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The stratigraphic record shows a considerable decrease in ammonoid taxonomic diversity and distinct changes of ammonite shell morphology toward end of the Jurassic in the Central Russian Basin. By the end-Volgian, ammonites were represented by only two genera, belonging to a single family Craspeditidae, which differ markedly from previous ammonite families in their shell form. The end-Jurassic decrease in ammonoid biodiversity started in the mid-Volgian and is mainly correlated with shallowing of the shelf.
By the end-Volgian, ammonites were represented by only two genera, belonging to a single family Craspeditidae, which differ markedly from previous ammonite families in their shell form. The end-Jurassic decrease in ammonoid biodiversity started in the mid-Volgian and is mainly correlated with shallowing of the shelf.