Hypselospinus fittoni ( Lydekker, 1889 ) Lydekker, 1889

Norman, David B., 2010, A taxonomy of iguanodontians (Dinosauria: Ornithopoda) from the lower Wealden Group (Cretaceous: Valanginian) of southern England, Zootaxa 2489, pp. 47-66 : 55-64

Hypselospinus fittoni ( Lydekker, 1889) comb. nov.

Nomenclatural history/synonymy

Iguanodon fittoni Lydekker, 1889: 354 .

Iguanodon hollingtoniensis Lydekker 1889: 355 .

Iguanodon fittoni [ Lydekker 1890a: 38].

Iguanodon hollingtoniensis [ Lydekker 1890a: 40].

Iguanodon fittoni Lydekker 1889 [ Lydekker, 1890b]. Iguanodon hollingtoniensis Lydekker 1889 [ Lydekker 1890b]. Iguanodon fittoni Lydekker, 1889 [in Steel 1969: 18]. Iguanodon hollingtoniensis Lydekker, 1889 [in Steel 1969: 19]. Iguanodon fittoni Lydekker, 1889 [in Norman 1987:164]. Iguanodon fittoni Lydekker, 1889 [in Norman & Weishampel 1990: 530]. Iguanodon fittoni Lydekker, 1889 [in Blows 1998: 31]. Iguanodon fittoni Lydekker 1889 [in Norman 2004: 416]. “ Iguanodon ” hollingtoniensis [ Paul 2008: 214].

“ Iguanodon ” hollingtoniensis [ Galton 2009: 245].

Holotype. NHMUK R 1635, ilium ( Fig . 5) and associated elements ( Fig. 6 ) comprising a partial posterior sacrum ( Fig. 6 A–C), anterior caudal centrum ( Fig. 6 D, E) and (more dubiously) a water-worn proximal ischial fragment ( Fig. 6 F).

Referred material. The holotype of Iguanodon hollingtoniensis Lydekker , which includes the following registered material: NHMUK R1148 (femur, Fig. 7 ), R1629, R1632, R811 (sacrum, Fig. 8 A, B), R811b (ilium, Fig. 8 C, D), R604 ( Fig. 9 ). Additional referred specimens include NHMUK R1834, R33, R1627, R1636 (including specimens accessioned as R2357, R2358), and R1831 ( Fig. 10 ), which includes specimens accessioned as NHMUK R1832, R1833, R1835, and R1836. Additional referred material will be described elsewhere.

Locality and horizon of holotype. Quarry site at the village of Shornden, near Hastings, East Sussex (the same locality that yielded the type material of B. dawsoni ). Hastings Sub-Group, Wadhurst Clay Formation (Valanginian). Referred material was also recovered from the Wadhurst Clay Formation.

Diagnosis based on the holotype. Iguanodontian with the following unique character combination (* indicates autapomorphies - all other characters are apomorphic within basal iguanodontians, even if they occur sporadically within ornithopods more generally),), summarised by element:

Ilium ( Fig. 5 ): preacetabular process (prp) vertically orientated, laterally compressed and comparatively deep proximally*; ventral edge of the preacetabular process is thicker than the dorsal edge*; a low-relief, curved, ridge is present on the medial surface of the preacetabular process; the main portion of the iliac blade is flat; the dorsal edge of the iliac blade is transversely compressed* ( Fig. 5 D, nde); the dorsal edge of the central portion of the iliac blade, in profile view ( Fig. 5 A), is straight (rather than being bowed dorsally)*; the postacetabular blade tapers to a blunt, thickened bar ( Fig. 5 A,B); strong medial inturning of the postacetabular blade to form a thick shelf that roofs a well-defined brevis fossa (brf); the brevis fossa is bordered laterally by a horizontal ridge (lr); the scarring for the posterior sacral ribs ( Fig. 5 C, srs) follows the ventral margin of the postacetabular blade as it rises obliquely toward the posterior end of the blade to merge with the more dorsally positioned facets along the lower border of the postacetabular blade.

Vertebrae ( Fig. 6 ): the ventral surfaces of the posterior centra of the sacrum bear midline keels*; midanterior caudal vertebral centrum ( Fig. 6 ), which bears a well-marked haemal facet posteriorly, has cylindrical proportions*.

Comparisons with closely related iguanodontians. This taxon appears (to date) to be restricted to the Valanginian stage and is representative of a medium-sized (body length of ~6 metres) and robust iguanodontian ornithopod. Comparisons will be drawn with the earlier (Kimmeridgian–Berriasian) camptosaur-grade iguanodontians, the exactly contemporary taxon Barilium dawsoni (see above), and the younger (Hauterivian–Lower Aptian) and geographically local Iguanodon bernissartensis and Mantellisaurus atherfieldensis .

Camptosaurs. The base of the preacetabular process is laterally compressed and deep with little evidence of axial twisting in both Hypselospinus fittoni and camptosaurs ( Gilmore 1909: figs 29, 30, pl. 14); however, the dorsal margin of this process is thickened and rounded transversely in camptosaurs, unlike H. fittoni . The dorsal edge of the main iliac plate is horizontal above the acetabular region in both taxa; however, unlike H. fittoni , this border in camptosaurs develops an oblique (dorsolaterally orientated) bevelled surface that becomes steadily deeper over the postacetabular blade and increasingly laterally everted so that the upper part of the blade comes to overhang the ischial peduncle and brevis fossa regions. In H. fittoni the dorsal edge of the iliac blade remains narrow and vertically orientated along its length; toward the posterior end of this dorsal edge, in camptosaurs, the thickened bevelled surface forms a small dorsally deflected lip, before descending obliquely to the medially inflected posterior bar which, as in H. fittoni , forms the roof to a substantial brevis fossa. The brevis fossa is bounded medially by a thin wall of the ilium in camptosaurs that is clearly visible in lateral aspect (this is far more prominent than the small medial ‘lip’ seen in H. fittoni : Fig. 5 A), but the lateral ridge that encloses the brevis fossa is similar in both taxa. Camptosaur ilia have a dorsal edge that becomes increasingly thick transversely and forms a bevelled structure that increases steadily in size posteriorly; the lateral surface of the iliac blade above the ischial peduncle also curves laterally so that it overhangs the ventral part of the ilium. The sacrum of camptosaurs lacks a ventral keel ( Gilmore 1909), which is present in H. fittoni .

Barilium dawsoni . The ilium of B. dawsoni is larger and more massive than that of H. fittoni (compare Figs 3 , 5 ). The form and orientation of the preacetabular process is completely different, as is the outline of the dorsal margin, as well as the structure of the postacetabular blade in all respects. The only anatomical similarity, in a very general sense, is that the iliac blade is essentially flat, and appears to lie vertically against the sacral yoke, rather than having a blade that curves laterally towards its dorsal margin so that it overhangs the lower portion of the blade. The dorsoventrally compressed (subrectangular) structure of anterior caudals of B. dawsoni differs strikingly from the form of the anterior caudals in H. fittoni .

Iguanodon bernissartensis . The ilium of I. bernissartensis differs significantly from that of H. fittoni in the form of the preacetabular process (which is very robust, broad and essentially triangular in cross-section: Norman 1980: Fig. 64), the bowed and ultimately sinuous dorsal margin, the dorsal portion of the iliac blade being thickened transversely and expanded laterally to overhang the lower part of the blade, and the postacetabular process is narrow and tapering and far more elongate (proportionally) than is the case in H. fittoni .

Mantellisaurus atherfieldensis . The ilium of M. atherfieldensis differs from that of H. fittoni in its curved dorsal margin, the progressive thickening and expansion of the dorsolateral portion of the iliac blade ( Norman 1986: figs 53, 54) and the comparatively narrow brevis fossa.

Taxonomic discussion. Hypselospinus fittoni represents a smaller but relatively robustly constructed iguanodontian that was a sympatric contemporary of Barilium dawsoni (above). As with the latter species, H. fittoni appears, on present evidence, to be restricted to the Valanginian. The holotype material is far from ideal (based as it is on a small fraction of the skeleton), but it is nevertheless diagnostic, as proposed by Lydekker (1889). Of considerable interest (and importance) in relation to our understanding and interpretation of these Wadhurst Clay Formation species is the status of other iguanodontian material that was originally referred to a third species, Iguanodon hollingtoniensis , by Lydekker (1889).

Iguanodon hollingtonensis was based on skeletal remains recovered from Little Ridge Quarry in the village of Hollington, near Hastings (the quarry also yielded portions of a large theropod and an ankylosaur at around the same time). The iguanodontian skeleton was articulated, but was excavated over a period of two years and donated to the British Museum in batches (as a consequence a variety of registered numbers were attributed to the holotype) that are re-united here.

The new species was established upon a nearly complete, but somewhat distorted, femur (NHMUK R1148, Fig. 7 ). Lydekker (1890a) claimed that this differed from a femur assigned by him to I. dawsoni (NHMUK R1627); unfortunately, the femur to which Lydekker was referring is so badly damaged that no meaningful comparisons are possible. In addition, Lydekker argued that the sacral vertebrae of I. hollingtoniensis (registered under numbers NHMUK R811 and R1632) could be distinguished from those assigned to I. fittoni ( NHMUK R1635, Fig. 6 ), because the former had sacral centra that were flattened ventrally and not co-ossified. In fact the flattened and unfused sacrals of NHMUK R1632 identified by Lydekker proved to be broken fragments of several cervical vertebrae, while NHMUK R811 ( Fig. 8 A,B) is a post-mortem dorsoventrally flattened set of fused sacrals that exhibit the remnants of a ventral midline keel ( Norman 1977). It was also claimed that the fragmentary ilium (NHMUK R811b, Fig. 8 C,D) associated with the holotype skeleton differed (in an unspecified way) from the holotype ilium of I. fittoni ( NHMUK R1635). However, the features that can be observed in NHMUK R811b demonstrate clear and diagnostic affinities with the holotype ilium of I. fittoni (compare Figs 6 , 8 ). A summary by Lydekker (1890a) of this new taxonomic assessment included illustrations of various “iliac types ” that were intended to draw clear distinctions between new and established iguanodontian taxa. Unfortunately the ilium of I. hollingtoniensis that was illustrated was that of NHMUK R1636 (not the holotype as claimed in the text: Lydekker 1890a: 42).

In summary, the holotype material attributed to the taxa I. fittoni and I. hollingtoniensis are indistinguishable. The type material attributed to I. fittoni establishes the species and takes priority over I. hollingtoniensis by virtue of the sequence in which these two taxa appear in print; therefore, the latter species becomes a junior subjective synonym of I. fittoni . The material that is attributed to Iguanodon fittoni is renamed Hypselospinus fittoni ( Lydekker, 1889) comb. nov., based on the diagnosis provided above; in totality, the holotype and referred material represents the major part of the skeleton of an intermediate-sized styracosternan iguanodontian (Norman in preparation). The combination of the unusual form of the vertebral column, specialisations of the pelvic girdle and robust structure of the forelimb and manus, as well as the distinctive pattern of ridging on the enamelled surface of the crowns (below), serve to distinguish H. fittoni clearly and unambiguously from the other contemporary species ( B. dawsoni ) as well as the iguanodontian species that are known from younger horizons (Weald Clay Formation, Hauterivian–Lower Aptian) within the Wealden Group of southern England.

Anatomical discussion. Axial skeleton. The neural spines of the dorsal, sacral and proximal caudal series of vertebrae are very striking because of their oblique, narrow and elongate form ( Fig. 9 ). Cervical and dorsal centra have the shape of low cylinders, with notably thickened articular margins (em); viewed laterally, the cervicodorsal vertebral centra are tilted posteriorly, while proximal caudal centra are strongly anteroposteriorly compressed and have a vertical, rather than tilted, orientation, even though their neural spines are inclined posteriorly.

Appendicular skeleton. The forelimb ( Owen 1872: pl. I–III) and shoulder girdle are remarkably powerful and robust, and show evidence of partial fusion, some of which may have a pathological origin. The manus and carpus display fusion of components and also bear a very large, transversely compressed (triangular in lateral aspect) and very robust pollex ungual ( Owen 1872; Norman 1977, in preparation). Sternal plates appear to be fused in the midline in at least one example ( Norman 1977, in preparation). The femur is relatively stout and bowed, and has a thick, pillar-like, anterior trochanter; there is also a narrow dorsal exposure for the cylindrical anterior intercondylar groove.

A note on NHMUK R1831. A right dentary ( Owen 1874: pl. 1, fig. 1; see Figs 10 A, B) in the Samuel Husbands Beckles Collection is nearly complete in overall length, but very badly fractured and crushed. All of the functional dentition has also been broken away and many of the replacement crowns, notably at the anterior of the series, have also been lost by crushing and breakage. In medial view ( Fig. 10 A, B) the dentary symphysis (ds), though much broken, can be judged to have been essentially horizontal; there is a horizontal slot-and-lip toward its posterior end that formed part of the interlocking junction between adjacent dentaries. Farther anteriorly (beyond and just lateral to the symphyseal region), there is a short, finger-like projection (pr) that would have abutted the lateral edge of the mid-section at the root of the posteriorly bifid predentary flap; the lateral surface of the projection is smooth and forms a lip-like structure that curves posterodorsally leading to a compressed edge that backed on to the lateral, occlusal portion, of the predentary. After a short, smooth edge this surface becomes badly broken and irregular and the next third of the upper portion of the dentary ramus is eroded and broken away – indeed the great majority of the dorsal edge of the dentary is sheared off ( Fig. 10 B, br) with the consequent loss of the upper worn teeth, leaving some of the unworn replacement crowns located more posteriorly and ventrally within the dental ‘magazine’. The only remnant of the dorsal scalloped, and therefore true, alveolar margin (am) is preserved at the extreme posterior end of the preserved part of the dentition (even the coronoid process has been sheared off and re-adhered in an unnatural position – it is clear that preparation of the material subsequent to completion of the lithograph has taken place resulting in the removal of sediment and a dentary crown adhering to the coronoid process; Fig. 10 A, mt). The lateral surface of the dentary is very crazed and crushed; the outline of three alveolar scallops indicates the true position of the upper margin of the dentary and indicates that the dentary (despite appearances to the contrary) was deep and robust, in addition to being reasonably straight (see Figs 10 A, B).

The upper portion of the coronoid process only is preserved, but has been glued to the adjacent part of the dentary. As a result, the coronoid process ( Fig. 10 B) appears to be both shorter and more oblique than it would have been in its natural (and complete) condition. It seems that the friable condition of all the material collected meant that the specimen was very unstable at the time it was excavated and needed urgent and not necessarily expert on-site conservation/preparation. Medially, the dentary ramus displays a matrix-filled Meckelian fossa that becomes shallow and forms an indented horizontal groove ( Fig. 10 A, B, mgr) anteriorly; part of the sutural surface for the splenial is still preserved above the Meckelian groove on the medial wall of the dentary shelf that supported the dentition.

Form of the dentary and the ‘diastema’. In the lithograph provided by Owen (1874: pl. I, fig. 1; see Fig. 10 A), the form of the majority of the dentition is accurately illustrated; however, a small and clearly anomalous crown and root are shown projecting obliquely from the dentary adjacent to the most anterior securely embedded successional crown. This small tooth ( Fig. 10 A, at) is anomalous both in position and form; it appears to have no legitimate association with this jaw in this position, although it might have been found loose in sediments nearby (coming, as it may well have done, from closer to the anterior end of the magazine). When this lower jaw was re-examined and described (by the author) in the mid 1970s no trace of this anomalous tooth (or of the crown illustrated in the lithograph adhering to the side of the coronoid process) was discovered.

Without looking at the original material it would be easy to draw the mistaken conclusion from the illustration in Owen (1874) that this tooth indicates the approximate position of the anterior limit to the dental magazine and that the jaw has, as a consequence, a very substantial ‘diastema’ (viz. Paul 2008). The original illustration ( Owen 1874: see Fig. 10 A) reflects, uncritically, the remodelling that was done to this fragile and crushed specimen after it was first excavated.

General and comparative comments. In general shape and proportions NHMUK R1831 resembles that of the ‘Brickenden jaw’ (NHMUK 28660, Fig. 10 C) in the Mantell Collection. NHMUK 28660 is preserved ‘in the round’ and has suffered far less from general crushing and breakage than NHMUK R1831 – even though a large portion of its posterior dental shelf, anteromedial to the base of the coronoid process, has been sheared away ( Fig. 10 C, sh.alv).

The crowns, or remnants of crowns, of 15 teeth are preserved in varying states of eruption in NHMUK R1831 (see Fig. 10 A, B); none are part of the occlusal dentition, indicating significant loss/breakage across the upper portion of the dental magazine. In medial (= lingual, in terms of tooth orientation) view the enamelled faces of the crowns are slightly recurved, broad and lozenge-shaped, and fringed by curved, tongue-like marginal denticles forming oblique ledges that run around leading and trailing edges of the crown; the ledges formed by these denticles are irregularly mammillated. The base of the distal margin of the crown appears (as is the case in many known basal styracosternan iguanodontians) to have been inrolled to create a narrow ‘cingulum’ that is oriented obliquely toward the root (judged by the form of the broken base of one, potentially functional, tooth). The enamelled lingual face is dominated by a distally off-set primary ridge that is well-formed and has a distinctly flattened (truncated) ridge apex in the better preserved examples (Norman in press). In the more posteriorly positioned crowns within the ‘magazine’ there is a low, and indistinct, mesially positioned secondary ‘ridge’ or swelling that lies parallel to the primary one and divides the crown into two vertical and shallow troughs (with a third present between the primary ridge and the distal crown margin). The upper mesial rim of the enamelled face is marked by an array of parallel ridges that descend across the crown surface, starting off as buttressing ridges to the cusps from which they originate; most gradually merge with the crown surface ventrally, but some of the more apically positioned ones form longer ‘strand-like’ ridges of enamel that meander along or run parallel to the secondary ‘ridge’. This combination of characters appears to be generally typical of many Hastings Sub-Group iguanodontian teeth.

Interestingly, the slightly distorted large unworn crown preserved in the dentary of the ‘Brickenden jaw’ differs from those seen in NHMUK R1831 (Norman in preparation).

Figures

FIGURE 3. Barilium dawsoni (Lydekker, 1888) comb. nov. NHMUK R 802, holotype ilium in lateral (A), dorsal (B), and medial (C) views. Abbreviations: acet – acetabulum; ip – ischial peduncle; pp – pubic peduncle; prp – preacetabular process; srf – sacral rib facet; srs – posterior sacral rib scar. Broken surfaces indicated by cross-hatching.

FIGURE 5. Hypselospinus fittoni (Lydekker, 1889) comb. nov. NHMUK R 1635, holotype ilium. A. Lateral view. B. Posterior view to show the inflected shelf, lateral ridge and brevis fossa. C. Medial view showing the form of the sacral rib scars. D. Dorsal view showing the transversely narrow dorsal margin of the ilium compared with that of B. dawsoni (Fig. 3 B). The mediodorsal edge of the ilium shows an abscess-like excavation that may be pathological in origin. Even though the dorsal margin is eroded laterally posterior to the ischial peduncle (cross-hatched area) there is little indication of a laterally everted edge. Abbreviations: acet – acetabulum; brf – brevis fossa; lr – lateral ridge that bounds the brevis fossa; nde – narrow dorsal edge to the iliac blade; path? – probable pathological structure; prp – preacetabular process; srs – sacral rib scar on medial surface of iliac blade. Broken surfaces indicated by cross-hatching.

FIGURE 6. Hypselospinus fittoni (Lydekker, 1889) comb. nov. NHMUK R 1635, additional postcranial remains recovered from the same locality and horizon as the holotype ilium (Fig. 5) at Shornden. A – C. Posterior portion of sacrum (lateral, ventral and dorsal views respectively). D, E. Anterior caudal centrum, in right lateral and anterior views respectively. F. Proximal left ischium, a water-rolled and polished fragment. The ischium is of different preservation and is very probably transported to a greater extent than the other type specimens – and in any case shows no diagnostic features beyond being the proximal fragment of an ornithopod ischium. Abbreviations: cr – caudal rib; hf – haemal arch (chevron) facet; il – base of iliac peduncle; k – keel on ventral surface of sacral centrum; nc – floor of canal for spinal cord; obt – base of obturator process; pu – base of pubic peduncle; sr – fragments of the sacral ribs. Broken surfaces indicated by cross-hatching.

FIGURE 7. Hypselospinus fittoni (Lydekker, 1889) comb. nov. NHMUK R 1148, referred specimen. The right femur, illustrated as part of the holotype of Iguanodon hollingtoniensis Lydekker, 1889. A. Anterior view. B. Anteromedial view. Abbreviations: aig – anterior intercondylar groove almost occluded by shoulders developed from the distal articular condyles; at – anterior trochanter; 4 t – fourth trochanter and associated muscles scars associated with the caudifemoral complex. Broken surfaces are indicated by cross-hatching.

FIGURE 8. Hypselospinus fittoni (Lydekker, 1889) comb. nov. Referred material. NHMUK R 811, crushed sacrum (part of the holotype of Iguanodon hollingtoniensis) in ventral (A; note remains of midline keel is preserved despite crushing) and dorsal (B) views. NHMUK R 811 b, crushed ilium (part of the holotype of I. hollingtoniensis) in lateral (C) and medial (D) views. Note the deep, narrow preacetabular process with a low curved medial ridge, the flat iliac blade with a narrow dorsal edge and that the ventral postacetabular margin is indented and curves upward away from the ischial peduncle. Abbreviations: k – keel on ventral surface of the sacrum; mr – low, curved ridge on the medial surface of the preacetabular process; nc – floor of canal for the spinal cord; nde – narrow dorsal edge to the ilium; prp – preacetabular process; sr – fragments of the sacral ribs. Broken surfaces indicated by cross-hatching.

FIGURE 9. Hypselospinus fittoni (Lydekker, 1889) comb. nov. NHMUK R 604, referred specimen. A, B. Anterior – mid dorsal vertebra in left lateral and anterior views respectively. C. Anterior caudal vertebra in left lateral view. Abbreviations: cr – caudal rib; dia – diapophysis; em – everted articular margin of the centrum; hf – haemal arch (chevron) facet; par – parapophysis; poz – postzygapophysis; prz – prezygapophysis.

FIGURE 10. Hypselospinus fittoni (Lydekker, 1889) comb. nov. Referred material. NHMUK R 1831, right dentary. A. Medial view, reproduced from Owen (1874: pl. 1, fig. 1). B. Medial view, sketch drawing taken from the original dentary (December 2009). C. The ‘ Brickenden jaw’ (NHMUK 28660). Sketch (November 2009) in medial view for comparison with NHMUK R 1831 (above). This is the right dentary of a large iguanodontian of robust build (attribution unconfirmed at present but it may prove to be attributable to B. dawsoni – Norman in preparation) collected by Col. Brickenden from a quarry near Cuckfield, Sussex. Abbreviations: am – level of alveolar margin; at – ‘ anomalous tooth’ (no longer associated with the jaw); br – extensively broken and crushed dorsal portion of the dentary ramus; ds – dentary-dentary symphysis; m – matrix infill of the Meckelian canal, the medial surface of the postdentary bones and the alveoli; mgr – Meckel’s groove; mt – matrix and tooth crown removed since the lithograph was prepared; pr – ‘ prong’ at the anterior end of the dentary ramus that locks against the lateral surface of the median ventral flap of the predentary; sh. alv – large area of the posterior alveolar shelf that has been sheared away post-mortem.