Acta Musei Moraviae

Discussion on measuring dimensional parameters of deformed planispirally coiled Lower Cretaceous ammonites (Ammonoidea)

One of the most important taxonomic features for species determination in undeformed planispirally coiled ammonite shells is the measurement of their size parameters, including shell diameter, whorl height, umbilical width, and whorl width. In Early Cretaceous deposits of Europe, ammonite shells exhibit various types of deformation, as the ammonite-bearing strata primarily consist of clayey-marly limestones. After sediment containing ammonite shells is deposited on the seabed, diagenesis follows. The degree of sediment compaction is controlled by pressure exerted by the height of the overlying water column. Clayey deposits are compressed more than sediments with a predominance of a calcareous component. This process results not only in sediment compaction but also in the planar deformation of ammonite shells. During compaction, the whorl width undergoes substantial changes, rendering its measurement irrelevant for taxonomic purposes. Regions with complex geological structures commonly contain specimens further affected by varying degrees of lateral pressure. The original logarithmic spiral coiling acquires an elliptical shape. This type of deformation partially or even completely alters the original parameters of the ammonite shell. The present contribution, based on an analysis of specifically selected ammonite specimens, demonstrates the consequences of these occurrences. Measurements of shell parameters in specimens deformed by strong lateral pressure are essentially unusable for taxonomic purposes. Such deformation is usually not considered in taxonomic studies of Early Cretaceous ammonites.