When Abnormality Becomes Perennial in a Reduced Population: The Case of Altudostephanus longicostis gen. et sp. nov. (Valanginian Ammonites, South-Eastern France)

Abstract

The discovery of the new ammonite Altudostephanus longicostis gen. et sp. nov. around the lower/upper Valanginian boundary (Lower Cretaceous) with a true longitudinal ribs pattern oriented in the direction of coiling is reported here for the first time for the Cretaceous. This character rises questions as this type of ornamentation is mainly represented in the Paleozoic or old Mesozoic taxa. Its study shows that it is not a ‘shell accident’ and that it belongs to a particular lineage of Olcostephaninae, in a restricted geographical area and in a progenetic evolutionary context derived from Olcostephanus, which justifies the introduction of the new monophyletic genus Altudostephanus gen. nov. The discovery of a specimen of Passendorferia sp. (Oxfordian, Upper Jurassic) with the same pattern allows us to consider that this capacity could have a genetic cause. It seems that the fixation of this character, otherwise strongly recessive, in A. longicostis gen. et sp. nov. could take place thanks to a founder effect and the drastic numerical reduction in the population. The geological reference section PIG5 (Moriez area, South-Eastern France) is described, and the problem of the conservation of ‘pyritic’ s.l. ammonites is addressed with a proposed methodology allowing their long-term conservation.