Review of the early Aptian (Early Cretaceous) ammonoid species Deshayesites deshayesi (dʹOrbigny, 1841)

This study is focused on the ontogeny, phylogeny, population variability and sexual dimorphism, of the early Aptian ammonoid species Deshayesites deshayesi (dʹOrbigny, 1841). The studied material consists of an assemblage of ninety-seven specimens collected from three different stratigraphic sections of the Maestrat Basin, Morella sub-basin, Spain. The lectotype of Deshayesites deshayesi and other specimens of the type locality (Bailly aux Forges, Paris Basin, France) were used to build a diagnosis of the true Deshayesites deshayesi and also to compare with the specimens studied in this work. In addition, a phylogenetic analysis was carried out to differentiate similar species from those specimens considered to be true Deshayesites deshayesi. We reviewed in detail Deshayesites deshayesi as described by different authors in the literature, as well as some references that we consider to correspond to this species. This work clarifies the taxonomy of the species Deshayesites deshayesi and by extension, the taxonomy of the Family Deshayesitidae. This is very significant because the lower Aptian ammonite biochronostratigraphic scheme is based on the stratigraphic ranges of representatives of this family. Poor taxonomic knowledge about some species of this family can cause biostratigraphic issues. One example occurs between the species Deshayesites deshayesi and Deshayesites forbesi; this has resulted in the allocation of the Oceanic Anoxic Event 1a through both biozones where these species are index forms. This work improves the precision of the Mediterranean standard ammonite biozonation for the lower Aptian.