On Mongolian ornithopods (Dinosauria:Ornithischia). 1. Iguanodon orientalisRozhdestvensky 1952

DAVID B. NORMAN F.L.S.

Sedgwick Museum of Geology, Department of Earth Sciences, University of Cambridge,Downing Street, Cambridge CB2 3EQ

Received September 1994, accepted for publication June 1995

The type material of the Cretaceous ornithopod Iguanodon orientalis Rozhdestvensky, 1952 is redescribed.The characters which were originally used to establish a new species are shown to be either incorrect orindistinguishable from Iguanodon bernissartensis Boulenger, 1881. Additional material, previouslyunrecognized, further confirms the similarity of I. orientalis to I. bernissartensis. It is proposed that I. orientalisshould be relegated in synonymy as a junior subjective synonym of I. bernissartensis Boulenger, 1881. Theidentification of the Barremian species I. bernissartensis in Asia indicates an unusually wide geographicspread for this species, and provides a potential biostratigraphic indicator for beds in Mongolia whichhad been previously dated variously between the Cenomanian and Maastrichtian stages.

©1996 The Linnean Society of London

CONTENTS

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 303Systematics . . . . . . . . . . . . . . . . . . . . . . . . . . 305Material . . . . . . . . . . . . . . . . . . . . . . . . . . . 305Description . . . . . . . . . . . . . . . . . . . . . . . . . . 305Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 311Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 313Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 314References . . . . . . . . . . . . . . . . . . . . . . . . . . . 314

INTRODUCTION

In 1952 A.K. Rozhdestvensky described the fragmentary remains of a dinosaurwhich had been collected at a locality named Khamarin Khural (Fig. 1). which isapproximately 30 km south-east of Buyant-Ukhaa, East Gobi Province, Mongolia. Inthe original article the right maxilla was illustrated and described, as was the leftscapula. The structure of the teeth in the maxilla and the shape of the scapula, whencompared with described specimens of Iguanodon bernissartensis (Dollo, 1882, 1883a,1883b, 1884, 1923) and I. atherfieldensis (Hooley, 1925) were thought to indicategeneric affinity with the genus Iguanodon; however the high maxillary tooth count (27— the maximum thought to be present in the commensurate species I. bernissartensis

Zoological Journal of the Linnean Society (1996), 116: 303–315. With 9 figures

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was 23), shape of the crowns and curvature of the tooth row were suggested assufficient reason for assigning the material to a new species: Iguanodon orientalis.

In 1966, Rozhdestvensky reported on new material of ornithopod dinosaurscollected from Mongolia and, in the comparative discussion, commented (in afootnote) on the maxillary tooth count of I. orientalis which he considered to havebeen less than 27 as originally stated, but not less than 25. Norman (1980) re-described the type and referred material of I. bernissartensis and noted that themaximum tooth count of the maxilla in known specimens was 29. SubsequentlyNorman (1986) observed that I. orientalis, as currently defined, appeared to beindistinguishable from I. bernissartensis. On a number of occasions during the pastdecade references have been made to additional material collected by Joint Soviet-Mongolian collecting trips to Mongolia during the early 1970s. Most notable andrelevant among these has been reference to a nearly complete skull of ‘Iguanodonorientalis’ (see Norman, 1985:115); this latter material has now been studied(Norman & Kurzanov, in preparation) and clearly is not referable to the taxonnamed Iguanodon orientalis.

The material assigned to I. orientalis is of taxonomic, stratigraphic andbiogeographic importance, bearing as it does on the distribution and evolutionaryhistory of the ornithopod dinosaurs of the Cretaceous (Milner & Norman, 1984).Ignoring evidence based either on isolated teeth (Hasegawa et al. 1995 — which areof limited taxonomic value below the level of Iguanodontia) and footprints (Heintz,1963 — which cannot be used to identify the print maker below the familial levelwith any real confidence), the genus Iguanodon has been reported on the basis ofreasonable skeletal evidence from North American (Weishampel & Bjork, 1990),widely across Europe (Belgium (Norman, 1986), England (Hooley, 1925), France(Martin & Buffetaut, 1992), Germany (Norman, 1987), and Spain (Sanz, Casanovas& Santafe, 1984). So the description of Iguanodon from Asia would provide a northernhemisphere circum-global distribution. Since some of the earliest known hadro-saurian ornithopods (which temporally and geographically seem to replaceiguanodontians during the early Late Cretaceous) come from the earliest LateCretaceous of Asia (Brett-Surman, 1979; Weishampel & Horner, 1986), the spatialand phylogenetic relationship between the late iguanodontians and early hadrosaurs

Figure 1. Outline map of Mongolia showing the approximate location of Khamarin Khural at which theholotype of Iguanodon orientalis was collected.

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would appear to have become focused on Asia generally, even though the dinosaursthemselves are (to date) rather poorly known.

The purpose of this short series of papers is to review the anatomy, taxonomy andsystematic position of the ornithopod fauna which has been collected from a numberof localities in Mongolia; all of these taxa have a direct bearing on our currentunderstanding of the range of problems outlined above.

Repository abbreviations: PIN: Palaeontology Institute Nauk (Moscow); BMNH: NaturalHistory Museum (London); IRSNB: Royal Institute of Natural Sciences (Brussels).

SYSTEMATICS

Indented classification hierarchyOrnithischia (Seeley, 1887)

Cerapoda (Sereno, 1986)Ornithopoda (Marsh, 1881)

Iguanodontia (Dollo, 1888)

Binomial: Iguanodon bernissartensis Boulenger, 1881Junior subjective synonym. Iguanodon orientalis Rozhdestvensky, 1952.

MATERIAL

Holotype. PIN 559-1/1, 559-1/2, 559-1/3, 559-1/7. Right maxilla, left scapula,several large rib fragments, vertebral neural arch fragments, portion of rightposterior premaxillary ramus, distal metatarsal fragment, partial right nasal.

Locality. Khamarin Khural (30 kilometres due South of Buyant-Ukhaa, in Dornogov'(East Gobi Province), Mongolia — S.M. Kurzanov personal communication).Described by Rozhdestvensky as “50 km south-east of Sajn-Shandy”.

Horizon. Probably Barremian/Aptian on the basis of comparative anatomy.Previously dated: ?Cenomanian (Rozhdestvensky, 1952, 1966). ?Cenomanian/?Maastrichtian (Norman & Weishampel, 1990).

DESCRIPTION

Maxilla. PIN 559-1/1. The maxilla is from the right side of the skull, is 400 mm longas preserved, and is mostly complete (Figs 2, 3). There are 27 tooth positionsidentifiable and, despite some areas being broken or cracked, the general shape andproportions of the maxilla are well shown and can be compared quite readily to themaxillae of other, known species: specifically I. bernissartensis (Norman, 1980) withwhich it is commensurate, and the somewhat smaller I. atherfieldensis (Norman,1986).

The maxilla is long and wedge-shaped at the caudal end, forked at the rostral endand along the ventral edge is a notched arcade of tooth sockets; in lateral view (Fig.2) this latter margin is interrupted somewhat by breakages in this specimen. Themedial surface of the maxilla is vertical and smooth; above the alveolar margin there

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is an arcade of small foramina (Fig. 3, f) each one corresponding to a vertical toothfamily; these are by convention interpreted (Edmund, 1957) as foramina for the entryof nutritive blood vessels and sensory nerves to the dental lamina. All the foraminaare linked together by a shallow groove which presumably marks the path taken bythe main blood vessel and nerve trunk supplying these foramina. The area betweenthese nutritive foramina and the alveolar margin forms a thin alveolar wall orparapet (Fig. 3, al.p), which supports the roots of functional teeth and the crowns ofsuccessional teeth. The height of the alveolar parapet is greatest just caudal of themid-point along the upper jaw and it is here that the largest teeth emerge; the surfaceof this parapet, which has a fine irregular ornament, is totally unlike that of the restof the maxilla. This bone structure presumably relates to the metabolic needs of thisregion; the rapid replacement of teeth, which occurred in these animals, implies arapid turnover of minerals in the alveolar bone for the continuous remodelling whichmust have occurred as the teeth grew.

Rostral to the tooth row there is an incomplete, tapering process which extendsventrally, curving beneath the area originally occupied by the lateral margin of thepremaxilla (Fig. 2, lap) Along the mediodorsal edge of this process the bone ismarkedly rugose, marking an area for attachment of ligaments binding thepremaxilla to the maxilla. Laterally, this surface slopes away to form a smoothlyrounded lateral surface. Above the first few tooth positions, the dorsal edge of thisprocess is compressed and forms a relatively thin vertical wall which has a thickerand rugose dorsal edge. The anterior part of this dorsal edge projects forwards as adistinct finger-like process, which bears distinct facets laterally and medially (Figs 2,3,rug). The rugose facets on the medial edge of this rostral process are probably forarticulation against the median, posterior premaxillary process (evidence for whichcan be seen in I. atherfieldensis - Norman, 1986). Caudally, this dorsal edge runsroughly horizontally and merges with the main body of the maxilla — though thisarea is broken and crushed. Between the vertical septum which supports the dorsaledge, and the lateral surface of the maxilla, there is an obliquely inclined, elongatetrough (Fig. 2, tr), somewhat crushed and distorted in this specimen, into whichwould have fitted the lateral portion of the posterior premaxillary process. Caudallythe groove in the upper surface of the maxilla is interrupted by breakage in the dorsal

Figure 2. PIN 559-1/1. Holotype I. orientalis. Right maxilla, lateral view. Even tone = plaster of Paris;cross-hatching = broken surface. Abbreviations: ap, median anterior process of the maxilla; ect,ectopterygoid suture; jug. p, jugal process (broken away); la, contact area for lacrimal; lap, lateral anteriorprocess; rug, rugosities for attachment of premaxilla; tr, trough to receive the posterior lateral process ofthe premaxilla.

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lateral wall before continuing to an apex which represents a lamina of bone whichwould have overlapped the lacrimal (Norman, 1986: fig. 9A); the lacrimal processwhich would have been located immediately caudal to this apex of bone has beenbroken away.

Posterior to the area for attachment of the lacrimal, the maxilla is rather damaged.The dorsal profile of the maxilla curves quite sharply downward to its caudal end.Laterally, the area associated with the finger-like process for the attachment of thebasal portion of the jugal is broken away (Fig. 2, jug. p), and the area between thisprocess and the lacrimal process (which would be the area occupied by the antorbitalopening) is mutilated. The mediodorsal edge of the maxilla is rugose for attachmentof the palatine (Fig. 3, pal) and pterygoid bones, while the external surface facesposteriorly and somewhat laterally with a shallow facet for the attachment of theectopterygoid (Fig. 2, ect).

The external surface of the maxilla, though somewhat distorted is characterised bya broad horizontal swelling across the middle/upper portion. The ventral part of theexternal surface of the maxilla has a conspicuous longitudinal recess (Fig. 2), so thatthe main body of the maxilla overhangs the tooth row.

Nasal. PIN 559-1/7. The nasal (Fig. 4) was not previously identified and has beenreconstructed by the author from several isolated fragments. Though imperfect thespecimen is quite distinctive, comprising the central portion of the bone, and is inmost respects very similar to the isolated nasal of I. atherfieldensis (Norman, 1986: fig.11).

The nasal is thin and somewhat arched transversely and would originally haveformed a considerable portion of the roofing to the snout. The dorsomedial edge(Fig. 4B) of the nasal shows the nasal-nasal suture as a diagonal butt-joint (Fig. 4B,nn.s); rostrally this suture was overlain by the median dorsal premaxillary process(Fig. 4B, pmx.s). Posteriorly and ventrally, its anterior border descends as a smooththough slightly irregular curve, forming the posteriorly upper margin of the bonynasal opening. Ventrally the extreme rostral part of the nasal-premaxillary suture ispreserved (Fig. 4B, pm.s), and shows that the margin of the nasal is recessed so thatit clasps the upper border of the premaxilla. Caudal to this small portion of the nasalthe lower edge is notched by what appears to be the edge of a small channel (Fig. 4A,

Figure 3. PIN 559-1/1. Holotype I. orientalis. Right maxilla, medial view. Even tone = plaster filling,cross-hatching = broken surface. Abbreviations: al, alveoli exposed by destruction of alveolar parapetand removal of teeth; al.p, alveolar parapet; ap, anterior median process of the maxilla; f, furrow andarcade of foramina; pal, palatine suture; rug, rugose facets for attachment of the premaxilla.

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ch), which is identical to the form of the preserved right nasal of a referred specimenof I. atherfieldensis (BMNH R.11521). The external surface (Fig. 4A) of the nasal of I.orientalis is smoothly curved with a slightly puckered surface and marked by a curioussmall ‘boss’ near its rostro-dorsal margin; near the narial margin there is a prominentvascular opening (Fig. 4A, for) which again is positioned very similarly to that seenin specimens of I. atherfieldensis. The caudomedial surface of the nasal appears to showsigns of a sutural contact which might represent the anterior extent of the prefrontal(Fig. 4B, prf.s).

Premaxilla. PIN 559-1/7. Another isolated bone previously unidentified, is tentativelysuggested as a central portion of the right, lateral, posterior premaxillary process ofthe premaxilla (Fig. 5). The bone is broken at both ends and rather crushedtransversely, but has a characteristic axial twist, and sutural surfaces which wouldseem to help place this bone with a measure of confidence. It is also possible to

Figure 4. PIN 559-1/7. Holotype I. orientalis. Nasal, right. A, lateral view. B, medial view. Abbreviations:ch, channel; for, foramen; nn.s, nasal-nasal suture; pr.s, prefrontal suture; pm.s, premaxillary suture;pmx.s, recess for dorsal median process of premaxilla.

Figure 5. PIN 559-1/7. Holotype I. orientalis. Right nasal and premaxilla shown in lateral view andarticulated. Cross-hatching = broken bone. Abbreviation: e.n, external naris.

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articulate crudely the newly identified nasal with this premaxillary fragment asshown in Figure 5.

The lateral surface (Fig. 5) shows a post mortem depressed area which hasdistorted the general appearance of the caudal half of this bone. The dorsal edge israised and rugose (for attachment to the nasal). Rostrally, from the nasal suture, theupper surface curves gradually downward and splays laterally forming the lower partof the narial opening above the premaxillary beak. The medial surface has a twistedappearance with the rostral area showing the possible sutural area for attachment tothe inner wall of the maxilla.

Teeth. A number of tooth crowns, in various stages of eruption, are preserved in theright maxilla and this allows their form to be described, and comparison of theseteeth with those of other species.

The crowns of maxillary teeth are narrow, tall and asymmetric in lateral (labial)view (Fig. 6). The asymmetry of the crown is emphasised by a prominent primaryridge which bisects, unequally, the labial surface vertically. The larger rostral (mesial)portion of the crown forms a smooth, tapered surface, which expands from the apextoward the junction between the root and crown. The apical portion of this surfaceis developed into a ‘shoulder’ which serves to broaden the mesial half of the crown;the mesial edge of this surface is delimited by a coarse denticulate margin whichforms a slightly raised and thickened edge to the crown; small tertiary ridges arevisible originating from the bases of marginal denticles near the apex of the crown,but this is a variable trait in the crowns exposed here. The smaller caudal (distal)surface of the crown is similarly delimited by the primary ridge and marginaldenticles. The medial (lingual) surface of the crown is smooth, slightly concavevertically, and strongly convex mesiodistally.

The marginal denticles are well developed, being formed from curved, irregularlycrenelated ledges which are wrapped around the leading and trailing edges of thecrown (rather than being small pointed structures as is the case in the majority of

Figure 6. PIN 559-1/1. Holotype I. orientalis. Maxillary teeth of I. orientalis. Stereoscopic view in obliquelateral view to show the structure of the labial surfaces of the crowns.

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Length Width(prox) Width(dist) Width(min)

I. orientalis 940mm 290mm 210mm 130mmI.bernissartensis 970mm 370mm 210mm 150mm

smaller ornithischians). The mesial denticles are rather more strongly rolled on to thelabial surface of the crown than the distal ones.

The manner of replacement and interlocking of the functional and replacementteeth in the maxillary magazine is, as far as can be ascertained, identical with thatseen in the two well-described species of Iguanodon (Norman, 1980, 1986). Wornteeth, of which there are three examples in the maxilla, show an abruptly truncatedcrown, the triturating surface approximates to a ‘D’ shape, with the straight edge ofthe ‘D’ marked by perpendicular spine formed by the sectioned primary ridge,flanked on either side by the raised denticulate margins.

Scapula. PIN 559-1/2. The scapula (Fig. 7) is almost complete (parts of the distalmargin are eroded) and very large. As can be seen from the comparativemeasurements tabulated below, the size and relative dimensions of this specimen aresimilar to those of I. bernissartensis (measurements based on the holotype IRSNB 1534;Norman, 1986). The main difference lies in the broader base of the scapula seen inI. bernissartensis.

Figure 7. PIN 559-1/2. Holotype I. orientalis. Scapula in (A) lateral and (B) medial view. Abbreviations:bl, blade of scapula; but, buttress above the glenoid; cor. s, coracoid suture; gl, glenoid; ac. pr, acromionprocess.

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The scapula is typical of that of ornithopod dinosaurs: it is broad and flatteneddistally and curved both medially (to fit against the ribcage) and caudally. Theproximal end is expanded to support the glenoid and to provide a sutural surface forthe coracoid (Fig. 7A, cor.s). The coracoid suture is slightly curved and occupiesmuch of the proximal end; rostrally the surface curves upwards into the main bladeof the scapula and is supported by a thickened acromial ridge (Fig. 7A, ac.pr), whichis prominent proximally but gradually merges indistinguishably with the blade (bl).The caudal corner of the proximal end has a semicircular embayment which formsthe humeral socket (Fig. 7, gl); this is supported distally by a curved and laterallyprominent buttress (Fig. 7, but). Distally the buttress contracts inwards against theblade. Between the acromial process and the glenoid buttress the lateral surface ofthe scapula forms a shallow embayment. Distal to the coracoid and humeral areas ofthe scapula the scapular blade has its minimum width, but is comparatively thicktransversely, progressing distally the blade thins transversely and broadens rostro-caudally to form the typical blade of the scapula. The upper (distal) end of the bladeis eroded both rostrally and caudally, though a small segment of its upper marginremains in the middle. The slight thickening and irregular surface of the distal endof the blade is suggestive of a suprascapular cartilage.

Ribs. PIN 559-1/7. Portions of two quite well-preserved dorsal ribs are amongnumerous other fragments. These show no distinctive characters, being typical ofthose seen in many ornithopod ornithischians, and thus broadly similar to thosealready described in other species of Iguanodon.

Metatarsal. PIN 559-1/7. A distal fragment, unremarkable, and typical in form of thatof ornithopod ornithischians generally.

Other. There are likewise a number of indeterminable fragments of bone, and somebroken pieces of neural arch from the dorsal series of vertebrae.

DISCUSSION

In his original description (Rozhdestvensky, 1952) a number of features werementioned which were held to justify the separate specific identity of this then newmaterial.

(1) ‘Number of tooth positions’. The count of 27 tooth positions (revised downwards to25 [incorrectly] by Rozhdestvensky, 1966) does not serve to distinguish I.orientalis from I. bernissartensis — there can be as many as 29 tooth positions inthe maxilla of the latter species (Norman, 1980).

(2) ‘Replacement teeth’. The growth pattern of teeth in the maxilla was described byRozhdestvensky (1952: 1244) as consisting of three teeth (an initial crown, alarger tooth, and above these a functional tooth). Examination of the maxilladid not allow this point to be verified and it is considered that the replacementpattern is as seen in other species of Iguanodon, with a single replacement crowngrowing beneath a freshly emergent or functional tooth.

(3) ‘Tooth form’. Teeth of I. orientalis were distinguished from European species (I.atherfieldensis and I. bernissartensis) by having a single longitudinal ridge on theflattened (labial) surface of the crown. This feature is in fact one of the primarycharacters of iguanodontian dinosaurs.

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(4) ‘Curvature of the tooth row’. Smaller curvature of the tooth row in I. orientaliscompared with other known species. Rozhdestvensky’s use of the term ‘toothrow’ is ambiguous. Crushing of the maxilla has straightened the tooth rowunnaturally so this character is not significant. Alternatively the curvature ofthe vertical series of teeth in each alveolus (if this was the intended feature) isagain not significantly different from those of closely related species ofIguanodon.

Rozhdestvensky (1952) claimed that these differences were correlated with specificadaptations to differences in the environments inhabited by known species ofIguanodon, and, in some slightly less obvious way, to ‘evolutionary trends’. Thediscussion in the 1952 paper makes it clear that Rozhdestvensky understood that thisnew species had an important bearing on the evolution of ornithopod dinosaurs,particularly in relation to the origin of the closely similar hadrosaurs, which were alsoknown in Asia (Gilmore, 1933; Weishampel & Horner, 1986).

Additional informationThe holotype material, now it has been re-examined, refutes all of Rozhdest-

vensky’s specifically distinct characters, thus throwing doubt upon the validity of thetaxon. The only anatomical feature of possible significance is shown by the scapulawhich, although it is large, well-preserved and comparable to that of I. bernissartensishas a proximal width which is smaller than that seen in the latter species; in theopinion of the author this difference is unlikely to be taxonomically robust.

Furthermore examination of the holotype has enabled identification of one otherfragment which is of material importance to the identity of not only this taxon, butalso to the identity of material which has been in the recent past referred to Iguanodonorientalis. The nasal bone (Fig. 4), though incomplete, conforms in all respects to thatwhich is seen in the European species I. bernissartensis; this further strengthenssimilarities to this latter species. The skull bones identified in the holotype of I.orientalis are shown superimposed on the skull outline of I. bernissartensis (Fig. 8, shadedarea).

The form of the nasal seen in the holotype of I. orientalis and other species of thegenus Iguanodon differ radically from that seen in an ornithopod collected during theSoviet-Mongolia Expeditions of the early 1970s by Dr S.M. Kurzanov from the

Figure 8. Skull of Iguanodon bernissartensis in right lateral view, with the skull bones of I. orientalis indicatedin position as the shaded portions (from Norman, 1980).

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locality known as Khuren Dukh (Norman & Kurzanov, in preparation). This latterhas been frequently referred to the species I. orientalis; this is incorrect. These twonasals are shown for simple visual comparison in Figure 9 and indicate the existenceof a new species of iguanodontian ornithopod.

CONCLUSIONS

(1) The holotype, and referred material of Iguanodon orientalis Rozhdestvensky,1952 have been examined and the holotype is redescribed.

(2) Iguanodon orientalis differs in no significant anatomical respect from theEuropean species of Iguanodon bernissartensis and on the basis of the holotypeshould be regarded as a junior subjective synonym of Iguanodon bernissartensis.

(3) No other material should be referred to Iguanodon orientalis unless newdiscoveries are made at or near the original site of discovery at KhamarinKhural.

(4) If correctly identified, these remains confirm that the species Iguanodonbernissartensis had an exceptionally broad Eurasian distribution, and that itmight be expected that the species I. atherfieldensis might eventually berecognized in similarly aged beds in this region.

(5) Identification of I. bernissartensis in Mongolia is suggestive of a Barremian (lateLower Cretaceous) age for the ‘greenish-grey clays bearing pebbles’ fromwhich this specimen was collected in the Dornogov' (East Gobi Province) ofMongolia at the locality known as Khamarin Khural.

(6) Extensive new material, including skull and postcranial elements collected fromKhuren Dukh and previously referred to I. orentalis are referable to a new taxonof ornithopod dinosaur.

Figure 9. Nasal bones in lateral view. A. New species of iguanodontian from Khuren Dukh (PIN 3386/7).B. PIN 559-1/7. Holotype I. orientalis. Abbreviations: e.n, external naris; pm.s, premaxillary suture.

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ACKNOWLEDGEMENTS

This research would not have been possible without the support offered by theRoyal Society of London and the Russian Academy of Science, I gratefullyacknowledge their assistance. I thank Dr Sergei Kurzanov for considerable help inlocating ornithopod material in the collections housed at the PalaeontologicalInstitute of the Academy of Sciences, Moscow. I also thank Academician L.P.Tatarinov for allowing me free use of his research room and facilities during myresearch visit to the Palaeontology Institute Nauk, Moscow. I am very grateful toProf. T. Koren' who kindly translated A.K. Rozhdestvensky’s 1952 paper onIguanodon orientalis.

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