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 : 117-122

HYPSELOSPINUS FITTONI

1. NHMUK R604 & R604a

A partial skeleton collected by Dawson from Hollington quarry (old specimen cards associated with this collection of bones indicate that it was collected at Ridge Farm quarry). Most of this collection represents the vertebral column: one cervical centrum and fragments of a neural arch, 12 dorsal vertebrae, 16 caudal vertebra, several fragmentary ribs, and the proximal end of a chevron bone. Other associated remains include: three worn and somewhat damaged maxillary crowns, a well-preserved pollex ungual, a partial ulna, and some bones of the pes. The specimen was initially referred to I. dawsoni by Lydekker (1888a, b), but a little later Lydekker (1889: 355) transferred it, without explanation, to I. hollingtoniensis .

Axial skeleton

The cervical fragments exhibit typical features such as opisthocoely of the centrum, a thick ventral keel, and a parapophysis located, just posterior to the margin of the convex anterior articular surface, at midcentrum height on a raised ridge located.

Dorsal vertebrae: The dorsal vertebrae are mostly well preserved and comprise a series of 12 (close to a complete dorsal vertebral count of 16). The numbering system adopted here is for guidance only.

The most anterior of the preserved series is probably a first or second dorsal (d1/2; Fig. 20A–C ). It retains a number of cervical morphological attributes: a low broad centrum, with a thick ventral keel and opisthocoely (and a modest convex anterior articular surface). Crucially (for positioning in the series), it has a large oval parapophysis (par) on the ventrolateral surface of the neural arch pedicel (clearly above the neurocentral suture – ncs). The transverse processes are robust and angled obliquely dorsolaterally. The prezygapophyses (prz) are separated from the midline by a shallow embayment and do not project forward; this is a standard configuration seen in cervicals ( Fig. 19A, B ). The pedicels that support the postzygapophyses (poz) are elongate and therefore overlap the succeeding vertebra substantially and the neural spine (ns) is posterodorsally inclined. Unfortunately, the spine is broken so its actual length is unknown. Neural spine length may have been substantial, judged by the shape of its base, and the spine length attained by succeeding dorsals).

The next in the series is probably a third dorsal (d3; Fig. 21 ). It resembles the former in that the centrum is comparatively low and broad and it retains slight opisthocoely, although its anterior face is slightly concave ( Fig. 21C ). The ventral keel (k) is thick, albeit narrower than the previous example ( Fig. 20 ). The parapophysis (par) is smaller and positioned higher on the neural arch pedicel above the neurocentral suture (ncs) than in the previous example. The prezygapophyses (prz) project anteriorly, are closer together on either side of the midline and the articular faces are more steeply inclined ( Fig. 21C ). The transverse processes are robust, elongate, and dorsolaterally directed, terminating in a well-developed tubercular facet (dia); the anteroventral surface of the transverse process is scarred (rs) by ligaments that helped to anchor the neck of the dorsal rib. The postzygapophyses (poz) do not overlap the succeeding vertebra so extensively as in the previous example and the neural spine (ns) is little damaged, showing it to have been remarkably tall, slender, and obliquely inclined (rising to a rugose, slightly expanded, apex).

The fourth dorsal (d4; Fig. 22 ) is less complete, but continues the morphological transition: the centrum is taller than wide, the ventral keel (k) is narrower ( Fig. 22B ), and the anterior articular surface of the centrum is gently concave ( Fig. 22C , the posterior half of the centrum is not preserved). The parapophysis is positioned higher on the pedicel, so that its upper border is now adjacent to the top edge of the prezygapophysis ( Fig. 22A ) and the transverse processes are massive and ligament scarred (rs) but less upswept than in the previous example.

The seventh/eighth (d7/8) and ninth/tenth (d9/10) dorsals ( Fig. 23 ) have centra of a more rectangular outline and smaller, more rounded parapophyses (par) compared with previous examples. The parapophysis (par) can also be seen to commence its lateral migration along the transverse process. These centra have a narrow keel and have shallowly concave articular surfaces [and the seventh/eighth example ( Fig. 23A ) is most similar to the dorsals of the holotype NHMUK R1148 : Fig. 12 ]. Judged by their shape these centra (particularly d7/8) resemble ‘keystones’ at the centre of the span of an arched dorsal series. Centrum (d9/ 10 – Fig. 23B ) leans more posteriorly and has rather thicker and more prominent articular margins .

The ninth/tenth dorsal ( Fig. 23B ) includes a substantial portion of its neural spine. The transverse process is less robust. The centrum is has thickened, rugose articular margins.

The most posterior dorsals (in the range d13−16: Fig. 24 ) have substantially larger, almost circular, articular faces; the anterior articular face of the centrum is shallowly concave, whereas the posterior face has become more obviously opisthocoelous. The articular margins of the centra form thickened rims that are more flared than previous examples. The centra also lean posteriorly. The last preserved dorsal (probably d16) has a more regular rectangular profile ( Fig. 24 C−C3) and is anteroposteriorly compressed compared with the previous two examples, and has an almost circular articular face (C1). A ventral keel (k) is present in the first two examples, but is lost in the most posterior in the series ( Fig. 24C 2 ). The parapophyses (par) are small, forming something akin to a ‘notch’ on the leading edge of the transverse processes. The transverse processes are less robust than earlier dorsals, horizontally directed as well as twisted along their length such that the dorsal surface faces anterodorsally ( Fig. 24 ).

Associated dorsal ribs: A few examples of partial dorsal ribs (NHMUK R604a) are illustrated ( Fig. 25 ). The larger examples ( Fig. 25A, B ) are representative of those from the anterior of the dorsal series. They have robust shafts with a well-marked longitudinal ridge (ar) running down the anterolateral margin; this probably reflects the attachment area for the intercostal ligaments and musculature. The articular rib heads [the capitulum (cap) and tuberculum (tub)] are prominent and separated by a distinct ligament-scarred neck (n – reflecting the wide separation of parapophyses and diapophyses seen in the anterior dorsal series). Two more posterior dorsal ribs ( Fig. 25C, D ) are preserved and have more slender rod-shaped shafts and rib heads that are smaller and connected via a ligamentscarred ridge; this shows that the entire articular region (incorporating capitulum, tuberculum, and intervening neck) was securely fastened to its transverse process.

Caudal vertebrae: The caudals in this collection include examples from the anterior, middle, and posterior sections of the tail, each of which have their own distinctive features that are generally indicative of progressive changes in shape along the length of the tail. The anterior caudals ( Figs 26 , 27 ) include one with an intact neural spine of considerable height.

The most anterior caudal preserved (c2) has, when compared with others in the series, a relatively elongate centrum ( Fig. 26A ), which is slightly anteriorly inclined (more so dorsally). Its anterior articular face ( Fig. 26A 1 ) exhibits a modest convexity dorsally and shallow concavity ventrally (which is similarly reflect- ed in the morphology of the posterior face: Fig. 26A 2 ). The centrum lacks an obvious haemal arch facet anteriorly, but a slight crease on the posteroventral rim ( Fig. 26A 3 ) may indicate a haemapophysis (articular facet for a diminutive first haemal arch). The ventral surface of the centrum displays a pair of shallow sulci separated by a smooth midline keel and flanked lat- erally by similarly smooth ridges ( Fig. 26A 3 ). The neural spine is broken off, but the prezygapophyses (prz) are anterodorsally directed prongs ( Fig. 26A 4 ). The neural arch is squat and has very thick pedicels that enclose a relatively narrow neural canal. The pedicels flare laterally where they are fused to the bases of robust caudal ribs (cr), which are also sheared off.

Succeeding caudals ( Figs 26B – 28 ) show a graduat- ed series of changes: the centra become initially more axially compressed, the chevron facets (cf) become far more prominent on the anterior and posterior ventral rims, and the articular faces of the centrum tend to shift from an almost circular outline to more dorsoventrally elongate ( Fig. 27 ). One of these caudals (?c5; Fig. 27 ) is well preserved, apart from relatively minor fracturing, displaying the full development of the caudal rib and structure of the neural spine. The latter is very elongate, slightly sinuous in profile and leans posteriorly; the lower half of the spine has thickened lateral flanks that are separated by grooves from midline ridges anteriorly and posteriorly (asr, psr).

Farther behind the anterior caudals, the centra become more elongate, and have less oblique prezygapophyses ( Fig. 28 ) and progressively less prominent caudal ribs. Later caudals become generally more rectangular in form and lose the prominent anterior chevron facet, as they also lose the caudal rib, which becomes reduced to a ridge on the side of the centrum. Posterior caudals ( Fig. 29 ) become lower, lose the elongate neural spine, and, in proportion, their centra become more elongate and develop a hexagonal crosssection and a shallow ventral midline sulcus; these features are well displayed in NHMUK R1148 (R1632; Fig. 29B, C ). The middle and posterior caudals of

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 12. Hypselospinus cf. fittoni (Iguanodon hollingtoniensis holotype). NHMUK R1148. A, B, dorsal centra as preserved in lateral view. A1, centrum A illustrated and pseudo-articulated with a neural arch; A2, centrum and neural arch in ventral view; B1, similar pseudo-articulation and B2, ventral view of the same. Hatching indicates broken surfaces. Abbreviations: dia, diapophyseal facet; k, keel; par, parapophysis; poz, postzygapophysis; prz, prezygapophysis. Scale bars = 10 cm.

Figure 19. Hypselospinus cf. fittoni (Iguanodon hollingtoniensis holotype). NHMUK R1148 (R1632). Cervical vertebrae. A−C, anterior cervical vertebra in lateral, dorsal, and anterior views, respectively (NB: image A is of the right-hand side and has been reversed). D−F, more posterior cervicals, demonstrating the increasing depth of the centrum and the enlargement of the parapophyseal facet. Abbreviations: dia, diapophysis; k, thick midline keel; par, parapophysis; poz, postzygapophysis; prz, prezygapophysis. Hatching indicates broken bone. Scale bar = 10 cm.

Figure 20. Hypselospinus cf. fittoni. NHMUK R604. First or second dorsal. A, lateral; B, anterior; C, ventral. Abbreviations: dia, diapophysis; k, midline keel; ncs, neurocentral suture; ns, neural spine; par, parapophysis; poz, postzygapophysis; prz, prezygapophysis. Scale bar = 10 cm.

Figure 21. Hypselospinus cf. fittoni. NHMUK R604. Third dorsal. A, A1, lateral (A is a reversed image of the right side); B, ventral; C, anterior. Abbreviations: dia, diapophysis; k, midline keel; ncs, neurocentral suture; ns, neural spine; par, parapophysis; poz, postzygapophysis; prz, prezygapophysis; rs, rugose surface for ligamentous attachment of the neck of the rib. Scale bar = 10 cm.

Figure 22. Hypselospinus cf. fittoni. NHMUK R604. Fourth dorsal. A, lateral; B, ventral; C, anterior. Abbreviations: dia, diapophysis; k, midline keel; par, parapophysis; poz, postzygapophysis; prz, prezygapophysis; rs, rugose surface for ligamentous attachment of the neck of the rib. Scale bar = 10 cm.

Figure 23. Hypselospinus cf. fittoni. NHMUK R604. Middle dorsals (range: 7–10). A, (seventh) lateral; A1, ventral; A2, anterior. B, (ninth) lateral; B1, ventral; B2, anterior. Abbreviations: dia, diapophysis; k, midline keel; par, parapophysis; rs, rugose surface for ligamentous attachment of the neck of the rib. Scale bar = 10 cm.

Figure 24. Hypselospinus cf. fittoni. NHMUK R604. Posterior dorsals (range: 14–16). A, A1, A2, 14th dorsal in lateral, ventral, and dorsal views, respectively. B, B1, B2, 15th dorsal in lateral, ventral, and dorsal views, respectively. C, C1, C2, C3, 16th (last free dorsal) in lateral, anterior, ventral, and dorsal views, respectively. Abbreviations: dia, diapophysis; k, midline keel; par, parapophysis; prz, prezygapophysis. Scale bar = 10 cm.

Figure 25. Hypselospinus cf. fittoni. NHMUK R604a. Dorsal ribs. A, right anterior; B, right middle; C, right middleposterior; D, right posterior. Abbreviations: ar, anterior curved ridge (for intercostal ligaments/muscles); cap, capitulum (rib head); n, neck of the rib; tub, tuberculum. Scale bar = 10 cm.

Figure 26. Hypselospinus cf. fittoni. NHMUK R604. Anterior caudals (range: 1–4). A, A1−A4, lateral, anterior, posteri- or, ventral, and dorsal views, respectively. B, B1, B2, lateral, anterior, and posterior views, respectively. Abbreviations: cf, chevron facet; cr, caudal rib; poz, postzygapophysis; prz, prezygapophysis; sul, sulcus. Hatching indicates broken bone. Scale bar = 10 cm.

Figure 27. Hypselospinus cf. fittoni. NHMUK R604. Anterior caudal (range: 3–5). A, A1−A4, lateral, anterior, posterior, ventral, and dorsal views, respectively. Abbreviations: asr, anterior slot-and-ridge margin to the neural spine; cf, chevron facet; cr, caudal rib; poz, postzygapophysis; prz, prezygapophysis; psr, posterior slot-and-ridge margin to the neural spine. Scale bar = 10 cm.

Figure 28. Hypselospinus cf. fittoni. NHMUK R604. Anterior-middle caudals (range: 7–13). A, A1, A2, lateral, anterior, and ventral views, respectively. B−E, lateral views of typical anterior-middle caudal vertebrae. Caudal ribs and chevron facets well-developed, ventral surface of the centrum convex transversely. Abbreviations: asr, anterior slot-andridge margin to the neural spine; cf, chevron facet; cr, caudal rib; psr, posterior slot-and-ridge margin to the neural spine. Scale bar = 10 cm.

Figure 29. Hypselospinus cf. fittoni. NHMUK R1632. Middle and posterior caudals. A, A1, A2, middle caudal centrum in lateral, ventral, and anterior views respectively. B, C (from top to bottom), posterior caudal centra in lateral, ventral, and anterior views. Abbreviations: cf, chevron facet; sul, midline sulcus. Scale bar = 10 cm.