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The First Description of Halisaurus (Reptilia Mosasauridae) from Europe, from the Upper Cretaceous of Belgium

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The First Description of Halisaurus (Reptilia Mosasauridae) from Europe, from the Upper Cretaceous of Belgium BULLETIN DE LTNSTITUT ROYAL DES SCIENCES NATURELLES DE BELGIQUE SCIENCES DE LA TERRE, 66: 129-136, 1996 BULLETIN VAN HET KONINKLIJK BELGISCH INSTITUUT VOOR NATUURWETENSCHAPPEN, AARDWETENSCHAPPEN, 66: 129-136, 1996 The first description of Halisaurus (Reptilia Mosasauridae) from Europe, from the Upper Cretaceous of Belgium Theagarten LINGHAM-SOLIAR Abstract Abbreviations This study represents the first extensive description of the skull of Repository abbreviations: species of the genus Halisaurus Marsh 1869. Halisaurus ortliebi BMNH, The Natural History Museum, Cromwell Road, London SW7 (= Phosphorosaurus ortliebi of Dollo, 1889), from the Maastrichtian 5BD, UK. of Belgium, shows, several character states, including a unique ventral IRSNB, Institut Royal des Sciences Naturelles de Belgique, Rue fossa of the parietal. Comparisons are made with the North American Vautier 29, B-1000 Bruxelles, Belgium. H. platyspondylus. H. sternbergi (Wiman. 1920) is no longer conside­ GPIT, Institut für Geologie und Paläontologie, der Universität Tübin­ red as belonging to the genusHalisaurus. gen, Sigwartstrasse 10, D-72076 Tübingen, DBR. Key-words: Halisaurus, Clidastes sternbergi, taphonomy, taxonomy, Cranial abbreviations used in the text: phytogeny. a, angular; ar. articular; che, cerebral hemispheres; cor, coronoid; d, dentary; en, external naris; f, frontal; gl, intermandibular articulation; j, jugal; mx, maxilla; of, olfactory lobe; p, parietal; paf, parietal foramen; pmx, premaxilla; pof, postorbitofrontal; prf, prefrontal; pt, pterygoid; Résumé ptte, pterygoid teeth; q, quadrate; qpp, quadratic process of pterygoid; sa, surangular, sp, splenial; sq, squamosal; st, supratemporal; sta, stapes; tym, quadratic tympanum. Le crâne d'espèces appartenant au genre Halisaurus Marsh, 1869 est présenté en détail pour la première fois.Halisaurus ortliebi (= Phos­ phorosaurus ortliebi Dollo, 1889) du Maastrichtien belge montre plusieurs caractères intéressants dont une fosse ventrale unique dans Stratigraphie position of the material from The le parietale. Une comparaison avec l’espèce nord-américaineH. pla- tyspondylus est faite. H. sternbergi (Wiman, 1920) n’est plus considéré Netherlands and Belgium comme appartenant au genre Halisaurus. Halisaurus ortliebi came from the Phosphatic Chalk of Mots-clefs: Halisaurus, Clidastes sternbergi, taphonomie, taxinomie, phylogénie. Ciply, near Mesvin (Hainaut, Belgium) and Halisaurus vertebrae, from St. Pietersberg, near Maastricht, (The Netherlands) (see L in g h a m -S o l i a r & N o l f, 1989 and L i n g h a m -S o l i a r 1994a respectively for the geology, Introduction palaeoecology and palaeoenvironment) Halisaurus material described here represents only the second record of the genus outside of the U.S.A. (the first Systematic Palaeontology was recorded in Nigeria byA z z a r o l i et a l, 1975) and even in the U.S.A. as B a ir d noted (1986b p. 72) “ Despite Order S q u a m a t a more than eleven decades of collecting effort,Halisaurus Family M osasauridae G e r v a i s , 1853 remains one of the rarest and least well-known of the SubfamilyP lioplatecarpinae ( W i l l i s t o n , 1897) mosasaurs.” The sum-total of the available material consists of no more than isolated and scanty remains, Plioplatecarpinae (W il l i s t o n , 1897) mainly vertebrae and a few fragments of skulls. Primarily on the basis of the unique frontal bone of Halisaurus (see B aird & C ase 1966 which includes D. A. 1884 Plioplatecarpidae D o l l o; 6 5 3 . R ussell’s identification of a frontal as that ofHalisaurus 1890 mosasauriens microrhynquesD o l l o: 163. platy spondylus), D o llo’s (1889) Phosphorosaurus ort­ 1897 Platecarpinae W il l is t o n : 177. liebi (IRSNB R34, formerly R4671) is reassigned as 1967 PlioplatecarpinaeR u s s e l l: 148 Halisaurus ortliebi. The skull described here, although fragmentary, represents the most substantial material of D ia g n o s is Halisaurus know n yet. See R u s s e l l (1967). 130 Theagarten LINGHAM-SOLIAR Halisaurus M a r s h , 1869. cending crest to synapophysis; zygosphenes and zygantra absent; hypapophyseal peduncle located posteriorly on H alisaurus M a r s h , 1869: 395. ventral surface of cervical centra, articulation for hypa- Baptosaurus M a r s h , 1870: 3. pophysis flat and lenticular, slightly inclined posteriorly, Phosphorosaurus D o l l o ,1 889: 68. slight tilt of condyle anteriorly. Generic type. Halisaurus polyspondylus M a r s h , 1869. Halisaurus ortliebi (D o l l o, 1889) (Figs. 1-5; PI. 1) E m e n d e d g e n e r i c d ia g n o s is Very narrow frontal, subrectangular. Articulating surfa­ 1889 Phosphorosaurus ortliebi D o l l o :279-286, p i. 10, ces of cervical and anterior dorsal vertebral centra nearly tig. 6. twice as wide as deep, subrectangular or kidney bean 1991 Halisaurus ortliebi Lingham-Soliar: 663. shaped, may be slightly upturned; synapophyses located in centre of lateral surface of cervical centra, occupies H o l o t y p e somewhat more posterior position in anterior thoracics;IRSNB R34, fragmentary skull, frontal, parietal, prefron- ventral border of antero-ventral extension of synapophy­tal, postorbitofrontal, quadrate, fragments of pterygoid, sis weak and horizontal in anterior cervicals, becomes fragments of dentary and splenial. much enlarged in posterior cervicals and anterior thora­ cics, extending far below flattened undersurface of cen­ H o r iz o n a n d l o c a l i t y o f h o l o t y p e trum; anterior zygapophyses of cervical and anteriorPhosphatic Chalk of Ciply, Upper Maastrichtian, near thoracics connected by gently rounded, posteriorly des-Mesvin, Hainault, Belgium. & FF Fig. 1 — Restored skull ofHalisaurus ortliebi IRSNB R34. A, lateral view; B, dorsal view. Scale bar = 100 mm. Halisaurus from Belgian Upper Cretaceous 131 Fig. 2 — Skull table of IRSNB R34. A, dorsal view of parietal and frontal; B, ventral view of the parietal and frontal. Scale bar = 50 mm. D ia g n o s is Frontal Large prefrontal, almost entirely overlain by maxilla The frontal in IRSNB R34 (Figs. 2B. D) is exceedingly and frontal. Rounded crest on frontal extends almost narrow with an exceptionally short and straight frontopa­ the entire length of the bone; sides virtually parallel, rietal suture. The lateral margins are relatively straight slight emargination over orbits. Little evidence of and only slightly emarginate above the orbits. A promi­ consolidation along mesokinetic axis. Enormous subcir­nent median crest extending almost the entire length of cular parietal foramen, most of the margin formed by the bone, dissipates into a delta of fine ridges near the the parietal; parietal narrow, elliptical excavation on posterior margin. The latter condition has not been des­ ventral surface behind the parietal foramen. Robust pte­ cribed in mosasaurs before. Anteriorly, a long slender rygoid, approximately eight posteriorly recurved teeth.medial process forms part of the intemarial bar and Fused supra and infrastapedial processes in a moderate bounds the postero-medial portion of the external nares. sized quadrate rather than in the usually large quadrate, The ventral surface of the bone is considerably eroded but relatively shallow quadratic tympanum; pinched stape­ along its midpoint a slight excavation marks the position dial pit. of the olfactory lobes. Shallow excavations along the lateral borders of the frontal indicate the location for D escriptions a n d comparisons the prefrontal and postorbitofrontal wings. A cleft at the Descriptions and restorations are based essentially on the midpoint of the posterior boundary of the frontal repre­ holotype IRSNB R34. Comparisons are made with the sents a small anterior portion of the enormous parietal North American type speciesHalisaurus platyspondylus foramen. Descending processes of the frontal are appa­ (based on R u s s e l l , 1967) from the Upper Cretaceous, rently little developed. Greensand, Navesink Formation of New Jersey, Horners- Prefrontal town. R u s s e l l ’ s (1970, p. 369) assignment ofClidastes The prefrontal (Figs. 3A, B) is relatively large and the sternbergi toHalisaurus is questioned (discussed later) prefrontal ala or shelf is relatively small. An excavation and it is consequently not considered for generic compa­ on its external dorsomedial surface indicates extensive rative purposes. overlap by the frontal. The maxilla overlapped the pre­ frontal on the lateral surface as far as a distinct dorsola­ S k u l l teral ridge, leaving very little of the prefrontal exposed. The skull (Fig. I and Pl.l) is fragmentary, essentially Poor preservation makes it impossible to determine whe­ lacking the dental rami, maxilla, postmandibular units ther the frontal bordered the orbit or was precluded by the and braincase. The available bones nevertheless indica­ prefrontal and postorbitofrontal wings. However, the ex­ te a delicate skull approximately 420 mm long that pro­ cavations on the ventral surface of the frontal and the bably tapered to a slender pointed snout (Fig. 1 and small size of the prefrontal wing to the maxilla suggest P I. 1). that the prefrontal and postorbitofrontals fitted close to 132 Theagarten LINGHAM-SOLIAR Jugal The vertical arm of the jugal is poorly preserved. The most notable characters are the laterally broad ala at the junction of the horizontal and vertical axes and absence of a postero-ventral process (Fig. 1A), features similar to those ofPlioplatecarpus houzeaui (L in g h a m - S o l i a r 1994a). On the postero-medial surface a striated excavation marks the point of a ligamentous connection. Squamosal. The squamosal (Fig. 4C) is poorly preserved and the only condition of note that can be observed is the heavily striated process leading to the supratemporal. Parietal The parietal (Figs. 2A, C) shows several unusual charac­ Fig.
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