Pseudoskirroceras, a remarkable but poorly known Early Pliensbachian Tethyan ammonite genus: New data from the High Atlas (Morocco)

Abstract

The discovery of new Early Pliensbachian ammonite faunas in Central High Atlas (Morocco) allows the re-examination of the taxonomic, stratigraphical and palaeogeographical framework of the genus Pseudoskirroceras, an intriguing but until now poorly known Tethyan taxon. For the first time, several specimens of Pseudoskirroceras mastodon (Fucini, 1935) the type species of the genus, have been collected in a well-known stratigraphical context. This material allows the evaluation of intra-specific variability and consequently the assessment of the taxonomical relevance of various geometrical and ornamental features. The best diagnostic features are the clearly evolute sub-serpenticone shell, associated with the very distinctive low location of the latero-ventral tubercles. In contrast, features such as rib density or the extent of the juvenile Coeloceras-like stage vary greatly with no taxonomic implications. These observations considerably reduce the scope of Pseudoskirroceras, which is in fact a rare taxon, probably localised in the western Mediterranean Tethys, and not a pantropical genus. Thus, the presence of Pseudoskirroceras in North and/or South America is unconvincing. These results confirm that Mediterranean (West Tethyan) ammonite palaeobiodiversity includes more rare and localised species than can be found in NW Europe.