The Toarcian ammonite genus Porpoceras Buckman, 1911 is a cosmopolitan taxon with an uncertain stratigraphic range. The oldest known occurrence of the genus was reported from the Falciferum Subzone (Early Toarcian) in Morocco, although based on a unique specimen whose taxonomic assignment to the genus Porpoceras remains doubtful. The youngest occurrence was a simple mention of Porpoceras sp. from the Variabilis Subzone (Middle Toarcian) of the Thouars area (Western France), but it was later regarded as doubtful because no specimen was figured. We here provide the first robust evidence for the youngest known occurrence of the genus Porpoceras, from the Variabilis Subzone of the Thouars area. The known revised stratigraphic range of the genus therefore extends from the Falciferum Subzone to the Variabilis Subzone. In turn, the genus Porpoceras likely went extinct in the Illustris Subzone together with the genera Catacoeloceras Buckman, 1923 and Mucrodactylites Buckman, 1928, coinciding with the drastic loss of diversity observed between the Bifrons and Variabilis Zones in northwestern Europe (also known as the Bifrons-Variabilis extinction). Finally, in the light of our new data and of previous reports, the genus Porpoceras is not a foolproof biostratigraphic marker of the NW European Bifrons Horizon, although it remains an important taxon for identifying this biostratigraphic unit.
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Vol. 38 • No. 4