AN UNUSUAL NEW NEOSAUROPOD DINOSAUR FROM THE LOWER CRETACEOUS HASTINGS BEDS

AN UNUSUAL NEW NEOSAUROPOD DINOSAUR FROM THE LOWER CRETACEOUS HASTINGS BEDS GROUP OF EAST SUSSEX,ENGLAND

by MICHAEL P.TAYLOR and DARREN NAISH

Palaeobiology Research Group,School of Earth and Environmental Sciences,University of Portsmouth,Burnaby Road,Portsmouth PO13QL,UK;

e-mails dino@https://www.wendangku.net/doc/0018119611.html,;eotyrannus@https://www.wendangku.net/doc/0018119611.html,

Typescript received2June2006;accepted in revised form18December2006

Abstract:Xenoposeidon proneneukos gen.et sp.nov.is a neosauropod represented by BMNH R2095,a well-preserved partial mid-to-posterior dorsal vertebra from the Berriasian–Valanginian Hastings Beds Group of Ecclesbourne Glen,East Sussex,England.It was brie?y described by Lydekker in 1893,but it has subsequently been overlooked.This speci-men’s concave cotyle,large lateral pneumatic fossae,complex system of bony laminae and camerate internal structure show that it represents a neosauropod dinosaur.However,it differs from all other sauropods in the form of its neural arch, which is taller than the centrum,covers the entire dorsal sur-face of the centrum,has its posterior margin continuous with that of the cotyle,and slopes forward at35degrees rela-tive to the vertical.Also unique is a broad,?at area of fea-tureless bone on the lateral face of the arch;the accessory infraparapophyseal and postzygapophyseal laminae which meet in a V;and the asymmetric neural canal,small and round posteriorly but large and teardrop-shaped anteriorly, bounded by arched supporting laminae.The specimen cannot be referred to any known sauropod genus,and clearly represents a new genus and possibly a new‘family’.Other sauropod remains from the Hastings Beds Group represent basal Titanosauriformes,Titanosauria and Diplodocidae; X.proneneukos may bring to four the number of sauropod ‘families’represented in this unit.Sauropods may in general have been much less morphologically conservative than is usually assumed.Since neurocentral fusion is complete in R2095,it is probably from a mature or nearly mature ani-mal.Nevertheless,size comparisons of R2095with corre-sponding vertebrae in the Brachiosaurus brancai holotype HMN SII and Diplodocus carnegii holotype CM84suggest a rather small sauropod:perhaps15m long and7600kg in mass if built like a brachiosaurid,or20m and2800kg if built like a diplodocid.

Key words:Dinosauria,Sauropoda,Neosauropoda,Xenopo-seidon proneneukos,Wealden,Hastings Beds Group,Lower Cretaceous.

The remains of sauropod dinosaurs have been known from the Lower Cretaceous Wealden strata of the English mainland since the1840s.Although sauropods were not recognized as a distinct dinosaurian group until some-what later(Phillips1871;Marsh1878a),the?rst named sauropod species,Cetiosaurus brevis Owen,1842,was coined for Wealden material(Naish and Martill2001; Upchurch and Martin2003).

Most Wealden sauropods are from the Barremian Wessex Formation of the Isle of Wight.Far less well represented are the sauropods of the older Berriasian–Valanginian(Allen and Wimbledon1991)Hastings Beds Group of the mainland Wealden.Specimens have been collected from Cuck?eld,West Sussex(Owen 1841;Mantell1850),Hastings,East Sussex(Mantell 1852),and most recently from Bexhill,East Sussex (Anonymous2005).There are indications that a taxonomic diversity similar to that of the Wessex Formation is present among these forms,as discussed below.

Here we describe a Hastings Beds Group specimen?rst reported,brie?y,by Lydekker(1893a).This specimen was collected by Philip James Rufford and subsequently acquired by the British Museum(Natural History), now the Natural History Museum,London,where it is deposited as specimen BMNH R2095.

Though consisting only of a single incomplete vertebra, R2095preserves many phylogenetically informative characters that allow it to be con?dently identi?ed as a neosauropod.Furthermore,it is highly distinctive,pos-sessing several autapomorphies.While it is generally dif?-cult to assess the af?nities of isolated bones,sauropod vertebrae,especially dorsal vertebrae,are highly diagnostic (Berman and McIntosh1978,p.33;Bonaparte1986a, p.247;McIntosh1990,p.345),and this is particularly true of the specimen described here.

[Palaeontology,Vol.50,Part6,2007,pp.1547–1564]

aThe Palaeontological Association doi:10.1111/j.1475-4983.2007.00728.x1547

Lydekker(1893a,p.276)reported that this specimen was discovered in‘the Wealden of Hastings’(Text-?g.1), but beyond that no locality or stratigraphic data were recorded.Watson and Cusack(2005,p.4)con?rmed that Rufford generally collected‘from the Wealden beds of the Hastings area,East Sussex’.Speci?c plant fossils known to have been collected by Rufford came from East Cliff(Wat-son and Cusack2005,p.75)and from the Fairlight Clays of Ecclesbourne Glen(Watson and Cusack2005,pp.64, 80,87,107,112,125,128,138,152–153),both in the Fair-light area.The units exposed at both East Cliff and Eccles-bourne Glen are part of the Ashdown Beds Formation, which straddles the Berriasian?Valanginian boundary (Text-?g.2).The vertebra was probably collected from Ec-clesbourne Glen since(1)it is closer to Hastings than is East Cliff and Lydekker(1893a)stated that the specimen was collected near Hastings;and(2)the majority of Ruf-ford’s documented specimens came from there.The part of the Ashdown Beds Formation exposed at Ecclesbourne Glen is Berriasian in age(e.g.Watson and Cusack2005), so this is the most likely age of R2095.

Anatomical nomenclature.The term‘pleurocoel’has been widely used to refer to the lateral excavations in the centra of sauropods and other saurischian dinosaurs.However, the blanket use of this term obscures the morphological diversity of these cavities,which varies considerably between taxa,encompassing everything from broad,shal-low fossae to small,deep foramina;and some taxa have both of these.Furthermore,the term has been used incon-sistently in the literature,so that characters such as‘pleu-rocoels present’in cladistic analyses are dif?cult to interpret.For example,in the analysis of Wilson(2002), character78is de?ned as‘Presacral centra,pneumatopores (pleurocoels):absent(0);present(1)’(Wilson2002, p.261),and Barapasaurus Jain,Kutty and Roy-Chowdhu-ry,1975is scored as0(‘pleurocoels absent’).While Barapasaurus does indeed lack pneumatic foramina,it has shallow lateral fossae(Jain et al.1979,pls101–102),a fea-ture that is not conveyed by the traditional terminology. Accordingly,we recommend that the ambiguous term

‘pleurocoel’(and Wilson’s equivalent‘pneumatopore’)be deprecated in favour of the more explicit alternatives ‘lateral fossa’and‘lateral foramen’(Britt1993,1997; Wedel et al.2000b;Wedel2003,2005).The EI(elongation index)of Upchurch(1998)is here used as rede?ned by Wedel et al.(2000b),being the length of the centrum divided by the height of the cotyle.

Anatomical abbreviations.ACDL,anterior centrodiapophyseal lamina;ACPL,anterior centroparapophyseal lamina;CPOL, centropostzygapophyseal lamina;CPRL,centroprezygapophyse-al lamina;PCDL,posterior centrodiapophyseal lamina; PCPL,posterior centroparapophyseal lamina;PODL,post-zygodiapophyseal lamina;PPDL,paradiapophyseal lamina; PRDL,prezygadiapophyseal lamina;PRPL,prezygoparapophy-seal lamina.We follow the vertebral lamina nomenclature of Janensch(1929)as translated by Wilson(1999)except in using capital letters for the abbreviations,a convention that allows plurals to be more clearly formed. Institutional abbreviations.BMNH,the Natural History Museum, London,England;CM,Carnegie Museum of Natural History, Pittsburgh,Pennsylvania,USA;FMNH,Field Museum of Natu-ral History,Chicago,Illinois,USA;HMN,Humboldt Museum fu¨r Naturkunde,Berlin,Germany;MIWG,Museum of Isle of Wight Geology(now Dinosaur Isle Visitor Centre),Sandown, A

B

Isle of Wight,England;MPEF,Museo Palaeontolo′gico Egidio Feruglio,Trelew,Argentina.

SYSTEMATIC PALAEONTOLOGY

DINOSAURIA Owen,1842

SAURISCHIA Seeley,1888

SAUROPODOMORPHA Huene,1932

SAUROPODA Marsh,1878a

NEOSAUROPODA Bonaparte,1986b

Genus XENOPOSEIDON gen.nov.

Derivation of name.Greek,xenos,strange or alien,and Poseidon, the god of earthquakes and the sea in Greek mythology,the latter in reference to the sauropod Sauroposeidon Wedel,Cifelli and Sanders,2000a.Intended pronunciation:ZEE-no-puh-SYE-d’n.

Type species.Xenoposeidon proneneukos sp.nov.

Diagnosis.As for the type and only species,X.proneneu-kos.

Xenoposeidon proneneukos sp.nov.

Text-?gures3–5;Tables1–2

Derivation of https://www.wendangku.net/doc/0018119611.html,tin,pronus,forward sloping,describing the characteristic morphology of the neural arch.Intended pro-nunciation:pro-nen-YOO-koss.

Holotype.BMNH R2095,the Natural History Museum,London.

A mid posterior dorsal vertebra consisting of partial centrum and neural arch.

Type locality and horizon.Near Hastings,East Sussex,England; probably Ecclesbourne Glen,about2km east of Hastings.Has-tings Beds Group(Berriasian–Valanginian,earliest Cretaceous); probably Berriasian part of the Ashdown Beds Formation.Pre-cise locality and stratigraphic information either has been lost or was never recorded.

Diagnosis.Differs from all other sauropods in the follow-ing characters:(1)neural arch covers dorsal surface of centrum,with its posterior margin continuous with that of the cotyle;(2)neural arch slopes anteriorly35degrees relative to the vertical;(3)broad,?at area of featureless bone on lateral face of neural arch;(4)accessory infra-parapophyseal and postzygapophyseal laminae meeting ventrally to form a V;(5)neural canal is asymmetric: small and circular posteriorly but tall and teardrop-shaped anteriorly;(6)supporting laminae form vaulted arch over anterior neural canal.

Description.BMNH R2095(Text-?gs3–4)is a partial dorsal ver-tebra from the middle or posterior portion of the dorsal col-umn.Most of the centrum and neural arch are preserved,but the condyle is broken,and the neural spine and dorsal part of the neural arch are missing,as are the pre-and postzygapophy-ses and diapophyses.However,suf?cient laminae remain to allow the positions of the processes to be inferred with some certainty(Text-?g.5).Measurements are summarized in Table1.

The most striking features of this specimen are the extreme height,anteroposterior length and anterodorsal inclination of the neural arch.These are clearly genuine osteological features and not the result of post-mortem distortion.Although the dor-salmost preserved part of the neural arch is ventral to the dia-pophyses,the height even of the remaining portion(160mm above the anterodorsal margin of the centrum,measured per-pendicular to the anteroposterior axis of the centrum)is equal to that of the cotyle.The centrum is190mm long measured along its dorsal margin;its anteroventral portion is missing but a maximum length of200mm is indicated,assuming that the curvature of the condyle is approximately equal to that of the cotyle.The base of the neural arch is170mm in anteroposterior length,85per cent of the estimated total length of the centrum, and its posterior margin is continuous with that of the cotyle, forming a single smooth curve when viewed laterally.The angle of the neural arch’s inclination relative to the vertical cannot be precisely ascertained due to the absence of the condyle,but

was

approximately35degrees and cannot have varied from this by more than5degrees or so unless the condyle was shaped very differently from that of other sauropods.

A clean break of the condyle exposes within the centrum the dorsal part of a median septum and a pair of ventromedially directed lateral septa,indicative of an extensively pneumatized

A B

C D

T E X T-F I G.3.Xenoposeidon proneneukos gen.et sp.nov.holotype in A,left lateral,B,right lateral,C,anterior,and D,posterior views.Scale bar represents200mm.

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T A Y L O R A N D N A I S H:U N U S UA L N E W E A R L Y C R E T A C E OU S N E O S A U R O P OD D I N O S A U R1551

A B

C D

T E X T-F I G.4.Interpretive drawing of Xenoposeidon proneneukos gen.et sp.nov.holotype in A,left lateral,B,right lateral,C, anterior,and D,posterior views.Scale bar represents200mm.Breakage is indicated by diagonal hatching.The PPDL(preserved only on the left side)is a sheet of bone projecting anterolaterally from the neural arch with its anterolateral margin running dorsoventrally, but which is broken off just dorsal to the parapophysis.

centrum with camerate,rather than camellate to somphospondy-lous,internal structure.The ventral portion of the broken con-dyle cannot be described as it is obscured by a catalogue note. The cotyle is slightly concave,its central portion indented10–15mm relative to its margin.It is160mm tall and170mm wide.A very subtle keel is present on the ventral surface of the centrum,and the ventral border of the centrum is gently arched in lateral view.

On the better preserved left side of the vertebra,a shallow lateral fossa is positioned dorsally on the centrum,and about mid-way between the anterior and posterior margins of the neural arch,onto which it intrudes.It is very roughly triangular in shape, taller anteriorly than posteriorly,with a maximum height of 80mm and a total length of95mm.Set within this is a deeper lateral foramen,oval,anteroposteriorly elongate and measuring80 by40mm.The fossa and foramen share their ventral borders.On the right side,the lateral fossa is situated even more dorsally,but is taller posteriorly than anteriorly,with a maximum height of 55mm and a total length of90mm.The lateral foramen is much smaller on this side,measuring only20by15mm,and is antero-ventrally placed within the fossa.

On the left side,the dorsal border of the lateral fossa is formed by a prominent sharp-lipped lateral ridge,which extends anterodorsally for90mm;this is absent on the right side,appar-ently due not to damage but to intravertebral variation.Instead, an irregularly shaped and sharp-lipped border separates the fossa from a more dorsally placed subcircular‘accessory fossa’30mm in diameter.On this side,an accessory lamina connects the ante-rior part of the border between the main and accessory fossae to a prominent boss positioned on the anterior margin of the neu-ral arch,50mm above the anterodorsal margin of the centrum. This is not a parapophysis or a diapophysis but seems to be an aberrant feature of this individual.Neither the accessory fossa nor the anterior boss has been reported in any other sauropod vertebra;however,these features are not considered taxonomi-cally signi?cant as their occurrence on only one side of the vertebra suggests that they are either pathological or a develop-mental aberration.Pneumatic features vary wildly and may be

T A B L E1.Measurements(in mm)of Xenoposeidon proneneukos gen.et sp.nov.holotype,BMNH R2095,and comparison with mid-posterior dorsal vertebrae of other neosauropods.The suf?x‘e’indicates an estimation;‘+’indicates a minimum possible value,e.g. the length of the preserved portion of a broken element.Measurements for Brachiosaurus altithorax FMNH P25107are taken from Riggs(1904,p.234):D?7and D?11are the vertebrae described by Riggs as presacrals VI and II,respectively,on the assumption than B.altithorax had12dorsal vertebrae.Measurements for Brachiosaurus brancai HMN SII are taken from Janensch(1950,p.44)except those suf?xed‘t’,which were omitted from Janensch’s account and so measured by MPT.Measurements for Diplodocus carnegii CM 84are taken from Hatcher(1901,p.38).Those suf?xed‘i’were interpolated by measuring from Riggs(1904,pl.72)for B.altithorax, Janensch(1950,?g.56)for B.brancai and Hatcher(1901,pl.7)for D.carnegii.

Xenoposeidon BMNH R2095Brachiosaurus altithorax

FMNH P25107

D?7D?11

B.brancai

HMN SII

D7

Diplodocus carnegii

CM84

D7D8

Total height of vertebra300+900800770+980i970i Total centrum length including condyle200e440350330264275 Total centrum length excluding condyle190294

Cotyle height160270280220t

Cotyle width170300310320t

Average cotyle diameter165285295270t280309 Centrum length?cotyle height(EI)1?251?631?251?500?940?89 Depth of cotylar depression108070

Anteroposterior length of lateral fossa95–––?––Dorsoventral height of lateral fossa80–––?––Anteroposterior length of lateral foramen8019016097i120i130i Dorsoventral height of lateral foramen401007058i85i95i Anteroposterior length of base of neural arch170220i155i170i180i165i Neural arch base length?centrum length0?850?500?440?520?680?60 Height of neural arch above centrum160+

Height of neural arch pedicels,posterior130+

Thickness of neural arch pedicels,posterior30

Height of neural canal,posterior35

Width of neural canal,posterior35

Height of neural arch pedicels,anterior80+

Thickness of neural arch pedicels,anterior25

Height of neural canal,anterior120

Width of neural canal,anterior55

Height of hyposphene above centrum90+

Height of postzygapophyses above centrum140e

Height of prezygapophyses above centrum140e

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opportunistic,if Witmer(1997,p.64)is correct that‘Pneumatic diverticula are…opportunistic pneumatizing machines,resorb-ing as much bone as possible within the constraints imposed by local biomechanical loading regimes.’

The remaining features are described from the left side of the vertebra.The right side is consistent with this morphology, although not all features are preserved.

From a point anterior to the anterodorsal margin of the lateral fossa,a vertically orientated ACPL extends dorsally70mm to a cross-shaped junction of laminae near the anterior margin of the arch,and may also have extended a similar distance ventrally although damage makes it impossible to establish this.The cross-shaped junction is interpreted as the location of the parapophysis. In sauropods,the position of the parapophysis migrates dorsally in successive dorsal vertebrae,being located ventrally on the cen-trum of anterior dorsals,dorsally on the centrum in mid to ante-rior dorsals,and on the neural arch of mid to posterior dorsals, level with the prezygapophyses in the most posterior dorsals:see, for example,Hatcher(1901,pl.7).The high position of the parap-ophysis on the neural arch of R2095indicates a mid to posterior placement of the vertebra within the dorsal column,but,because the prezygapophyses must have been dorsal to it,it was probably not among the most posterior vertebrae in the sequence.

In addition to the ACPL,three further laminae radiate from the parapophysis:part of an anteriorly directed PRPL,the ventral portion of a dorsally directed lamina,which is interpreted as a PPDL,and a posteroventrally directed accessory lamina support-ing the parapophysis.This is presumably homologous with a PCPL,but cannot be so named as it does not approach the cen-trum,and indeed extends only30mm.Where the latter lamina merges with the neural arch,another accessory lamina arises. Directed posterodorsally,it presumably extended to the postzy-gapophysis and is here regarded as an accessory postzygapophyse-al lamina similar to that found in posterior dorsal vertebrae of Diplodocus carnegii Hatcher,1901(Hatcher1901,pl.7).The PPDL,accessory infraparapophyseal and accessory postzygapo-physeal lamina form three sides of a quadrilateral fossa;the fourth side,presumably formed by a PODL,is not preserved, although a very low and unobtrusive accessory lamina does join the dorsalmost preserved part of the PPDL to the accessory post-zygapophyseal lamina.The near-vertical orientation of the PPDL indicates that the diapophysis was located some distance directly dorsal to the parapophysis,further extending the inferred height of the neural arch and ruling out an interpretation of the acces-sory postzygapophyseal lamina as the ACDL or as the‘accessory PCDL’of Salgado et al.(2005).Finally,a broken ridge of bone extends up the posterior margin of the lateral face of the neural arch.Its identity is problematic:it cannot be a PCDL owing to the anterior position inferred for the diapophysis.

Between the ACPL and the posterior lamina,above the dorsal margin of the lateral fossa and below the accessory laminae described above,the lateral face of the neural arch is a?at fea-tureless area measuring90mm anteroposteriorly and50mm dorsoventrally.This feature is not observed in any other sauro-pod vertebra.

In posterior view,the pedicels of the neural arch are robust pillars,leaning somewhat medially,measuring30mm in width, extending at least130mm dorsally,and merging into the CPOLs before damage obscures their further extent.They enclose a neu-ral canal that is almost exactly circular,35mm in diameter. There is no trace of the postzygapophyses or hyposphene,and no indication that these structures were attached to the preserved portion of the arch.It must be assumed,then,that these features were located on the lost,more dorsal,part of the neural arch. The hyposphene,if present,was located at least90mm dorsal to the centrum(measured from the?oor of the neural canal), and the postzygapophyses at least140mm dorsal to the

centrum.

In anterior view,too,the pedicels are robust,being25mm in width.They merge gradually into the CPRLs and extend dorsally for at least80mm,dorsal to which they are broken.In this aspect,however,the neural canal has no roof,instead forming a large teardrop-shaped vacuity120mm tall and55mm wide. The dorsal portion of this vacuity is bounded by a pair of gently curved,dorsomedially directed laminae unknown in other sauro-pods,which meet at a55degree angle to form an arch dorsal to the neural canal.The vacuity is?lled with matrix,so the extent of its penetration posteriorly into the neural arch cannot be assessed.The prezygapophyses are absent;their articular surfaces were probably about140mm above the?oor of the neural canal,judging by the trajectory of the PRPL.

The most anterodorsal preserved portion of the vertebra is obscured by a?at,anterodorsally directed‘apron’of matrix, 15mm thick and120mm wide,which hampers interpretation of the prezygapophyseal area.

COMPARISONS AND INTERPRETATION The large size of the specimen,combined with its concave cotyle,lateral foramina and complex system of bony lami-nae,indicate that it is a sauropod vertebra(Salgado et al. 1997,p.6;Wilson and Sereno1998,pp.42–43).Within this group,the deep excavation of the anterior face of the neural arch and the height of the neural arch exceeding that of the centrum(Upchurch1998,char.B7,B6)place the specimen within the clade(Barapasaurus+Eusauro-poda).The deep lateral foramen indicates that the speci-men is within or close to Neosauropoda(Salgado et al. 1997,pp.8–9;Wilson and Sereno1998,p.44;Upchurch 1998,char.B5),as does the camerate internal structure of the centrum(Wedel2003,p.354).Possession of an ACPL suggests placement with Neosauropoda(Upchurch1998, char.H3),a group of advanced sauropods consisting of diplodocoids,macronarians(camarasaurids,brachio-saurids and titanosaurs),and in some phylogenies Hap-locanthosaurus Hatcher,1903a.This identi?cation is corroborated by the fact that no de?nitive non-neosauro-pod sauropods are known from the Cretaceous(Upchurch and Barrett2005,p.119):Jobaria tiguidensis Sereno,Beck, Dutheil,Larsson,Lyon,Moussa,Sadleir,Sidor,Varricchio, Wilson and Wilson,1999from the Lower Cretaceous or Cenomanian of Niger,Africa,was recovered as a non-neosauropod by Sereno et al.(1999)and Wilson(2002), but as a basal macronarian by Upchurch et al.(2004). ACPLs are also present,apparently by way of conver-gence,in mamenchisaurids,i.e.the mostly Chinese radia-tion of basal eusauropods including Mamenchisaurus Young,1954and Omeisaurus Young,1939(Upchurch 1998,char.D4),suggesting an alternative identity for R2095.[Upchurch termed these animals‘euhelopodids’, but since Euhelopus Romer,1956itself is recovered outside this group in some analyses(Wilson and Sereno 1998;Wilson2002),this name is misleading.Of the other available names for this group,we prefer the older name Mamenchisauridae Young and Zhao,1972over Wilson’s (2002)Omeisauridae,as now does Wilson himself(pers. comm.2006to MPT)].The posterior dorsal vertebrae of the mamenchisaurid Mamenchisaurus hochuanensis Young and Zhao,1972indeed have ACPLs,but they do not at all resemble those of R2095,being much shorter and less de?ned.The vertebrae resemble R2095in having tall neu-ral arches;however,they lack lateral foramina entirely and their centra are amphiplatyan(Young and Zhao 1972,?g.7),thereby ruling out a mamenchisaurid iden-tity for R2095.

We now consider each neosauropod group in turn, investigating the possibility of X.proneneukos’s member-ship of these groups.

Diplodocoidea

Tall neural arches are not unusual in the dorsal vertebrae of diplodocoids;and forward-sloping neural arches are known in this group,for example in dorsals6–8of CM84,the holotype of Diplodocus carnegii(Hatcher1901,pl.7). Taken alone,these gross morphological characters of the neural arch suggest that R2095may represent a diplodo-coid.However,the length of the centrum,especially in so posterior a dorsal vertebra,argues against this possibility: the posterior dorsal centra of diplodocoids typically have EI<1?0,compared with1?25for R2095.Furthermore,the lateral foramina of diplodocoids are more anteriorly located on the centrum and not set within fossae(e.g. Hatcher1901,pl.7;Ostrom and McIntosh1966,pl.19). Among diplodocoids,rebbachisaurids differ in dorsal morphology from the better known diplodocids and dicr-aeosaurids,and in some respects R2095resembles the dorsal vertebra of the type specimen of Rebbachisaurus garasbae Lavocat,1954.As shown by Bonaparte(1999a,?g.39),that vertebra has a tall neural arch whose poster-ior margin closely approaches,though it is not continu-ous with,that of the centrum.However,it differs from R2095in many respects:for example,possession of a very prominent PCPL(LIP of Bonaparte’s usage),large and laterally diverging prezygapophyses,depressions at the base of the neural arch(Bonaparte1999a,p.173),lateral foramina not set within fossae,and a strongly arched ven-tral border to the centrum.There is,then,no basis for assigning R2095to this group.

In some phylogenies(e.g.Wilson2002,?g.13A),Hap-locanthosaurus is recovered as a basal diplodocoid close to Rebbachisauridae,and its dorsal vertebrae are quite simi-lar to those of Rebbachisaurus(compare Hatcher1903b, pl.1with Bonaparte1999a,?g.39).R2095therefore

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bears a super?cial resemblance to the dorsal vertebrae of Haplocanthosaurus,but a close relationship with that genus is precluded for the same reasons that R2095is excluded from Rebbachisauridae.The dorsal vertebrae of Haplocanthosaurus,and some rebbachisaurids[e.g.Limay-saurus(?‘Rebbachisaurus’)tessonei Calvo and Salgado, 1995],have asymmetric neural canals,but in the opposite sense from R2095:they are circular anteriorly,and tall and arched posteriorly.Furthermore,the posterior arches of the neural canals in these taxa,composed of dorsome-dially inclined CPOLs that meet below the zygapophyses, are very different from the anterior arch of R2095,which is composed of novel laminae that enclose the neural canal,laterally bound by the CPRLs.

Macronaria

The concave cotyle of R2095in so posterior a dorsal sug-gests a macronarian identity(Salgado et al.1997,p.9). The concavity is suf?ciently deep to rule out the possibil-ity of the vertebra being amphicoelous,i.e.it must have had a convex condyle;this is also interpreted as a macro-narian synapomorphy(Upchurch1998,char.J6).How-ever,the shallowness of the cotyle’s curvature makes this only a weak indication,since in brachiosaurids,camara-saurids and titanosaurs,even the posterior dorsals are strongly opisthocoelous(Wilson and Sereno1998,p.51). Among macronarians,the dorsally arched ventral margin of the centrum in lateral view suggests either a brachio-saurid or camarasaurid identity rather than a titanosauri-an one(Wilson and Sereno1998,p.51). Camarasauridae

The name Camarasauridae has been widely used(e.g. Bonaparte1986a;McIntosh1990),even though its mem-bership now seems to be restricted to Camarasaurus Cope,1877.Other putative camarasaurid genera such as Morosaurus Marsh,1878a and Cathetosaurus Jensen,1988 are currently considered synonymous with Camarasaurus (Osborn and Mook1921;McIntosh et al.1996),although morphological differences between specimens suggest that the genus may have been over-lumped.Various other genera have been referred to Camarasauridae but most of these are no longer considered to be closely related to Ca-marasaurus:for example,Opisthocoelicaudia Borsuk-Bia-lynicka,1977was considered camarasaurid by its describer and by McIntosh(1990),but is now considered titanosaurian(Salgado and Coria1993;Upchurch1998); and Euhelopus is now considered either a mamenchisaurid (Upchurch1995,1998)or closely related to Titanosauria (Wilson and Sereno1998;Wilson2002).However,remaining possible camarasaurids include Janenschia Wild,1991,considered camarasaurid by Bonaparte et al. (2000)but titanosaurian by Wilson(2002,p.248)and Upchurch et al.(2004,p.310);the unnamed proximal ?bula described by Moser et al.(2006,p.46)as camara-saurid based on the shape of the tibial articular face;and Datousaurus bashanensis Dong and Tang,1984(Peng et al.2005)and Dashanpusaurus dongi Peng,Ye,Gao, Shu and Jiang,2005.Since Camarasaurus morphology differs so characteristically from that of other sauropods, it is useful to refer to‘camarasaurid’morphology,and to that end we provisionally use the name Camarasauridae to refer to the clade(Camarasaurus supremus Cope,1877 not Saltasaurus loricatus Bonaparte and Powell,1980),i.e. the clade of all organisms sharing more recent ancestry with Camarasaurus than with Saltasaurus.

The posterior dorsals of Camarasaurus have somewhat dorsoventrally elongated neural arches(Osborn and Mook 1921,pl.70),and some posterior dorsal vertebrae of Camarasaurus have a tall infraprezygapophyseal vacuity similar in size to that of R2095(e.g.Ostrom and McIntosh 1966,pls23–25).However,the oval shape of this vacuity is very different,and there are no internal supporting lam-inae.The neural arches of camarasaurid dorsal vertebrae are typically very close to vertical,giving the vertebrae an ‘upright’appearance very different from that of R2095 (Osborn and Mook1921,?g.37;McIntosh et al.1996, pls5,9);and the small,subcircular,anteriorly placed lateral foramina of camarasaurids contrast with the medium-sized,anteroposteriorly elongate,centrally placed lateral foramen of R2095.Furthermore,camarasaurid centra are proportionally short,and their neural arches feature prominent infradiapophyseal laminae(Osborn and Mook1921,pl.70)that are absent in R2095.In summary, R2095does not closely resemble Camarasaurus,and a camarasaurid identity may be con?dently ruled out. Instead,the length of the centrum relative to the cotyle height,with an EI of1?25,suggests a titanosauriform iden-tity for X.proneneukos(Upchurch1998,char.K3).This is corroborated by the shape of the lateral foramen,which is an anteroposteriorly elongate oval(Salgado et al.1997, pp.18–19)with its posterior margin slightly more acute than its anterior margin(Upchurch1998,char.M1). Brachiosauridae

The long centrum particularly suggests a brachiosaurid identity,as Brachiosaurus Riggs,1903has the proportion-ally longest posterior dorsal centra of all sauropods.Bra-chiosaurids are the best represented sauropods in the Lower Cretaceous of England(e.g.the‘Eucamerotus’co-type specimens BMNH R89?90,the unnamed cervical vertebra MIWG7306and the undescribed partial skeleton

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MIWG BP001),so this identity is also supported on pal-aeobiogeographical grounds.

The cladistic analysis of Salgado et al.(1997)recovered a‘Brachiosauridae’that is paraphyletic with respect to Ti-tanosauria,a?nding that has been widely quoted(e.g. Wedel et al.2000b;Naish et al.2004).However,since only two putative brachiosaurids were included in the analysis(Brachiosaurus brancai Janensch,1914and Chub-utisaurus Corro,1975),this paraphyly amounts to the recovery of Chubutisaurus closer to titanosaurs than to B.brancai,which is not a particularly surprising result as its brachiosaurid af?nity has only ever been tentatively proposed(McIntosh1990,p.384),with an alternative ti-tanosaurian identity also mentioned.Furthermore,the (Chubutisaurus+Titanosauria)clade of Salgado et al.is supported only by a single synapomorphy,‘Distal end of tibia broader transversely than anteroposteriorly(rever-sal)’.That is,the distal end of the tibia of Brachiosaurus brancai is supposed to be longer than broad(Salgado et al.1997,p.26);but this seems to be contradicted by Salgado et al.’s own?gure11.In order to demonstrate that Brachiosauridae as traditionally conceived is para-phyletic,it would be necessary to perform an analysis that includes many putative brachiosaurids,such as B.altitho-rax,B.brancai,Cedarosaurus weiskopfae Tidwell,Carpen-ter and Brooks,1999;Atlasaurus imelakei Monbaron, Russell and Taquet,1999;Sauroposeidon proteles,the French‘Bothriospondylus’material,the‘Eucamerotus’co-type specimens BMNH R89?90,Pleurocoelus Marsh,1888, the Texan‘Pleurocoelus’material,Lapparentosaurus mada-gascariensis Bonaparte,1986a and the unnamed Argen-tinian brachiosaurid MPEF PV3098?9(Rauhut2006). Such an analysis would most likely indicate that some of these taxa are indeed not in the clade Brachiosauridae sensu Wilson and Sereno(1998)?(Brachiosaurus not Saltasaurus),but that a core remains.So far,the analysis that has included most putative brachiosaurids is that of Upchurch et al.(2004),which recovered a Brachiosaurus-Cedarosaurus clade,Atlasaurus as a basal macronarian and Lapparentosaurus as an indeterminate titanosauri-form.Pending restudy of this group,we assess likely membership of Brachiosauridae primarily by morphologi-cal similarity to the two Brachiosaurus species.

While the overall proportions of R2095are a good match for those of brachiosaurid dorsals,its lateral exca-vations are not characteristic of brachiosaurids.In this specimen,a deep foramen is located within a large, shallow fossa,a character usually associated with titano-saurs(Bonaparte and Coria1993,p.272),and not found in the Brachiosaurus altithorax holotype FMNH P25107 (Riggs1904,pl.72;MPT,pers.obs.2005).Only two dor-sal vertebrae belonging to Brachiosaurus brancai can be interpreted as having this feature:dorsal7of the B.bran-cai holotype HMN SII appears to have its lateral foramina located within slightly broader fossae,but its centrum is so reconstructed that this apparent morphology cannot be trusted;and the isolated dorsal vertebra HMN AR1 has a complex divided excavation that could be inter-preted in this way,but this vertebra is different from the other B.brancai material in several ways and may have been incorrectly referred(MPT,pers.obs.2005).R2095 also differs from brachiosaurid dorsal vertebrae in the dorsal placement of its foramina and its lack of infradia-pophyseal laminae.

Titanosauria

Although the lateral fossae and contained foramina of R2095are a good match for those of titanosaurs (Bonaparte and Coria1993,p.272),the specimen is in most other respects incompatible with a titanosaurian identi?cation.The neural spines of titanosaurs are pos-teriorly inclined by as much as45degrees and although the neural spine of R2095is not preserved,the35 degree anterior inclination of the neural arch makes such a posterior slope of the spine very unlikely.What remains of the neural arch does not have the‘in?ated’appearance characteristic of titanosaurs:the laminae are gracile and clearly delineated,whereas those of titano-saurs are more robust and tend to merge into the wall of the neural arch.The sharp-edged,vertical ACPL of R2095,for example,does not at all resemble the more robust and posteroventrally orientated centroparapophy-seal lamina of titanosaurs(Salgado et al.1997,p.19,?g.2).Xenoposeidon proneneukos also lacks the thick, ventrally forked infradiapophyseal laminae of titanosaurs (Salgado et al.1997,p.19).Finally,the camerate inter-nal structure of the centrum does not resemble the ‘spongy’somphospondylous structure characteristic of titanosaurs,although Wedel(2003,p.351)pointed out that there are exceptions such as Gondwanatitan Kellner and Azevedo,1999,a seemingly camerate titanosaur. The overall evidence contradicts a titanosaurian identity for R2095.

The origin of titanosaurs has traditionally been inter-preted as a vicariance event precipitated by the Late Jurassic break-up of Pangaea into the northern supercon-tinent of Laurasia and the southern supercontinent of Gondwana(e.g.Lydekker1893b,p.3;Bonaparte1984, 1999c;Bonaparte and Kielan-Jaworowska1987;Le Loeuff 1993).Wilson and Upchurch(2003,p.156)rejected this model,in part on the basis that titanosaur fossils are known from before the Pangaean break-up.However,the pre-Late Jurassic record of titanosaurs is dominated by trace fossils:‘wide-gauge’trackways(Santos et al.1994; Day et al.2002,2004;see Wilson and Carrano1999).Ti-tanosaurian body fossils from this era are in short supply

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and very fragmentary:the earliest titanosarian body-fossil known from adequate material is Janenschia from the Kimmeridgian Tendaguru Formation of Tanzania,Africa. We therefore have very little idea what the Middle Juras-sic ur-titanosaur,or its Laurasian descendants,looked like.Good Cretaceous titanosaur body fossils are known from Laurasian continents(e.g.Alamosaurus Gilmore, 1922from North America and Opisthocoelicaudia from Mongolia),but only from the Maastrichtian,and these may be interpreted as end-Mesozoic immigrants from Gondwana.The body-fossil record of endemic Laurasian Early Cretaceous titanosaurs remains extremely poor, consisting only of suggestive scraps.In this context,it is possible that Xenoposeidon proneneukos may represent a titanosaur belonging to the hypothetical endemic Laur-asian radiation,in which case it would be the?rst such known from presacral vertebral material.

In conclusion,while R2095can be con?dently identi-?ed as a member of Neosauropoda,its unusual combina-tion of characters,its wholly unique characters and the paucity of comparable Wealden or other Early Cretaceous Laurasian material preclude assignment to any more speci?c group within that clade.

Phylogenetic analysis

In light of the uncertain result of group-by-group com-parisons,and despite the fragmentary material,a preli-minary phylogenetic analysis was performed in the hope of elucidating the phylogenetic position of Xenoposeidon. We used the data of Harris(2006)and added the new taxon,yielding a matrix of31taxa(29ingroups and two outgroups)and331characters.Because of the paucity of material,Xenoposeidon could be scored for only13char-acters,4per cent of the total(Table2).Following Harris (2006),PAUP*4.0b10(Swofford2002)was used to per-form a heuristic search using random stepwise addition with50replicates and with maximum trees?500,000. The analysis yielded1089equally parsimonious trees with length?785,consistency index(CI)?0?5248, retention index(RI)?0?6871,and rescaled consistency index(RC)?0?3606.

The strict consensus tree(Text-?g.6A)is poorly resolved,with Neosauropoda,Diplodocoidea and Macro-naria all collapsing,and only Flagellicaudata and its subclades differentiated within Neosauropoda.This represents a dramatic loss of resolution compared to the results without Xenoposeidon(Harris2006,?g.5A),indi-cating the instability of the new taxon’s position.In the 50per cent majority rule tree(Text-?g.6B)all the stan-dard sauropod clades were recovered.This majority rule tree recovers Xenoposeidon as a non-brachiosaurid basal titanosauriform,the outgroup to the(Euhelopus+Titan-osauria)clade.However,various most-parsimonious trees also recover Xenoposeidon in many other positions, including as a brachiosaurid,basal titanosaur,basal litho-strotian,saltasaurid and rebbachisaurid.In none of the

T A B L E2.Character scores for Xenoposeidon in the matrix used for the phylogenetic analysis in this paper.Apart from the addition of Xenoposeidon,the matrix is identical to that of Harris(2006).Xenoposeidon is unscored for all characters except those listed.Con-ventional anatomical nomenclature is here used in place of the avian nomenclature of Harris.

Character Score

123Lateral fossae in majority of dorsal centra2Present as deep excavations that ramify into centrum and

into base of neural arch(leaving only thin septum in body

midline)

124Position of lateral foramina on dorsal centra2Set within lateral fossa

125Anterior face of dorsal neural arches1Deeply excavated

127Single midline lamina extending ventrally from

hyposphene in dorsal vertebrae

0Absent

134Morphology of ventral surfaces of anterior dorsal centra0Ventrally convex[inferred from posterior dorsal]

137Ratio of dorsoventral height of neural arch:

dorsoventral height of dorsal centrum

1>1?0

139Anterior centroparapophyseal lamina on middle

and posterior dorsal neural arches

1Present

140Prezygaparapophyseal lamina on middle and

posterior dorsal neural arches

1Present

141Posterior centroparapophyseal lamina on middle and posterior dorsal neural arches 1Present[as the homologous accessory infraparapophyseal lamina]

149Orientation of middle and posterior dorsal neural spines0Vertical[rather than posterodorsally inclined]

150Morphology of articular face of posterior dorsal centra1Opisthocoelous

151Cross-sectional morphology of posterior dorsal centra1Dorsoventrally compressed

153Position of diapophysis on posterior dorsal vertebrae1Dorsal to parapophysis

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most parsimonious trees does Xenoposeidon occur as a non-neosauropod,a camarasaurid or a?agellicaudatan, although in24trees it is the outgroup to Flagellicaudata. Two further steps are required if Xenoposeidon is con-strained to fall outside of Neosauropoda,and one further step if it is constrained to be a https://www.wendangku.net/doc/0018119611.html,parison to the50per cent majority rule tree calculated without Xenoposeidon(Harris2006,?g.6)shows that the inclu-sion of the new taxon greatly reduces the support for all neosauropod groups outside Flagellicaudata.The phyloge-netic instability of Xenoposeidon is a result of not only the large amount of missing data but also the unusual combi-nation of character states which,together with its autapo-morphies,prevents it from sitting comfortably within any known group.Conclusion

While X.proneneukos is clearly a neosauropod,it cannot be referred to any existing neosauropod genus,nor even to any‘family’-level or‘superfamily’-level group,a conclu-sion?rst reached by means of group-by-group compari-sons and then veri?ed by the phylogenetic analysis.Its unique characters indicate that it is either a highly derived member of one of the known groups,or,more likely,the ?rst representative of a previously unknown group.While we consider this specimen to represent a new‘family’-level clade,raising a new monogeneric family name would be premature;and the indeterminate position of the new genus within Neosauropoda means that no useful phylo-genetic de?nition could be

formulated.

Although we are reluctant to in?ict another vertebra-based taxon upon fellow sauropod workers,BMNH R2095is highly distinctive and can be separated from other sauropods,and so formal systematic recognition is appropriate.Although some workers have preferred not to raise new names for specimens represented only by limited material,a better criterion is how autapomorphic the preserved portion of the specimen is;and R2095’s suite of unique characters emphatically establishes it as distinct.In the light of its separation from all recognized major sauropod clades,failure to recognize it as a sepa-rate taxonomic entry would be misleading,as typically it is only named genera that participate in diversity surveys such as those of Holmes and Dodson(1997),Fastovsky et al.(2004)and Taylor(2006).

DISCUSSION

Historical taxonomy

While the specimen described here represents a diagnos-able taxon,the possibility that it is referable to one of the named sauropod taxa from the Hastings Beds Group must be considered.Two named sauropods are known from the Hastings Beds Group.‘Pelorosaurus’becklesii Mantell,1852is based on a humerus,ulna and radius with associated skin,discovered at Hastings.On the basis of the robustness of its limb bones,this taxon appears to be a titanosaur(Upchurch1995,p.380;Up-church et al.2004,p.308),and one of the earliest reported members of that clade.BMNH R2095therefore cannot be referred to it.[Since‘P.’becklesii is not conge-neric with the Pelorosaurus type species P.conybeari(see below)it should be given a new name,if it is suf?-ciently diagnostic.This decision falls outside the scope of the current work.]

The second taxon from the Hastings Beds Group has a complex nomenclatural history.Four proximal caudal vertebrae(BMNH R2544–2547)and three chevrons (BMNH R2548–2550)from the Hastings Beds Group of Cuck?eld,together with specimens from Sandown Bay on the Isle of Wight,were named Cetiosaurus brevis Owen, 1842.This is the?rst named Cetiosaurus species that is not a nomen dubium and thus is technically the type spe-cies.However,because the name Cetiosaurus is histori-cally associated with the Middle Jurassic Oxfordshire species C.oxoniensis Phillips,1871.Upchurch and Martin (2003,p.215)plan to petition the ICZN to make this the type species.Cetiosaurus brevis is clearly not congeneric with C.oxoniensis:accordingly,the former is referred to as‘C.’brevis from here on.The Isle of Wight‘C.’brevis material was demonstrated to be iguanodontian by Mel-ville(1849)who went on to provide the new name‘C.’conybeari Melville,1849for the Cuck?eld sauropod com-ponent of‘C.’brevis.As has been widely recognized,Mel-ville’s(1849)course of action was inadmissible as‘C.’brevis was still available for this material(Ostrom1970; Steel1970;Naish and Martill2001;Upchurch and Martin 2003)and,accordingly,‘C.’conybeari is a junior objective synonym of‘C.’brevis.

Discovered adjacent to the Cuck?eld‘C.’brevis vertebrae and chevrons was a large humerus.Mantell(1850)referred this to Melville’s(1849)name‘C.’conybeari,but decided that the taxon was distinct enough for its own genus, Pelorosaurus Mantell,1850.[As shown by Torrens(1999, p.186),Mantell considered the name Colossosaurus for this humerus].Though still discussed apart in most taxonomic reviews(e.g.Naish and Martill2001;Upchurch and Martin 2003),it is therefore clear that Pelorosaurus conybeari and ‘C.’brevis are objective synonyms,with the latter having priority.As part of the previously mentioned ICZN peti-tion,it is planned to suppress the latter name,and instead conserve the more widely used Pelorosaurus conybeari;for now,though,we continue to use‘C.’brevis.The identity and validity of this material remains problematic.The humerus lacks autapomorphies and,though it is brachio-saurid-like and,hence,conventionally identi?ed as repre-senting a member of that group(e.g.McIntosh1990),it differs in having a less prominent deltopectoral crest. Furthermore,the‘C.’brevis caudal vertebrae are titano-saur-like in at least one feature,the absence of a hyposphe-nal ridge.On this basis,Upchurch and Martin(2003) proposed that the material be referred to Titanosauriformes incertae sedis.It can be seen to be distinct from‘Pelorosau-rus’becklesii as the humeri of both species are preserved. Since R2095is similar in age and geography to‘C.’bre-vis,it is conceivable that it might belong to this species; indeed,Lydekker(1893a)assumed this to be the case, based on it being distinct from‘Eucamerotus’(‘Hoplosau-rus’of his usage)and on the unjusti?ed assumption that there were no more than two Wealden sauropods.How-ever,this assignment cannot be supported owing to the lack of overlapping material.

To confuse matters further,during part of the nine-teenth and twentieth centuries,‘C.’brevis was referred to by the name Morosaurus brevis;and it is under this name that R2095is catalogued.The description of Moro-saurus impar Marsh,1878a from the Morrison Forma-tion of Como Bluff in Wyoming initiated the naming of several new Morosaurus species,and the referral to this genus of species previously classi?ed elsewhere(Marsh 1878b,1889).Marsh(1889)evidently thought that Moro-saurus might occur in Europe,as‘Pelorosaurus’becklesii was among the species he referred to it.Nicholson and Lydekker(1889),regarding‘P.’becklesii as a junior syno-nym of‘Cetiosaurus’brevis and agreeing with Marsh’s referral of‘P.’becklesii to Morosaurus,then incorrectly

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used the new combination Morosaurus brevis.This name was now being used for assorted Lower Cretaceous Eng-lish sauropods belonging to quite different https://www.wendangku.net/doc/0018119611.html,e of M.brevis was perpetuated by Lydekker(1890,1893a) and Swinton(1934,1936).However,Marsh’s(1889)ori-ginal referral of‘Pelorosaurus’becklesii to Morosaurus was unsubstantiated as no unique characters shared by the two were identi?ed.The name Morosaurus was later shown to be a junior synonym of Camarasaurus(Osborn and Mook1921),so this name is not available for R2095because it is tied to a holotype now regarded as a junior subjective synonym.

In addition to the named taxa discussed above,a large sauropod metacarpal from Bexhill beach,derived from the Hastings Beds Group,has been identi?ed as diplodo-cid(Anonymous2005),an identi?cation con?rmed by Matthew F.Bonnan(https://www.wendangku.net/doc/0018119611.html,m.2006to DN).If cor-rectly identi?ed,this specimen indicates the presence of at least three higher sauropod taxa in the Hastings Beds Group(diplodocids,basal titanosauriforms and titano-saurs)or four if X.proneneukos indeed represents a new group.The presence of these several different taxa in coe-val or near-coeval sediments is not unexpected given the high genus-level sauropod diversity present in many other sauropod-bearing units(e.g.Morrison Formation, Tendaguru Formation).

Length and mass

Table1shows comparative measurements of R2095and the dorsal vertebrae of other neosauropods.We can reach some conclusions about the probable size of X.proneneu-kos by comparing its measurements with those of a typi-cal brachiosaurid and a typical diplodocid,reference taxa that bracket the known range of sauropod shapes.

The estimated total centrum length of R2095including the missing condyle is200mm,compared with330mm for the seventh dorsal vertebra of Brachiosaurus brancai HMN SII(Janensch1950,p.44):about60per cent as long.If R2095were built like a brachiosaurid,then it would be60per cent as long as HMN SII,yielding a length of15m based on Paul’s(1988)estimate of25m for that specimen.

The average cotyle diameter of R2095is165mm,com-pared with270mm for HMN SII:again,about60per cent.If the two animals were isometrically similar, R2095’s mass would have been about0?63?22per cent that of HMN SII.SII’s mass has been variously estimated as78,258kg(Colbert1962),14,900kg(Russell et al. 1980),46,600kg(Alexander1985),29,000kg(Anderson et al.1985),31,500kg(Paul1988),74,420kg(Gunga et al.1995),37,400kg(Christiansen1997)and25,789kg (Henderson2004).Of these estimates,those of Russell et al.(1980)and Anderson et al.(1985)can be discarded, as they were extrapolated by limb-bone allometry rather than calculated from the volume of models.The estimates of Colbert(1962)and Gunga et al.(1995)can also be dis-carded,as they are based on obviously overweight models. The average of the remaining four estimates is35,322kg. Based on this?gure,the mass of R2095might have been in the region of7600kg,about the weight of a large Afri-can bush elephant(Loxodonta africana).

R2095would have been longer and lighter if it were built like a diplodocid.Its centrum length and average cotyle diameter of200mm and165mm compare with measurements of270mm and295mm for corresponding vertebrae in Diplodocus carnegii CM84.Therefore,if X.proneneukos were diplodocid-like it would be perhaps 74per cent as long as a27-m Diplodocus,i.e.20m.Its volume can be estimated as proportional to its centrum length multiplied by the square of its average cotyle dia-meter,under which assumption it would have been23 per cent as heavy as Diplodocus:2800kg,based on Wedel’s(2005)mass estimate of12,000kg for CM84. While R2095represents an animal that is small by sauropod standards,neurocentral fusion is complete and the sutures completely obliterated,indicating that it belonged to an individual that was mostly or fully grown (Brochu1996).

Sauropod diversity

Historically,Sauropoda has been considered a morpho-logically conservative group,showing less diversity in body shape than the other major dinosaurian groups, Theropoda and Ornithischia(e.g.Wilson and Curry Rog-ers2005,pp.1–2).For many decades,the basic division of sauropods into cetiosaurs,mamenchisaurs,diplodoc-oids,camarasaurs,brachiosaurs and titanosaurs seemed established,and as recently as30years ago,Coombs (1975,p.1)could write that‘little information in the form of startling new specimens has been forthcoming for sauropods over the last40years’.Recent?nds are chang-ing this perception,with the discovery of previously unknown morphology in the square-jawed rebbachisaurid Nigersaurus Sereno,Beck,Dutheil,Larsson,Lyon,Moussa, Sadleir,Sidor,Varricchio,Wilson and Wilson1999,the long-legged titanosaur Isisaurus Wilson and Upchurch, 2003(originally‘Titanosaurus’colberti Jain and Bandyo-padhyay1997),the short-necked dicraeosaurid Brachy-trachelopan Rauhut,Remes,Fechner,Cladera and Puerta, 2005,and the truly massive titanosaurs Argentinosaurus Bonaparte and Coria,1993,Paralititan Smith,Lamanna, Lacovara,Dodson,Smith,Poole,Giegengack and Attia, 2001and Puertasaurus Novas,Salgado,Calvo and Agno-lin,2005.During the same period,Rebbachisauridae has

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emerged as an important group(Calvo and Salgado1995; Pereda Suberbiola et al.2003;Salgado et al.2004). Perhaps most interesting of all is the recent erection of two sauropod genera that arguably do not?t into any established group:Agustinia Bonaparte,1999b and Tendaguria Bonaparte,Heinrich and Wild,2000.Both of these genera are represented by specimens so different from other sauropods that they have been placed by their authors into new monogeneric‘families’,Agustiniidae and Tendaguriidae.Together with X.proneneukos,these taxa emphasize just how much remains to be discovered about the Sauropoda and how little of the full sauropod diversity we presently understand.It is hoped that the discovery of new specimens will allow the anatomy and relationships of these enigmatic new sauropods to be elucidated.

CONCLUSIONS

BMNH R2095is a highly distinctive dorsal vertebra with several features unique within Sauropoda,and as such warrants a formal name,Xenoposeidon proneneukos.It does not seem to belong to any established sauropod group more speci?c than Neosauropoda,and may repre-sent a new‘family’.Xenoposeidon adds to a growing understanding of the richness of sauropod diversity,both within the Hastings Beds Group of the Wealden,and globally.

Acknowledgements.We thank Sandra D.Chapman(Natural History Museum,London)for access to the specimen,and Nick Pharris(University of Michigan)for etymological assistance. Matthew F.Bonnan(Western Illinois University)and Jeffrey A. Wilson(University of Michigan)gave permission to cite per-sonal communications.We used English translations of several papers from the very useful Polyglot Palaeontologist website https://www.wendangku.net/doc/0018119611.html,/palaeo/palaeoglot/index.cfm and gratefully acknowledge the efforts of the site maintainer Matthew T.Carr-ano.Speci?c thanks are due to the following translators:Sebas-tia′n Apestegu?′a(Bonaparte1999a),Matthew T.Carrano (Bonaparte1986b),the late William R.Downs(Young and Zhao 1972),Matthew https://www.wendangku.net/doc/0018119611.html,manna(Corro1975;Bonaparte and Coria 1993;Lavocat1954)and Jeffrey A.Wilson(Salgado and Coria 1993).In addition,portions of Janensch(1914)were translated by Gerhard Maier.David M.Martill(University of Portsmouth), Jerald D.Harris(Dixie State College),Leonardo Salgado(Museo de Geolog?′a y Palaeontolog?′a,Buenos Aires)and two anony-mous reviewers provided thorough reviews of the manuscript which greatly improved its quality.Fiona Taylor’s careful proof-reading enabled us to correct several minor errors.Finally,we thank editor Oliver W.M.Rauhut(Bayerische Staatssammlung fu¨r Pala¨ontologie und Geologie)and Editor-in-Chief David J. Batten(University of Manchester)for their diligence and?exi-bility in dealing with this manuscript,even in the face of differ-ences of opinion.REFERENCES

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