Ammonoids of the Oxynotum Zone and Raricostatum Zone (Densinodulum Subzone) of Sinemurian, Lower Jurassic, in the Asturian and Lusitanian basins

Abstract

This research is focused on the ammonoids of Oxynotum Zone and Raricostatum Zone (Densinodulum Subzone) from Asturian (Spain) and Lusitanian (Portugal) basins. They come from expanded sections from where 216 specimens have been collected. These have been classified in eight genera and 15 species that belong to families Echioceratidae, Oxynoticeratidae and Eoderoceratidae. Most of the ammonoids are typical of the Northwest European Province. Echioceratidae is the most frequent in the Asturian Basin, whereas Oxynoticeratidae is the predominant in the Lusitanian Basin. Eoderoceratidae specimens are scarcer, and they are registered only between the middle part of Oxynotum Subzone and the lower part of Densinodulum Subzone. A new species is proposed for the upper part of Oxynotum Subzone of the Lusitanian Basin, Plesechioceras rochai sp. nov., which is relatively frequent. As well, also in Portugal, Cheltonia sp. is registered in Densinodulum Subzone, which is a form different from Cheltonia accipitris. This provides new evidence to support the hypothesis of the relation micro-macroconch between the genera Oxynoticeras (M) and Cheltonia (m). Remarkable differences in the studied area are the presence of Paroxynoticeras salisburgense and Plesechioceras cf. delicatum in the Asturias Basin, and Crucilobiceras densinodulum, associated to Oxynoticeras lymense, in the Lusitanian Basin.