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Les Reptiles Marins Du Toarcien (Jurassique Inférieur)

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Les Reptiles Marins Du Toarcien (Jurassique Inférieur) MINISTERE DES AFFAIRES ECONOMIQUES ISSN 0408-951 0 Mémoires pour servir à l'Explication Toelichtende Verhandelingen des Cartes Géologiques et Minières voor de Geologische en Mijnkaarten de la Belgique. van België. N° 39 Nr39 MINISTERIE VAN ECONOMISCHE ZAKEN LES REPTILES MARINS DU TOARCIEN (JURASSIQUE INFERIEUR) BELGO-LUXEMBOURGEOIS par Pascal GODEFROIT 1994 Mém. Exp!. Cartes Géologiques et Minières de la Belgique- Toelicht. Verhand. Geologische en Mijnkaarten van België 1994, N. 39, 98 p., 37 fig., Il pl., 6 tab. LES REPTILES MARINS DU TOARCIEN (JURASSIQUE INFERIEUR) BELGO-LUXEMBOURGEOIS par Pascal GODEFROIT Edition Uitgeverij Imprimerie du Ministère des Affaires Economiques Drukkerij van het Ministerie van Economische Zaken Rue de Mot, 24 - 1040 Bruxelles De Motstraat, 24- 1040 Brussel Septembre 1994 September 1994 Comité éditorial : L. Dejonghe, P. Laga, R. Paepe Comrnissie van uitgevers : L. Dejonghe, P. Laga, R. Paepe Service Géologique de Belgique Belgische Geologische Dienst Rue Jenner, 13- 1040 Bruxelles Jennerstraat, l3 - l 040 Brussel LES REPTILES MARINS DU TOARCIEN (JURASSIQUE INFERIEUR) BELGO-LUXEMBOURGEOIS par Pascal GODEFROIT (*) Résumé: Les affleurements toarciens (Jurassique Inférieur) de Belgique et, surtout, du Grand-Duché de Luxembourg ont livré de nombreux fossiles de reptiles marins Ichthyosauria, Thalattosuchia et Plesiosauria. Leur bon état de conservation permet, dans de nombreux cas, une détermination spécifique et la description de caractères anatomi­ ques mal connus chez ces animaux. L'étude des ichthyosaures a nécessité une révision systématique du genre Stenopterygius. Une synthèse paléogéographique des reptiles marins toarciens est proposée. Des différences quan­ titatives importantes ont été observées dans la composition des faunes de reptiles marins découvertes dans la région du Württemberg (Allemagne), du Yorkshire (Angleterre) et du Luxembourg. Cette dernière faune se carac­ térise par une nette prédominance de formes bien adaptées à la vie en pleine mer. Mots-Clés: Reptiles marins (lchthyosauria, Thalattosuchia et Plesiosauria), Toarcien, Lorraine belge, Gutland luxembourgeois, systématique, paléogéographie. Abstract: Toarcian (Lower Jurassic) outcrops have yielded many marine reptiles lchthyosauria, Thalattosuchia and Plesiosauria in Belgium and, principally, in Grand Duchy of Luxemburg. Their preservation allows a determination at a specifie leve! as also the description ofbadly known anatomical characters. The study of ichthyosaurs required a systematic revision ofthe genus Stenopterygius. A palaeogeographical synthesis ofToarcian marine reptiles is proposed. Important quantitative differences can be observed in the composition of marine reptiles' faunae from Württemberg (Germany), Yorkshire (England) and Luxemburg. This latterfauna is characterized by a predominance of animal weil adapted for the life in open sea. Key words: Marine reptiles (lchthyosauria, Thalattosuchia and Plesiosauria), Toarcian, Belgian Lorraine, Luxemburger Gutland, systematics, palaeogeography. (*) P. GODEFROIT, Institut royal des Sciences naturelles de Belgique, Département de Paléontologie, rue Vautier 29, 1040 BRUXELLES, BELGIUM. TABLE DES MATIERES 1. Introduction ...................................................................................................................................................................................................................................................... 2 Il. Cadre géologique ........................................................................................................................................................................................................................................ 2 III. Paléoenvironnement et conditions de dépôt ................................................................................................................................................................... 7 IV. Description des reptiles marins du Toarcien belgo-luxembourgeois .................................................................................................... 7 A. Collections étudiées ........................................................................................................................................................................................................................ 7 B. Définition des mesures ................................................................................................................................................................................................................ 8 C. Remarques sur la systématique adoptée dans ce travail: discussion et révision du genre Stenopterygius JAEKEL, 1904 ................................................................................................. 13 D. Paléontologie systématique ................................................................................................................................................................................................ 22 V. Paléogéographie des reptiles marins toarciens ........................................................................................................................................................ 66 A. Aperçu synoptique de la distribution des reptiles marins toarciens ......................................................................................... 66 B. Comparaisons entre les grandes faunes de reptiles marins toarciens ..................................................................................... 69 VI. Conclusions .................................................................................................................................................................................................................................................. 76 Remerciements .......................................................................................................................................................................................................................................... 77 Liste des abréviations ......................................................................................................................................................................................................................... 78 Références bibliographiques ................, ...................................................................................................................................................................................... 80 Planches ............................................................................................................................................................................................................................................................ 88 I. INTRODUCTION Les terrains toarciens (Jurassique Inférieur) d'Europe occidentale sont bien connus pour leurs grandes richesses paléontologiques. Les falaises de Whitby, le long des côtes du Yorkshire (Angleterre) et, surtout, les célèbres carrières d'Holzmaden, dans le Baden-Württemberg (Allemagne), sont les gisements toarciens les mieux connus. Cette dernière localité a notamment livré, depuis le 18ème siècle, des centaines de squelettes de reptiles marins ichthyosaures, plésiosaures et crocodiles mésosuchiens. Les conditions de fossilisations exceptionnelles de ce gisement ont permis la conservation particulièrement fine de nombreux spécimens. Certains ichthyosaures ont ainsi conservé l'empreinte de leur peau, sous la forme d'un fin film phosphaté (KELLER, 1992), rappelant la silhouette de l'animal vivant. De nombreux embryons d'ichthyosaures, à tous les stades du développement, ont également été découverts à l'intérieur du corps de leur mère, attestant l'ovoviviparité de ces animaux (BOTTCHER, 1990). La plupart des spécimens découverts à Holzmaden sont écrasés sur des plaques de schiste: la description ostéologique de leur crâne est de ce fait très compliquée et nécessite l'observation de nombreux spécimens. La présence de reptiles marins dans les terrains toarciens de Belgique et du Grand-Duché de Luxembourg est reconnue depuis le siècle dernier (MORIS, 1854; VAN BENEDEN, 1871; PURVES, 1884). Depuis plusieurs années, de nombreux paléontologues amateurs explorent systématiquement les grands travaux qui entament les terrains Toarcien lu:x.embourgeois. Ils ont pu rassembler d'impressionnantes collections (voir STREITZ, 1983) conservées, en grande partie, au Musée national d'Histoire naturelle de Luxembourg. Des nouveaux spécimens ont également été découverts récemment à l'occasion de fouilles organisées conjointement par l'Institut royal des Sciences naturelles de Belgique et le Musée national d'Histoire naturelle de Luxembourg, à Bascharage. L'étude des reptiles marins du Toarcien belgo-luxembourgeois est l'objet de cet article. L'intérêt de ces spécimens est leur conservation, en trois dimensions, dans des nodules calcaires: les détails anatomiques crâniens sont de ce fait beaucoup plus facilement observables que sur les spécimens découverts à Holzmaden. Les trois grands grou­ pes de reptile marins liasiques sont très inégalement représentés dans ces terrains. Les ichthyosaures sont les plus fréquents. L'identification des ichthyosaures a nécessité un important travail préliminaire de révision systémati­ que: les travaux antérieurs (v. HUENE, 1922b; Mc GOWAN, 1979), basés essentiellement sur une analyse biométrique de spécimens complets, ne permettaient pas l'identification de fossiles plus fragmentaires. De plus,
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