On the co-occurrence of Rubroceras and Vascoceras (Ammonoidea, Vascoceratidae) in the upper Cenomanian of the West Portuguese Carbonate Platform

Abstract

The upper Cenomanian and basal Turonian ammonite assemblages of the West Portuguese Carbonate Platform (WPCP) are characterised by the occurrence of many Tethyan species, with emphasis on Vascoceratidae. They include Vascoceras gamai, type-species of the genus, and several other taxa created in 1898, by Choffat, based on abundant materials collected from exposures of the Baixo Mondego region (West Central Portugal), in the northern range of the carbonate platform. As noted by Berthou et al., in 1975 and subsequent works, many of these Vascoceras are upper Cenomanian and form two distinct assemblages, respectively with Euomphaloceras septemseriatum and Pseudaspidoceras pseudonodosoides. The occurrence of this latter species can be correlated with the uppermost Cenomanian Neocardioceras juddii standard zone, since both species co-occur in assemblages with vascoceratid ammonites described by Cobban et al. (1989) in New Mexico (United States). It is precisely from the same assemblages that the genus Rubroceras was originally described, as an interesting example of the late Cenomanian evolutionary diversification of the Vascoceratidae, when highly ornamented forms with transverse ribs persisting in the body chamber appeared in Tethyan assemblages with Vascoceras, characterised by much more smooth morphologies.As in New Mexico, the occurrence of Rubroceras in the WPCP is restricted to a stratigraphic unit with Pseudaspidoceras pseudonodosoides, Vascoceras gamai, V. barcoicense and V. silvanense. Besides Rubroceras alatum and R. burroense, other additional taxa are Spathites (Jeanrogericeras) subconciliatus, Vascoceras adonense, Vascoceras sp. A and Parapuzosia (Austiniceras) sp. This assemblage mostly occurs in the outcrops of Nossa Senhora dos Olivais, near Tentúgal, in the northern range of the carbonate platform. It is found above beds with Euomphaloceras septemseriatum and Vascoceras gamai, and below the highest upper Cenomanian unit of the area, with P. pseudonodosoides, Spathites (Jeanrogericeras) subconciliatus, Fagesia catinus, and depressed Vascoceras, such as V. durandi, V. douvillei and V. kossmati.In addition to New Mexico and West Portugal, representatives of Rubroceras are presently known from other carbonate platforms of North Africa, Arabian Peninsula and Nigeria, where they have been found in ammonite assemblages correlative to the Neocardioceras juddii standard zone. They are apparently absent from the remaining basins of the Iberian microplate with abundance of Vascoceratidae representatives, namely the Iberian Through (Spain), whose late Cenomanian oceanic communication was more processed from the North. Further studies will provide a better understanding of this important biotic dispersal event across the Tethyan Realm, as an additional marker of uppermost Cenomanian ammonite assemblages.