Bardet & Suberbiola, 2001

C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 332 (2001) 395–402 2001 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS. Tous droits réservésS1251-8050(00)01486-5/FLA

Paléontologie / Palaeontology(Paléontologie des Vertébrés / Vertebrate Palaeontology)

The basal mosasaurid Halisaurus sternbergii from theLate Cretaceous of Kansas (North America): a reviewof the Uppsala type specimen

Nathalie Bardeta,∗, Xabier Pereda Suberbiolaa,b

a Laboratoire de paléontologie, UMR 8569 du CNRS, Muséum national d’histoire naturelle, 8, rue Buffon, 75005 Paris, Franceb Universidad del País Vasco/Euskal Herriko Unibertsitatea, Facultad de Ciencias, Departamento de Estratigrafía y Paleontología,Apartado 644, 48080 Bilbao, Spain

Received 15 September 2000; accepted 27 September 2000

Communicated by Philippe Taquet

Abstract – The type specimen ofClidastes sternbergii Wiman, 1920, a basal mosasauridfrom the Santonian of Kansas (USA), is reviewed. Its attribution toHalisaurus is confirmed.H. sternbergii is mainly defined on the basis of cranial characters: frontal with a smoothdorsal surface bearing a prominent median ridge; parietal with a triangular table extendingfar posteriorly and bearing a medium sized circular foramen, which is located at a distanceequal to twice its diameter from the frontal-parietal suture. The vertebral column andappendicular skeleton retain many plesiomorphies.H. sternbergii is the oldest species ofthe genus, also known in the Maastrichtian. 2001 Académie des sciences / Éditionsscientifiques et médicales Elsevier SAS

Squamata / Mosasauridae / Halisaurus sternbergii / Late Cretaceous / North America

Résumé – Le mosasauridé basal Halisaurus sternbergii du Crétacé supérieur du Kansas(Amérique du Nord) : une révision du spécimen type d’Uppsala. Le spécimen type deClidastessternbergii Wiman, 1920, un mosasauridé basal du Santonien du Kansas (États-Unis), est révisé. Sonattribution àHalisaurus est confirmée.H. sternbergii est défini essentiellement à partir de caractèrescrâniens, notamment un frontal à surface dorsale lisse, parcourue par une crête médiane marquée, etun pariétal à table triangulaire très étendue vers l’arrière, portant un foramen circulaire de taillemoyenne, situé à deux fois son diamètre de la suture fronto-pariétale. Sa colonne vertébrale etson squelette appendiculaire retiennent de nombreuses plésiomorphies.H. sternbergii est la plusancienne espèce du genre, connu également dans le Maastrichtien. 2001 Académie des sciences /Éditions scientifiques et médicales Elsevier SAS

Squamata / Mosasauridae / Halisaurus sternbergii / Crétacé supérieur / Amérique du Nord

Version abrégée

1. Introduction

Le mosasauridéClidastes sternbergii fut décrit en 1920par Wiman [10] à partir d’un squelette subcomplet pro-venant du Membre Smoky Hill de la Craie du Niobrara

∗ Correspondence and reprints.E-mail addresses: [email protected] (N. Bardet),

[email protected] (X. Pereda Suberbiola).

(Santonien) de Beaver Creek, comté de Logan (Kansas).Le spécimen, récolté par C.H. Sternberg, fut acheté par lePaleontologiska Museet de l’université d’Uppsala (Suède),où il est toujours conservé (UPI R 163). Russell [8] a étéle premier à suggérer que cette espèce puisse appartenirà un nouveau genre différent deClidastes, avant de la ré-attribuer au genreHalisaurus [9]. Depuis, le statut géné-

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N. Bardet, X. Pereda Suberbiola / C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences 332 (2001) 395–402

rique de cette espèce a été maintes fois discuté. DeBraga etCarroll [4] ont approuvé son exclusion deClidastes, touten questionnant son attribution àHalisaurus. Lingham-Soliar [7] a rapporté cette espèce àClidastes. Au contraire,Caldwell et Bell [3], Caldwell [2], Bell [1] et Holmes etSues [6] ont attribuéC. sternbergii à Halisaurus. Une ré-vision du spécimen d’Uppsala est ici faite ; elle permet dediscuter son attribution générique et de proposer une diag-nose émendée de l’espèceC. sternbergii.

2. Préservation du spécimen

UPI R 163 est un squelette subcomplet, d’environ2,60 m de long, appartenant à un même individu (figure 1),dont certains éléments ont été restaurés ou recouverts deplâtre [10]. Du crâne, seuls le maxillaire droit, le frontal,le pariétal, les carrés et une partie du préfrontal droit, dupostorbitofrontal droit, des squamosaux et des supratem-poraux sont disponibles (figure 2A). Les narines, les or-bites et les fosses temporales sont totalement restaurées.La branche mandibulaire gauche articulée est presque en-tièrement conservée (dentaire, angulaire, splénial, coro-noïde, surangulaire, articulaire), contrairement à la droite,les deux étant visibles en vue latérale (figure 2B). Toutesles dents sont artificielles. Les vertèbres sont articulées etpréservées en vue latérale droite, sauf quelques caudalesqui sont déplacées. La forme des surfaces articulaires etla condition du zygosphène–zygantrum ne sont donc pasaccessibles. Certaines vertèbres de la partie antérieure dutronc ainsi que des épines neurales et hémales des caudalesont été restaurées. Ainsi, la forme de l’aileron caudal (detype Mosasaurinae) est probablement artificielle. Les cein-tures et les membres sont bien conservés, mais certains ossont déplacés. Scapulas et coracoïdes reposent en vue mé-diale (figure 3A), de telle sorte que la cavité glénoïde n’estpas visible (contra [4]) et se situe à l’opposé de la tête hu-mérale. La ceinture pelvienne conserve les ilions, le pu-bis gauche et les ischions incomplets, tous exposés en vuelatérale (figure 3C). Les membres antérieurs sont articu-lés et également conservés en vue latérale, à l’exceptiondu premier métacarpien droit, préservé en vue médiale (fi-gure 3B). Le carpe inclut apparemment cinq et non quatreéléments [1, 2, 4]. Les deux fémurs ainsi que le tibia etla fibula gauches sont les seuls os conservés du membrepostérieur. Fémur, tibia et fibula gauches reposent en vuemédiale (figure 3D), le fémur droit en vue latérale.

3. Discussion

UPI R 163 est rapporté àHalisaurus à partir des syna-pomorphies suivantes ([1, 4, 6] et étude en cours) : frontalà bords latéraux presque droits et portant une crête lon-gitudinale médiane marquée ; pariétal court et large avecun foramen pariétal moyen à grand ; carré avec des pro-cessus infra- et suprastapédiaux coalescents, un processussuprastapédial fortement enflé distalement, un processusinfrastapédial médialement très proéminent et un bord tym-panique en forme de point d’interrogation (figure 2C) ; co-ronoïde long et étroit à aile postérieure très élevée ; articu-

laire formant la majorité de la surface glénoïde ; synapo-physes des vertèbres cervicales et dorsales étendues bienen dessous du bord ventral du centrum.

La forme très particulière du carré deH. sternbergii asouvent fait penser qu’il pourrait s’agir d’une déforma-tion [10] ou d’une pathologie [8]. Les mêmes argumentsont été invoqués concernant le carré deH. ortliebi (IRSNBR 34) du Maastrichtien de Belgique [7]. En fait, les carrésdeH. sternbergii et deH. ortliebi présentent une morpho-logie générale identique, également partagée parH. pla-tyspondylus, du Maastrichtien d’Amérique du Nord [6] etpar une nouvelle espèce deHalisaurus du Maastrichtien duMaroc, qui est en cours d’étude. Les caractères suscités ducarré sont considérés comme diagnostiques du genreHali-saurus.

Wiman [10] a attribué UPI R 163 àClidastes, à causede la fusion des arcs hémaux aux vertèbres caudales, et l’adifférencié des autres espèces du genre à partir de la mor-phologie de son pariétal et de la structure très primitive deses membres. De même, la présence de longs arcs hémauxfusionnés, d’épines neurales allongées sur les caudales mé-dianes et le développement d’une projection antérodistalesur le radius ont été considérés par Russell [8] comme descaractères propres àClidastes. Lingham-Soliar [7] a ex-clu l’espècesternbergii du genreHalisaurus et l’a rappor-tée àClidastes d’après la taille du foramen pariétal et sasituation par rapport à la suture fronto-pariétale. Cepen-dant, ces caractères ne permettent pas d’attribuer le spé-cimen d’Uppsala àClidastes. La fusion des arcs hémauxaux centres caudaux est une synapomorphie acquise de ma-nière convergente chez les Mosasaurinae,Prognathodon etHalisaurus [4]. Comme cela a déjà été discuté plus haut,l’élongation des épines neurales des caudales médianes estprobablement artificielle. La projection antérodistale du ra-dius est développée, mais pas autant que chezClidastes.Le pariétal deClidastes est très différent de celui deHali-saurus, avec une table rectangulaire, un très petit foramenet une suture fronto-pariétale ondulée. De plus,Clidastes(espècespropython et liodontus) diffère de l’espècestern-bergii par un préfrontal à processus supraorbitaire déve-loppé, un frontal à bords latéraux ondulés et crête médianefaible, un carré dont le processus suprastapédial est modé-rément grand et non coalescent avec le processus infras-tapédial, une formule vertébrale distincte (42 présacrées+ 7 caudales pygales + 26 caudales médianes et 46 cau-dales terminales), des synapophyses qui atteignent à peinela surface ventrale du centrum, un carpe composé de septéléments ossifiés et une scapula bien développée en formed’éventail. Un grand préfrontal et un frontal long et étroitsont des plésiomorphies partagées parClidastes et Hali-saurus.

Halisaurus sternbergii est un taxon valide, qui se diffé-rencie des autres espèces deHalisaurus à partir des auta-pomorphies crâniennes suivantes : frontal à surface dorsalelisse, parcourue par une crête médiane marquée, mais éten-due seulement sur les deux tiers antérieurs ; pariétal à tabletriangulaire très étendue vers l’arrière et portant un fora-men circulaire de taille moyenne, situé à deux fois son dia-

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mètre de la suture fronto-pariétale.H. sternbergii est aussicaractérisée par sa formule vertébrale, qui comprend septcervicales, 24 dorsales, quatre caudales pygales, 28 cau-dales médianes et au moins 41 caudales terminales, unequeue aussi longue que le tronc, par un humérus portantune grande cicatrice pour leM. latissimus dorsi, par un ra-dius à processus antérodistal développé, par un carpe pro-bablement composé de cinq éléments ainsi que par un tibialégèrement étendu antérodistalement et portant une surfacearticulaire distale rectiligne.H. sternbergii fournit des in-formations de valeur quant à la morphologie appendicu-laire deHalisaurus, inconnue chez les autres espèces dugenre. En attendant la découverte de matériel axial completet appendiculaire chez les autres espèces deHalisaurus, lescaractères postcrâniens suscités sont provisoirement consi-dérés comme des autapomorpies deH. sternbergii.

Halisaurus est actuellement considéré comme le mosa-sauridé le plus primitif [1, 4]. Il est plus dérivé que les« aigialosaures », car il possède entre 27 et 33 vertèbresprésacrées, des vertèbres sacrées non fonctionnelles (pasde contact entre l’ilion et les côtes sacrées), des épines hé-males fusionnées aux centres caudaux, des éléments desceintures non ossifiés entre eux, des os du propodium etde l’épipodium plus adaptés à la locomotion aquatique, un

carpe réduit, un premier métacarpien plus robuste que lesautres, un cinquième métacarpien d’environ 80 % de la lon-gueur du plus long et un pubis mince distalement. Tous cescaractères sont des synapomorphies de Mosasauridae [4].

Comparé aux autres mosasauridés,H. sternbergii retientles plésiomorphies suivantes [4, étude en cours] : grandpréfrontal, frontal long et étroit, suture fronto-pariétalerectiligne, branches suspensoriales du pariétal verticales,processus rétroarticulaire vertical et triangulaire, quatrecaudales pygales, caudales globalement plus longues quehautes, épines neurales et hémales inclinées postérieure-ment sur toute la longueur de la queue, scapula nettementplus petite que le coracoïde et peu développée en éventail,coracoïde avec une grande échancrure antérieure et un fora-men plus près du col que chez les autres mosasauridés, ilionrecourbé vers l’arrière, os propodiaux et épipodiaux longset minces à tête articulaire non ossifiée, humérus portantun sillon ectépicondylaire, ulna à olécrâne bien développé,foramens épipodiaux lenticulaires.

Halisaurus est connu du Santonien à la fin du Maastrich-tien et a été découvert en Amérique du Nord [1, 8, 10] etdu Sud [3], en Europe [7] et en Afrique [étude en cours].H. sternbergii est la plus vieille espèce du genre.

1. Introduction

In 1920, Wiman [10] described a new species of themosasauridClidastes Cope, 1868 asC. sternbergii.It was based on a nearly complete skeleton collectedin 1918 in the Smoky Hill Member of the NiobraraChalk (Late Cretaceous, Santonian) of Beaver Creek,Logan County (Kansas, USA), during a C.H. Stern-berg expedition. The specimen was purchased by theSwedish Paleontologiska Museet of the Uppsala Uni-versity, where it is still kept under the collection num-ber UPI R 163.

Although emphasizing Clidastes affinities ofC. sternbergii, Russell [8, p. 127] suggested that thisspecies could belong to a new genus distinct fromCli-dastes. Later, Russell [9, p. 369] transferredC. stern-bergii to Halisaurus Marsh, 1869, arguing from simi-larities between the Uppsala specimen and new mater-ial from Alabama, namelyC. sternbergii-like frontalsfound associated with typical halisaurine vertebrae.

Since that time, the generic status of this specieshas been subject to discussion by several authors,especially during the past few years. DeBraga andCarroll [4] agreed with the removal of this speciesfrom Clidastes but have questioned its inclusion in thegenusHalisaurus. Lingham-Soliar [7] disagreed withRussell’s generic attribution and again referred theUppsala specimen toClidastes. Conversely, Caldwelland Bell [3], Caldwell [2], Bell [1] and Holmes

and Sues [6] regardedC. sternbergii as belonging toHalisaurus.

The purpose of this paper is to provide a detaileddescription of specimen UPI R 163 and to discussits generic assignment. An emended diagnosis isproposed.

2. Preservation of the specimen

The specimen UPI R 163 is a nearly completeskeleton with skull, about 2.60 m long, mounted ona painted plaster slab (figure 1). Dollo [5, p. 198]suggested that this specimen was a composite butthere is currently no argument in favour of thisinterpretation. On the contrary, Wiman [10, p. 13]noted that all parts belong to a single specimen, butsome elements of the skeleton have been restored orcovered with plaster.

Of the skull, only the right maxilla, frontal, parietal,quadrates, and parts of the right prefrontal, rightpostorbitofrontal, squamosals and supratemporals areavailable (figure 2A). The pterygoids are present butremain unprepared. The nares, orbits and temporalfenestrae have been completely restored with plasterand are unreliable for description (figures 1B–1C).Thus, the precise length and width of the skull areunknown and can only be estimated as at least 11 cmwide and more than 30 cm long. From the mandible,most of the articulated left (dentary, angular, splenial,

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Figure 1. Halisaurus sternbergii, type specimen (UPI R 163, Uppsala, Sweden), Smoky Hill Member, Niobrara Chalk (Santonian), Beaver Creek,Logan County (Kansas, USA).A, global view;B, skull in dorsal view;C, skull and mandible in dorso-lateral view;D, pelvic girdle and hindlimbdetails;E, pectoral girdle and forelimb details. Scale= 5 cm.

Figure 1. Halisaurus sternbergii, spécimen type (UPI R 163, Uppsala, Suède), membre Smoky Hill, craie du Niobrara (Santonien), Beaver Creek,comté de Logan (Kansas, États-Unis).A, vue générale ;B, crâne en vue dorsale ;C, crâne et mandibule en vue dorso-latérale ;D, détails de laceinture pelvienne et des membres postérieurs ;E, détails de la ceinture pectorale et des membres antérieurs. Échelle= 5 cm.

coronoid, surangular, articular) and fragments of theright ramus are preserved but are visible only in lateralview (figure 2B). All teeth are artificial.

The vertebrae are preserved articulated in right lat-eral view, except those in the pelvic region and somefrom the caudal series, which have been slightly dis-placed (figures 1A and1D–1E). As a result, the shapeof the articulating surfaces and the condition of thezygosphene–zygantrum cannot be established. Somevertebrae from the anterior part of the trunk, as wellas some neural and haemal spines in the mid and ter-minal caudal region have been restored, so that themosasaurine-like caudal fin elevation is probably arti-ficial.

The girdles and limbs are well preserved, but somebones are displaced (figures 1D–1E). Wiman [10,p. 13] noted that “bones of the extremity are partlytransferred on plaster”. Both scapulae and coracoidslay in medial view (figure 3A), so that, in contrary to[4, fig. 15], the lateral glenoid cavity is not visible.This cavity is located opposite the humeral head. Thepelvic girdle consists of both ilia, the left pubis andincomplete ischia; all bones are preserved in lateralview (figure 3C).

Most of the forelimb elements are articulated (fig-ure 3B). They are exposed in extensor (i.e., lateral ordorsal) view, in agreement with the interpretation ofBell [1, fig. 8F]. As a result, the deltopectoral crest,

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Figure 2. Halisaurus sternbergii, type specimen UPI R 163, drawings of the skull and mandible. The recognised bones are indicated in grey.A,skull in dorsal view;B, mandible in lateral view;C, right quadrate in lateral view.An, angular;Ar, articular;c, quadrate condyle;Co, coronoid;ct, calcified tympanum;De, dentary;Fr, frontal; isp, infrastapedial process;Mx, maxilla; Pa, parietal;Pf, prefrontal;Pof, postorbitofrontal;Sa,surangular;sn, stapedial notch;Sp, splenial;Sq, squamosal;ssp, suprastapedial process;St, supratemporal;tr, tympanic rim. Scale bars= 5 cm(A, B) and 2 cm (C).

Figure 2. Halisaurus sternbergii, spécimen type UPI R 163, dessins du crâne et de la mandibule. Les os reconnus sont indiqués en gris.A, crâneen vue dorsale ;B, mandibule en vue latérale ;C, carré droit en vue latérale.An, angulaire ;Ar, articulaire ;c, condyle du carré ;Co, coronoïde ;ct, tympan calcifié ;De, dentaire ;Fr, frontal ; isp, processus infrastapédial ;Mx, maxillaire ;Pa, pariétal ;Pf, préfrontal ;Pof, postorbitofrontal ;Sa, surangulaire ;sn, échancrure stapédiale ;Sp, splénial ;Sq, squamosal ;ssp, processus suprastapédial ;St, supratemporal ;tr, bord tympanique.Échelles= 5 cm (A, B) et 2 cm (C).

Figure 3. Halisaurus sternbergii, type specimen UPI R 163, interpretative drawings of girdles and limbs.A, left pectoral girdle in medial view;B, right forelimb in lateral view;C, left pelvic girdle in lateral view;D, left hindlimb in medial view.Co, coracoid;c, carpal;ce, coracoid anterioremargination;cf, coracoid foramen;eg, ectepicondylar groove;F, fibula;Fe, femur;fh, femoral head;hh, humeral head;Hu, humerus;Il, ilium; Is,ischium;ld, scar for the musclelatissimus dorsi; Mc I–V, metacarpals I to V;Ph, phalanges;pp, preaxial ridge;Pu, pubis;R, radius;Sc, scapula;T, tibia; U, ulna. Scale= 5 cm.

Figure 3. Halisaurus sternbergii, spécimen type UPI R 163, dessins interprétatifs des ceintures et des membres.A, ceinture pectorale gauche en vuemédiale ;B, membre antérieur droit en vue latérale ;C, ceinture pelvienne gauche en vue latérale ;D, membre postérieur gauche en vue médiale.Co, coracoïde ;c, carpien ;ce, échancrure antérieure du coracoïde ;cf, foramen coracoïdien ;eg, sillon ectépicondylaire ;F, fibula ; Fe, fémur ;fh, tête fémorale ;hh, tête humérale ;Hu, humérus ;Il, ilium ; Is, ischium ; ld, cicatrice pour l’insertion du musclelatissimus dorsi ; Mc I–V,métacarpiens I à V ;Ph, phalanges ;pp, ride préaxiale ;Pu, pubis ;R, radius ;Sc, scapula ;T, tibia ; U, ulna. Échelle= 5 cm.

ectepicondyle (radial tuberosity), capitulum, trochleaand entepicondyle (ulnar tuberosity) of the humeriare not visible (contra [4, fig. 16C]). The right firstmetacarpal is present in flexor view. The right carpus

apparently includes five elements, not four [1, 2, 4].Four carpals are found distally to the epipodium andone proximally. In the left forelimb, only three carpalelements are preserved. Both femora, the left tibia and

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fibula are located near the pelvic girdle (figure 3D).The bones of the left hindlimb lay in flexor (i.e., me-dial or ventral) view, whereas the right femur is seenin lateral view.

3. Description

3.1. Skull (figures 1B–1C, 2A and C)

The maxilla is long and narrow (preserved length18.5 cm). The incomplete prefrontal is rather largeand its suture with the frontal is straight. The frontalis very long and narrow, with almost straight lateralborders. The posterolateral margins of the frontal areconcave and the lateral corners are sharp. A medianridge, more marked anteriorly, is present on the an-terior two-thirds of the bone. Otherwise, the dorsalsurface of the frontal is smooth. The presence of anarial emargination could not be attested. The fronto-parietal suture is straight. The parietal is very shortand broad. As noted by Wiman [10, p. 14], the pari-etal table is triangular as inPlatecarpus, but the pos-terior corner extends far posteriorly, overlapping thesuspensorial rami. The parietal foramen is subcircu-lar and of moderate size. The frontal-parietal sutureis separated from the parietal foramen by a distanceequal to twice its diameter. Anterior and posterior tothe foramen, is a narrow, longitudinal groove. Thesuspensorial rami are almost vertical. The supratem-poral articulates medially with the parietal and later-ally with the squamosal. Only the posterior part of theright postorbitofrontal is preserved, so it is not possi-ble to determine how it embraces around the fronto-parietal suture (contra [4, p. 76]). As pointed out byWiman [10, p. 15], the quadrate is very peculiar inshape, being characterised by the contact of the supra-and infrastapedial processes, a distally greatly swollensuprastapedial process, a strongly medially developedinfrastapedial process, and a ‘question mark’-shapedtympanic rim (figure 2C). The stapedial notch is largeand oval. The calcified tympanum is probably pre-served. Some elements of the palate, such as the ptery-goids, are present but they are not prepared.

3.2. Mandible (figures 1C and 2B)

Like the maxilla, the dentary is long and narrow.The splenial-angular articulation is visible. The angu-lar is well exposed in lateral view ventral to the suran-gular. The surangular is long and high, with an ex-panded articulation for the coronoid, extending overhalf of the length of the bone. The coronoid is longand narrow, with a distinct high posterior wing. Thearticular participates in most of the mandibular artic-ulation. The retroarticular process is almost verticaland triangular.

3.3. Vertebral column (figures 1A and 1D–1E)

The vertebral formula is consistent with that de-scribed by Wiman [10, p. 17]: atlas–axis complex,five other cervicals (three last reconstructed), 24 dor-sals (the two first reconstructed), four pygals thatdo not support the ilium (including the ‘sacral’ ofWiman [10]), 28 median caudals, and at least 41 ter-minal caudals. Thus, there is a total of 104 verte-brae, divided into 31 presacrals (and not 24 as listedelsewhere [4, p. 273]) and at least 73 caudals. Thetrunk is approximately as long as the tail; this is aunique character among mosasaurids, since the trunkis either longer (mosasaurines) or shorter (plioplate-carpines and tylosaurines) than the tail. Synapophy-ses are located anteriorly on the centra of cervicaland dorsal vertebrae and are connected to the prezy-gapophyses by a sharp crest. Their ventral borderprojects well below the ventral surface of the cen-trum. Haemal arches are fused to the centra. The cau-dal fin is not constructed in the typical mosasauridmanner for both neural and haemal spines are inclinedposteriorly throughout the tail, as in ‘aigialosaurs’ [4,car. 88a]. The centra of the anterior and terminal cau-dals are longer than high and those of the median onesare as high as long. The articular surfaces of the py-gals and first median caudals are roughly elliptical,while those of the terminal caudals are nearly circu-lar.

3.4. Pectoral girdle (figures 1E and 3A)

The scapulae and coracoids are not fused together.There is no evidence of an interclavicle or a sternum.The scapula is narrow and much smaller than the cora-coid; its length is about half that of the humerus. It isnot expanded above the neck and the posterior ala isonly slightly larger than the anterior one. The dorsalmargin is gently convex. The coracoid bears a typicalfan shape and includes a large anterior emargination.Its neck is very short and broad, as inClidastes. Theanterior border is apparently larger than the posteriorone. The ventral margin is regularly convex and ex-panded. The coracoid foramen is located closer to ar-ticular facet for the scapula than in other mosasaurids,and resembles the condition observed inVaranus andthe ‘aigialosaurs’ [4].

3.5. Forelimb (figures 1E and 3B)

Propodial and epipodial elements are long and slen-der. The humerus is about as long as the femur (6 cm);its proximal end is slightly larger than the distalone. The head is subspherical and not completely co-ossified to the shaft. The ectepicondylar foramen isrepresented by an oblique groove, located anterodis-tally. A large ovoid scar for theM. latissimus dorsi

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is present on the mid-length of the shaft. The radiusbears a well-developed preaxial process. The proxi-mal facet for the humerus is straight; the distal radialfacet is posteriorly located. The ulna has an hour-glassshape and is as long as the radius (ca 4 cm). The prox-imal end exhibits a prominent olecranon. The distalfacets for the carpals are undifferentiated. The radiusand ulna encloses an antebrachial lenticular foramen.The carpus consists of four or more, probably five, os-sified elements. Metacarpals and phalanges are longand slender. The first metacarpal is the largest, as isusual in mosasaurids [2]. The fifth metacarpal is about80 % the length of the others [4]. The phalangeal for-mula is unknown.

3.6. Pelvic girdle (figures 1D and 3C)

The acetabular surface is poorly ossified. The iliumis long and slender, with an expanded acetabularcavity. As preserved, the shaft is apparently inclinedposteriorly, asVaranus and ‘aigialosaurs’ [4, p. 280].There is no bony contact between the ilium and thepygal vertebrae. The pubis is long and spatulate; itsproximal end is slightly more expanded than the distalone. The pubic tubercle is broken. The two ischiaare broken at the level of the ischiatic tubercle. Theproximal neck is broad.

3.7. Hindlimb (figures 1D and 3D)

The bones of the hindlimb are long and slender.The femoral head is poorly ossified. The distal endis slightly more expanded than the proximal one andhas undifferentiated facets for the tibia and fibula. Theanterior edge of the shaft is less concave than theposterior one. The internal trochanter is not visible.The tibia is about as long as the fibula, but is morerobust. The tibia is slightly expanded anterodistally.Its distal end is not convex, as is usual in mosasaurids,but rather straight. The crural foramen is lenticular.

4. Discussion

The specimen UPI R 163 is referred to the genusHalisaurus on the basis of the following synapomor-phies [4, 6 and work in progress]: frontal with al-most straight lateral margins and a longitudinal me-dian keel; parietal short and broad with a medium tolarge foramen; quadrate with supra- and infrastape-dial processes in contact, a distally strongly swollensuprastapedial process, a medially prominent infra-stapedial process and a ‘question mark’-shaped tym-panic rim; coronoid long and narrow with a veryhigh posterior wing; articular forming the bulk of theglenoid surface; synapophysis of cervical and dorsal

vertebrae extend well below the ventral surface of thecentrum.

Wiman [10, p. 15] did not exclude the possibilitythat the peculiar form of the quadrate of UPI R 163was due to preservation and Russell [8, p. 127] notedthat the quadrate ofH. sternbergii looks pathologic.Lingham-Soliar [7, p. 133] also interpreted the differ-ences between the left and right quadrates ofH. or-tliebi (IRSNB R 34) from the Maastrichtian of Bel-gium as due to either poor preservation or patho-logic deformation. In fact, the quadrates ofH. stern-bergi and H. ortliebi are identical in shape and thedifferences result only from preservation. Moreover,they share the same general morphology than thoseof H. platyspondylus from the Maastrichtian of east-ern North America [6] and of a newHalisaurusspecies from the Maastrichtian of Morocco [work inprogress]. The above mentioned quadrate features areconsidered to be diagnostic of the genusHalisaurus.

Wiman [10, p. 17] based the attribution of UPI R 163to Clidastes on the fusion of the haemal arches tothe caudal vertebrae, and differentiated it from otherspecies of the genus by its parietal morphology andvery primitive limb structure. Russell [8, p. 127] listedthe presence of long fused haemal arches, neuralspine elongation in mid-caudals, and the developmentof a broad anterodistal flange on the radius asCli-dastes characters; the low number of thoracic verte-brae, the small scapula and slender limbs were how-ever considered by him as distinct fromClidastes.Lingham-Soliar [7, p. 134] excluded the speciesstern-bergii from Halisaurus and referred it toClidasteson the basis of the size of the parietal foramen andits location in relation to the fronto-parietal suture.However, these characters do not allow the referralof the Uppsala specimen toClidastes. The fusionof haemal arches to the caudal centra is a synapo-morphy convergently acquired by the Mosasaurinae,Prognathodon andHalisaurus [4, character 89a]. Aspreviously noted (see §2.), the elongation of the mid-caudal neural spines in UPI R 163 is probably artifi-cial. The anterodistal flange of the radius is developed,but not as much as inClidastes. The parietal ofCli-dastes is very different from that ofH. sternbergii,with a rectangular parietal table, a very small foramenand a sinuous fronto-parietal suture. Moreover,Cli-dastes (speciespropython andliodontus) differs fromH. sternbergii through a prefrontal with a large supra-orbital process, a frontal with sinuous lateral bordersand a weak median dorsal ridge, a quadrate with amoderately large suprastapedial process, which is notin contact with the infrastapedial process, a differentvertebral formula (42 presacrals + 7 pygals + 26 me-dian and 46 terminal caudals), synapophyses thatreach only to the level of the ventral surface of thecentrum, carpus composed of seven ossified elements,

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and a scapula with well-developed alae. The occur-rence of a large prefrontal and a long and narrowfrontal are plesiomorphies shared byClidastes andHalisaurus.

Halisaurus sternbergii is a valid taxon that canbe differentiated from otherHalisaurus species onthe basis of the following cranial autapomorphies: afrontal with a smooth dorsal surface bearing a markedmedian ridge extending across the anterior two-thirdsof the bone; a parietal with a triangular table extend-ing very far posteriorly and bearing a medium sizedcircular foramen, which is located at a distance equalto twice its diameter from the frontal-parietal suture.H. sternbergii is also characterised by a vertebral for-mula of 7 cervicals, 24 dorsals, 4 pygals, 28 mediancaudals and at least 41 terminal caudals; the tail aslong as the trunk; the humerus bears a large ovoid scarfor theM. latissimus dorsi; the radius has pronouncedanterodistal process; a carpus is probably composedof five elements; a tibia is slightly expanded anterodis-tally, with a straight distal articular surface.H. stern-bergii provides valuable information on the appendic-ular morphology ofHalisaurus, which is unknownin other referred species. Pending the discovery ofcomplete vertebral and appendicular material in otherHalisaurus species, the postcranial characters listedabove are provisionnally regarded as autapomophiesof H. sternbergii.

Halisaurus is currently regarded as the most prim-itive mosasaurid [1, 4]. It is more derived than ‘aigia-losaurs’ in possessing between 27 and 33 presacral

vertebrae, no functional sacrals (ilium not attached tosacral ribs), haemal spines fused to the caudal centra,girdle elements not co-ossified, propodial and epipo-dial bones more suited for aquatic locomotion, car-pus reduced, first metacarpal more robust than theothers, fifth metacarpal about 80 % the length of thelongest metacarpal, and pubis slender distally. All ofthese features are considered as synapomorphies ofthe Mosasauridae [4].

As compared to other mosasaurids,H. sternbergiiretains the following plesiomorphies [4, work inprogress]: a large prefrontal; a long and narrowfrontal; a straight fronto-parietal suture; the suspenso-rial rami of the parietal vertically oriented; a retroar-ticular process vertical in orientation and triangular inoutline; four pygal caudals; most caudal centra longerthan high; both neural and haemal spines inclined pos-teriorly throughout the tail; a scapula much smallerthan the coracoid and poorly developed into anteriorand posterior alae; a coracoid with a large anterioremargination and a foramen closer from the neck thanin other mosasaurids; an ilium posteriorly recurved;propodial and epipodial bones long and slender withpoorly ossified articular surfaces; humerus bearing anectepicondylar groove; ulna with a well developedolecranon; lenticular epipodial foramen.

Halisaurus is known from the Santonian to the endof the Maastrichtian and has been recorded in NorthAmerica [1, 8, 10], South America [3], Europe [7] andAfrica [work in progress].H. sternbergii is the oldestspecies of the genus.

Acknowledgements. We warmly thank Solweig Stuenes (Paleontologiska Museet, Uppsala Universitets) for her kindness and hospitality andfor the permission to study material in her care. Thanks are due to Philippe Loubry and Henri Lavina (CNRS, MNHN, Paris) for respectively thephotographs and drawings.

References

[1] Bell G.B., A phylogenetical revision of North American andAdriatic Mosasauroidea, in: Callaway J.M., Nicholls E.L. (Eds.),Ancient Marine Reptiles, Academic Press, New York, 1997, pp. 293–332.

[2] Caldwell M.W., Ontogeny and phylogeny of the mesopodialskeleton in mosasauroid reptiles, Zool. J. Linnean Soc. 116 (1996)407–436.

[3] Caldwell M.W., Bell G.B., Halisaurus sp. (Mosasauridae)from the Upper Cretaceous (?Santonian) of East–Central Peru,and the taxonomic utility of mosasaur cervical vertebrae, J. Vert.Paleontol. 15 (1995) 533–544.

[4] DeBraga M., Carroll R.L., The origin of mosasaurs as a modelof macroevolutionary patterns and processes, in: Hecht M.K. et al.(Eds.), Evolutionary Biology, Vol. 27, Plenum Press, New York,1993, pp. 245–322.

[5] Dollo L., Globidens alabamaensis, mosasaurien américainretrouvé dans la craie d’Obourg (Sénonien supérieur) du Hainaut, et

les mosasauriens de la Belgique en général, Arch. Biol. 34 (1924)167–213.

[6] Holmes R.B., Sues H.D., A partial skeleton of the basalmosasaurHalisaurus platyspondylus from the Severn Formation(Upper Cretaceous: Maastrichtian) of Maryland, J. Paleontol. 74(2000) 309–316.

[7] Lingham-Soliar T., The first description ofHalisaurus (Rep-tilia, Mosasauridae) from Europe, from the Upper Cretaceous of Bel-gium, Bull. Inst. Roy. Sci. Nat. Belgique, Sci. Terre 66 (1996) 129–136.

[8] Russell D.A., Systematics and morphology of Americanmosasaurs, Bull. Peabody Mus. Nat. Hist. 23 (1967) 1–240.

[9] Russell D.A., The vertebrate fauna of the Selma formation ofAlabama, Part VII: The mosasaurs, Fieldiana, Geol. Mem. 3 (1970)369–380.

[10] Wiman C., Some reptiles from the Niobrara group in Kansas,Bull. Geol. Inst. Uppsala 18 (1920) 9–18.

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