An example of taxonomic revision of an oversplitted cephalopod family: the Deshayesitidae

International audienceThe Deshayesitidae is one of the most significant Lower Aptian (Lower Cretaceous) ammonite families (fossil cephalopods) due to its wide paleogeographical extension, large morphological disparity and abundance. Moreover, this family provides almost all the species used for relative datation of the biostratigraphical units (zones and subzones) of the Lower Aptian. Unfortunately, this family has been taxonomicaly oversplitted in more than one hundred « species » and six genera. The diagnosis characters of these typological taxa are generally the rib density and the shell shape (characters known nowadays to be of potentially poor taxonomic significance due to high intraspecific variation) and the evolutionary characters were neglected in most of the cases. Due to its wide paleogeographical extension and its high typological diversity, a revision of this family using a “paleobiological” concept of species would be a considerable work. We performed a “first step” revision of this family based on literature data. In our opinion, the most complete literature data about this subject are the works of Casey (1961, 1963) and Casey et al. (1998) about the Deshayesitidae of Southern England. These authors recognized initially no less than 66 typological species and varieties and 3 genera. The results of this “first step” revision are as follows: (1) the general ontogenic sequence of the Southern England Deshayesitidae consist of five stages: post embryonic stages A and B, stage C with high growth in whorl height, narrow whorl section and slender ribbing, stage D with low growth in whorl height, broad whorl section and robust ribbing, and adult stage E with robust and smooth simple ribs. (2) Intraspecific variability of ornamentation and shell shape is similar throughout the Deshayesitidae evolutionary history and it is determined by three “rules”: thevariability of type 1 according to the Buckmann’s law of covariation and depending on the duration of the ontogenic stage C, and thevariability of type 2 which concerns the intensity of expression of the ornamental relief and a probable sexual dimorphism with macroconchs and microconchs. (3) The evolution of the Deshayesitidae is intepreted as being anagenetic. The evolutionary patterns are increasing complexity of suture lines, the emergence of a new ontogenic stage, and the emergence of three successive novelties on the inner whorls ventral area (smooth siphonal band, to subtabulate ventral area, then lateroventral tubercles) that evolve centrifugally through time (neotenic process). The group is reinterpreted as being composed by only one chronospecies divided into 2 genera, 8 successive paleospecies and 2 subspecies. Discordance in the “degree of evolution” of the English specimens attributed to one of the recognized species, Deshayesites deshayesi, and the type specimen of this species from the Paris Basin (France) is attributed to a diachronism between the English and French D. deshayesi