacral rib on the side of the centrum rather than intervertebrally, reduced prominence of the ventral keel, lack of arching of the keel, and less pronounced thickening at the fused intervertebral junctions.

Caudal vertebra: The anterior caudal centrum ( Fig. 8A– C ) is approximately commensurate with those of the sacrum (allowing for its more posterior position along the tail) and its general preservational state is similar. The vertebra is almost cylindrical and its sides only slightly contracted between the articular margins. The centrum is very slightly forwardly inclined and there is a prominent posterior haemapophysis (chevron facet), with little development of a discrete anterior facet (although such anterior facets are, as a general rule, less prominent). The ventral surface of the centrum is broadly convex, with no indication of either a midline keel or sulcus. The articular faces of the centrum display a swollen rim that encloses a very shallow central concavity. The caudal ribs (cr), broken at their bases on both sides, are positioned along the line of the neurocentral suture and appear to have been well developed: a feature seen specifically in anterior caudal vertebrae.

This caudal could not be identified and compared with the direct serial equivalent of one of the caudals of the sympatric contemporary B. dawsoni , but it differs substantially in size, structure, and proportions from those of the latter taxon ( Norman, 2011a).

Appendicular skeleton

Ilium: Although somewhat eroded and broken in places, and apparently lacking most of the preacetabular process that was illustrated by Lydekker (1890a: reproduced in Fig. 5 ), the ilium appears to be relatively little distorted ( Figs 3A, B , 9 ). The general preservation is very similar to that described in the sacrum and caudal. The preacetabular process (prp) is laterally compressed and its dorsal edge is flattened, whereas the ventral border is slightly thicker and smoothly rounded transversely. The lateral surface of the preacetabular process is shallowly concave dorsoventrally, whereas the medial surface is equivalently convex and there is a low, oblique ridge (mr) medioventrally that is associated with a shallow rugose depression; this indicates a probable area of contact with the distal end of the ‘free’ rib of the sacrodorsal vertebra (not preserved). Compared with B. dawsoni , the preacetabular process differs substantially in size, shape, and proportions ( Figs 3 , 9 ). The main portion of the iliac blade stands essentially vertically and its lateral surface is shallowly concave; the dorsal edge is narrow and flattened ( Fig. 9B , fdm). The dorsal edge and its muscle scar may have expanded slightly in the region above and behind the ischiadic peduncle, but this area is broken ( Fig. 9A , cross-hatching) and interpreted by reference to NHMUK R1834 ( Fig. 46 ). Posteriorly, the dorsal edge inclines posteroventrally before merging with a transversely thickened shelf at the posterior end of the iliac blade ( Fig. 9C ). This shelf reflects the abrupt medial deflection of the ventral portion of the iliac blade, which forms a shallow arched brevis fossa (brf) bounded by a distinct lateral ridge (lr). The medial edge of the brevis fossa curves ventrally and forms a thin sheet of bone that is visible in lateral view ( Fig. 9A ).

The ventral margin of the postacetabular process is sinuous and oblique, merging with the expanded ischiadic peduncle anteriorly. The latter, although somewhat eroded, expands laterally to form a stepped boss: having a prominent posterodorsal eminence that is separated – step-wise – from a flatter bevelled area adjacent to the acetabulum. The ischiadic sutural surface is obliquely offset (facing posteroventrally – Fig. 9A ). The dorsal margin of the acetabulum curves smoothly into the lateral surface of the iliac blade, although the remnant of the pubic peduncle (pp) shows that there was a distinct supra-acetabular crest (sac) developed as a ledge along the margin of that peduncle. The pubic peduncle has been sheared off, thereby obscuring its overall appearance and orientation. The medial surface of the ilium ( Fig. 9D ) has a midheight horizontal ridge punctuated by a line of thumbprint-like depressions; these mark the attachment points for the sacral transverse processes and dorsal parts of the sacral ribs. Beneath this ridge the surface is smooth before developing into a broader and more continuously scarred area (sy) for attachment of the sacral yoke (formed by the coalesced ventral portions of the sacral ribs). The posterior sacral rib scars are conjoined (srf), indicating the region where the sacral yoke and ventral portions of the sacral ribs have coalesced.

The orientation of the articular surface for the ischium (posteroventral) and the positioning of the supraacetabular crest (restricted almost exclusively to the pubic peduncle) suggest that ‘in life’ the ilium was articulated against the sacrum and orientated such that its dorsal edge was inclined posterodorsally so that the pubic peduncle, supported medially by a very robust first sacral rib, formed the dorsal rim of the acetabulum.

Ischium: Comprising the proximal end only of a (comparatively) small left ischium ( Fig. 10 ), this bone is missing almost all of its features; both peduncles have been broken off and worn smooth, the obturator process (obt) can be inferred only from the curvature of the preserved bone, the shaft is almost entirely missing, and its stump is worn smooth. The preservation of this specimen is such that it is permineralized, appears not to be strongly iron-stained, and has been very waterrolled. The suggested association of this specimen with the earlier-described specimens is regarded as conjectural at best, but as it contributes nothing to the determination of this taxon it can be disregarded safely.

Figures

Figure 1. The location of the town of Hastings within the county of East Sussex in the UK. Quarries (Q) and location names indicate sites that are known to have yielded Iguanodon-like ornithopod remains (most of which were collected by either Samuel H. Beckles or Charles Dawson). The ambiguity surrounding the location of ‘Hollington Quarry’ is genuine and reflects inconsistencies and vagueness introduced (probably by Dawson) when the documentation of these collections was being assembled in the late 1880s. Abbreviation: IoW, Isle of Wight.

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 3. Comparative ilia. A, Iguanodon (= Hypselospinus) fittoni NHMUK R1635, holotype in lateral view; B, NHMUK R1635 in medial view; C, Iguanodon (= Barilium) dawsoni NHMUK R802, holotype in lateral view; D, NHMUK R802, holotype in medial view. Abbreviations:?brf, brevis fossa non-existent (the area forming a smoothly curved overhang); brf, brevis fossa; lr, lateral ridge that demarcates the brevis fossa;?mr, medial ridge (much reduced); mr, medial ridge very prominent and forming an overhanging ledge; prp, preacetabular process; srf, sacral rib facet (prominent medially and clearly visible laterally). Scale bars = 10 cm.

Figure 4. Iguanodon hollingtoniensis (= Hypselospinus fittoni) holotype. NHMUK R1148. A, femur (right) nearly complete but crushed, as illustrated by Lydekker (1890a) (a, articular head; b, anterior trochanter; c, fourth trochanter, with exaggerated pendant tip; d, lateral distal condyle; e, medial distal condyle); B, C, the original specimen as preserved (May 2011) in dorsal and ventral views, respectively – the ventral view reveals the extent of longitudinal crushing postmortem. Abbreviations: 4t, fourth trochanter; at, anterior (lesser) trochanter; cr, crushing of the dorsal part of the medial condyle; icg, anterior intercondylar groove. Scale bar = 10 cm.

Figure 5. Iguanodon (= Hypselospinus) fittoni. Holotype. NHMUK R1635. The ilium as illustrated in Lydekker (1890a: fig. 1C). A, lateral; B, ventral portion of the postacetabular process. The original illustration indicates the existence of a separate anterior portion of the preacetabular process; this latter part has not been found in the collections of the NHMUK since it was first looked for in 1974. C, teeth with the same registered number as the holotype and, although not mentioned before, presumably associated: 1. The stump of a heavily worn dentary tooth; 2, 3. Worn maxillary crowns in labial view. Abbreviations: brf, brevis fossa; ch, channel for replacement crown; p, primary ridge; prp, preacetabular process; r, subsidiary ridges. Scale bar = 10 mm.

Figure 7. Iguanodon (= Hypselospinus) fittoni. Holotype NHMUK R1635. Partial sacrum. A, dorsal; B, lateral; C, ventral. Abbreviations: k, ventral keel, ln, aperture for lateral spinal nerve; na, broken base of the neural arch positioned suprarather than intervertebrally on the last sacral centrum; sr, sacral rib remnants. Scale bar = 10 cm.

Figure 8. Iguanodon (= Hypselospinus) fittoni. Holotype NHMUK R1635. Anterior-middle caudal centrum in: A, anteri- or; B, lateral; C, posterior views. Abbreviations: cf, chevron facet; cr, caudal rib (eroded basal portion). Scale bar = 10 cm.

Figure 9. Iguanodon (= Hypselospinus) fittoni. Holotype NHMUK R1635. Ischium (right) proximal end, eroded. Abbreviations: ac, acetabular margin; obt, obturator process (eroded base). Scale bar = 10 cm.

Figure 10. Hypselospinus fittoni. Holotype NHMUK R1635. Ilium illustrated in A, lateral; B, dorsal; C, posterior; D, medial views. Abbreviations: brf, brevis fossa; ip, ischiadic peduncle with laterally stepped surface; lr, lateral ridge that demarcates the brevis fossa; pth, probable pathology; srf, sacral rib facets; sy, scarred area for attachment of the sacral yoke. Shading: cross-hatching indicates broken surfaces. Scale bar = 10 cm.

Figure 46. Hypselospinus cf. fittoni. NHMUK R1834. Ilium (right, nearly complete). A, lateral; B, ventral view of postacetabular process; C, dorsal; D, medial. Abbreviations: ac, acetabulum; bl, abnormally truncated preacetabular process; brf, brevis fossa; fdm, flattened, narrow dorsal margin of the iliac blade; ip, ischiadic peduncle; lr, lateral ridge; mr, medial ridge; m.sc, blister-like strip of rugose tissue running parallel to the dorsal margin of the ilium; pp, pubic peduncle; prp, preacetabular process; th, dorsoventral thickening of the blister-like muscle scar. Scale bar = 10 cm.