The ultimate Pulchelliidae (Ammonoidea, upper Barremian)

Abstract

Pulchelliidae are one of the most important ammonite groups for the Barremian biostratigraphy, and its last species Gerhardtia provincialis was deemed to be an end of lineage with no descendants. However, presence of Pulchelliidae was mentioned several times in levels clearly assigned to the Hemihoplites feraudianus Interval Horizon, while this family was deemed to be extinct in the G. provincialis Subzone; these specimens have never been depicted or described. The description of the both two antidimorphs of Gerhardtia ultima sp. nov., discovered in several sections of southeastern France, fills this gap. In terms of evolution, this ultimate species continues the channeling initiated by Gerhardtia sartousiana and G. provincialis with the setting up of the over deepening of the ventral furrow, which widens in order to occupy its entire width between the peripheral structures in G. ultima sp. nov. In parallel, the peripheral structures are attenuated, the ornamentation disappears on the flanks, and the adult size decreases together with the wide opening of the umbilicus. This hyper-paedomorphic tendency (neoteny and progenesis) occurs suddenly at the end of the lineage, at the same time G. ultima sp. nov. is under dramatic quantitative rarefaction, announcing the complete and definitive disappearance of the Pulchelliidae.