Dansk Geologisk Forening / DGF OnLine Series / Discovery of the ichthyosaur Ophthalmosaurus (Reptilia) in the Late Jurassic of the Boulonnais

Discovery of the ichthyosaur Ophthalmosaurus (Reptilia) in the Late Jurassic of the Boulonnais

26/05/1996

Nathalie BARDET1, Sylvain DUFFAUD1, Michel MARTIN2, Xabier PEREDA SUBERBIOLA1,3, Jean-Pierre VIDIER4

Abstract
We report the discovery of ichthyosaur remains from the Late Jurassic (Tithonian) sediments cropping at Condette, near Boulogne-sur-Mer, Northern France. It consists on a desarticulated specimen, that includes a fragmentary jaw, several precaudal vertebrae, ribs, and an ischio-pubis complex. This material is referred to the genus Ophthalmosaurus. This discovery confirms the presence of Ophthalmosaurus in the Late Jurassic of the Boulonnais.1: Laboratoire de Paléontologie des Vertébrés, URA 1761 du CNRS, Université Pierre et Marie Curie, case 106, 4 place Jussieu, 75252 PARIS cedex 05.
2: Museum d’Histoire naturelle, 115 boulevard Eurvin, 62200 BOULOGNE-SUR-MER.
3: Universidad del País Vasco / Euskal Herriko Unibertsitatea, Facultad de Ciencias, Departamento de Estratigrafía y Paleontología, Apartado 644, 48080 BILBAO.
4: Centre des Sciences de la Terre, Laboratoire de Paléontologie analytique et de dynamique sédimentaire, URA 157 du CNRS, boulevard Gabriel, 21000 DIJON.

Introduction
The occurrence of marine reptiles in the Jurassic of the Boulonnais is known since the last century, essentially due to the work of the paleontologist Henri-Emile Sauvage (e.g., Sauvage 1913). The marine reptile fauna from the Mid to Late Jurassic of the Boulonnais includes ichthyosaurs, plesiosaurs, pliosaurs, crocodiles and turtles and is comparable to that known in more or less equivalent levels from Normandy and Great-Britain. During the Callovian-Tithonian times, these area were covered by a shallow epicontinental sea and were part of the Anglo-Paris Basin paleogeographical unit. Until recently, most of the fossils recovered from the Boulonnais came from sea-cliff or inland quarrie exposures. Since some years, civil engeenering works linked to the Channel Tunnel (motorway or railway cuttings) have permitted observations of not previously known or no more accessible series. In this context, several vertebrate skeletons have been discovered, for example the herewith described ichthyosaur.

Geographical and geological settings
The specimen has been discovered on a motorway cutting slope, near Condette, a small village located South to the City of Boulogne-sur-Mer, in Northern France. The specimen has been excavated on a surface of 1,5 m x 3m. The bones were in a disarticulated stage of preservation and were probably part of a single individual (fig. 1). The remains are well preserved but bear pyrite and show traces of crushing, especially the vertebral centra. Lamellibranch moulds, oyster lumachells, fish bones (hyomandibular, vertebra) and a plesiosaur tooth have been collected in association with the ichthyosaur skeleton.
The fragmentary ichthyosaur comes from the upper part of the Chatillon Clays, most probably from the Gigas Ammonite Zone, Lower Tithonian in age (Proust et al. 1993). This formation, which is well exposed on the coast and has yielded a great quantity of marine reptile remains in the last century, was not known in details inland.
ols_bardet1

Fgure 1. Disarticulated skeleton of Ophthalmosaurus sp., BHN2R-1200, Argiles de Chatillon Fm., Tithonian, Condette near Boulogne-sur-Mer, Northern France. The arrows indicate the ischio-pubis complex (left) and the premaxillar (right).

Description and taxonomic attribution
The ichthyosaur remains include a left premaxillar, eleven anterior thoracic vertebral centra including the atlas-axis complex, twelve neural arches, about fifty rib fragments and a right ischio-pubis complex. The material is kept at the Musée d’Histoire naturelle de Boulogne-sur-Mer under the number BHN2R-1200.
The premaxillar is a slender bone with the medial border of the alveolar groove thicker and slightly higher than that of the lateral border. The alveolar groove is narrow without traces of sockets. No teeth has been found. The vertebral centra, including the fused atlas-axis, are part of a more or less continuous serie from the most anterior part of the thoracic region. The articular faces are subpentagonal in shape, the diapophyses contact the neurapophyses and the parapophyses are located anteriorly on the median part of the lateral surface. The neural arches have been found isolated from the centra. They correspond to the most anterior thoracic region as the pre- and post-zygapophyses are well separated from each other. The ribs correspond to the precaudal vertebral region as they bears both capitulum and tuberculum. The ischium and pubis are fused in a complex by their proximal and distal ends, leading between them a small obturator foramen (fig. 2).
The slenderness of the jaw and the fusion of the ischium and pubis into a complex, with a small obturator foramen, are diagnostical features of the genus Ophthalmosaurus (Andrews 1907, Appleby 1956). Due to its fragmentary state of preservation, the material is referred to as Ophthalmosaurus sp.
Discussion
Ophthalmosaurus is currently well known in the Mid Jurassic (Callovian) of England (Kirton 1983). Further material suggests the occurrrence of this taxa in the Late Jurassic (Kimmeridgian) of Normandy. The ichthyosaur specimen found in the Boulonnais confirms the presence of Ophthalmosaurus in the Late Jurassic (Tithonian) of Northern France. Besides Europe, this genus is known in the Late Jurassic (Tithonian) of Argentina (Gasparini 1988). During the Late Jurassic, the great similarity of the marine reptiles faunas (including ichthyosaurs) from Tethyan and Eastern Pacific realms implicates the existence of dispersal routes (Gasparini 1992).

ols_bardet2

Figure 2. Ophthalmosaurus sp., BHN2R-1200, Tithonian of Boulonnais. Ischio-pubis complex in dorsal view. Key: PU, pubis; IS, ischium; ac, acetabulum; of, obturator foramen. Scale = 2.5 cm.

References

Andrews, C.W. 1907: Notes on the osteology of Ophthalmosaurus icenicus, an ichthyosaurian reptile from the Oxford Clay of Peterborough. Geological Magazine, V, IV, 515, 202-208.

Appleby, R.M. 1956: The osteology and taxonomy of the fossil reptile Ophthalmosaurus. Proceedings of the Zoological Society of London, 126, 403-447.

Gasparini, Z. 1988: Ophthalmosaurus monocharactus Appleby (Reptilia, Ichthyopterygia), en las calizas litograficas titonianas del area Los Catutos, Neuquén, Argentina. Ameghiniana, 25, 1, 3-16.

Gasparini, Z. 1992: Marine reptiles of the circum-Pacific region. In: The Jurassic of Circum Pacific. G. Westermann Ed., Cambridge University Press, 361-364.

Kirton, A. 1983: A review of British Upper Jurassic ichthyosaurs. Ph.D. Thesis University of Newcastle-upon-Tyne (inedite).

Proust, J.N., Deconinck, J.F., Geyssant, J., Herbin, J.P. & Vidier, J.P. 1993: Nouvelles données sédimentologiques dans le Kimméridgien et le Tithonien du Boulonnais (France). Comptes Rendus de l’Académie des Sciences de Paris, 316, II, 363-369.

Sauvage, H.E. 1913: Catalogue des reptiles jurassiques du Boulonnais. Bulletin de la Société Académique de Boulogne-sur-Mer, X, 253-264.