L'albien (crétacé) de la région de Troyes et ses ammonites : Hoplitidae et Douvilleiceratidae
Résumé
In the 'département de l'Aube' (France), numerous and beautiful ammonites were collected in the Lower and Middle Albian of the Paris basin. Some historical outcrops, such as Le Gaty, Géraudot, Unienville, Le Perchois, Mesnil-Saint-Père and Courcelles, are primarily outcropping on a narrow, 10 km long region which represent a vestige of a seaway that periodically connected the Boreal Realm and the Tethysian Realm during the Early Cretaceous. Among those, the Courcelles quarry, located at about 20km southeast of Troyes, is the most renowned. It attracted many collectors from all over Western Europe since the 1970s. These collectors uncovered ammonites mostly from nodules originating from a calcareous layer, the famous bed 'b', which contains beautifully preserved Douvilleiceras and Hoplites. Nowadays, fossil hunting is almost impossible. However, a overview of those localities and the ammonite faunas thereof is still relevant because numerous collectors owned some specimens which are difficult to determine because of the proliferation of ammonite species and subspecies, particularly for the Hoplitidae and Douvilleiceratidae. The pioneer studies of Hardouin Michelin (1834, 1838) and Alexandre Leymerie (1841-1842) recognized that the stratigraphic and palaeontological relevance of the Albian of Aube Subsequently, Alcide d'Orbigny defined the Cretaceous stage with the longest duration in his 1842 'Paléontologie Française'. Since the 1960s, this stratotype for the Albian was usually subdivided in three major lithostratigraphical units: the "Sables verts de l'Aube", the "Argiles tégulines de Courcelles" and the "Marnes de Brienne". In 2007, however, Amédro & Matrion (2007) assigned the first one to the Upper Aptian. The ammonites discovered at Courcelles, Le Perchois, Montreuil-sur-Barse... were all collected in the "Argiles tégulines de Courcelles", of which the stratigraphical range extends from the Floridum Zone (Lower Albian) to the Intermedius Subzone of Dentatus Zone (Middle Albian). The Le Perchois quarries mainly yielded the Lower Albian ammonites Douvilleiceras, Protanisoceras, Cleoniceras, Sonneratia, Pseudosonneratia and Neosaynella (Perchois-Ouest), followed by Douvilleiceras, Protanisoceras, Cleoniceras, Platiknemiceras, Tegoceras, Protohoplites and many 'species' of the Hoplitidae Otohoplites (Perchois-Est). At Montreuil-sur-Barse, several ammonite levels were exposed in the Bulliensis and Steinmanni Subzones (Lower Albian). At Courcelles, ammonites such as Phylloceras (Hypophylloceras), Douvilleiceras and numerous Hoplites (Hoplites) were mainly unearthed from the famous bed 'b' of the Benettianus Subzone (Middle Albian). About fifteen scientific names were used for the Hoplitidae found in the bed 'b' and the underlying and overlying silty marls (clays "a' and 'c'): ? H. dentatus, H. devisensis, H. pseudodeluci, H. baylei, H. benettianus and H. bullatus in the Lyelli Subzone, and H. dentatus, with H. robustus, H. densicostatus, H. spathi, H. paronai, H. escragnollensis, H. vectensis, H. mirabiliformis and H. rudis in the Dentatus Subzone. It appears that those names were only used for small specimens. Those species represent a 'morphological continuum' with a compressed shell with a quadratic whorl section (with dense and fine ornamentation) to a very inflated shell with a wide hexagonal whorl section (with sparse and coarse ornamentation) resulting in numerous, intermediate specimens that cannot be classified. Without assuming the true biological nature of those taxa, we can distinguish slender (dentatus, with 'morphotypes' robustus and densicostatus; devisensis; vectensis; escragnollensis), mediumthick (pseudodeluci; spathi; mirabiliformis) and thick forms (baylei; benettianus; bullatus; rudis; paronai). The largest specimens of Hoplites, having a diameter between 180 and 400 mm, were probably adults which appear to be less variable than the smaller specimens. Those 'macroconchs' converge to a 'medium-thick' type, with spinose tubercles in their early stages, short, weakening secondary ribs in the periventral area and a ventral furrow that is becoming less pronounced. In taking sexual dimorphism into account, for the Hoplitidae, the number of 'true' species for Hoplites of the Middle Albian of Courcelles will be probably restricted to H. benettianus for clays 'a' and bed 'b', H. dentatus for clays 'c', and possibly H. rudis for the top of clays 'c'. The genus Douvilleiceras shows during the Middle Albian a stable shell having an ornamentation with countless variations: from a dense, fine, regular and rapidly weakening ornamentation to a spaced, coarse, irregular and weakening ornamentation. As a result, it is almost impossible to determine most of the Douvilleiceratidae. At Courcelles, in particular for the specimens originating from the clays 'a' and bed 'b', it is possible to use Douvilleiceras clementinum for smooth forms, D. albense, D. monile and Douvilleiceras sp., for standard forms resembling to D. mammillatum, and D. inaequinodum (with D. paucicostatum, D. orbignyi, D. baylei and D. alternans) for strongly adorned forms. All these 'species' may belong to one biological species.