Inside the Orthoceratite limestone lithology of the Fluminimaggiore Formation, the nautiloid shell size variability is very high, with the maximum diameter (at the peristome) varying from few millimetres to 10.0 cm. The average diameter of the nautiloid shells tested by measurement of a dozen clastically deformed, transversally sectioned fragments, is 4.5 cm. Comparison of this datum with the thickness of specimens now preserved in the rock—after the various tectonic-diagenetic events (due to clastic and partially plastic deformation)—gives a thickness of 59 per cent. Based on these estimates, the reduction in thickness of the calcareous units bearing nautiloids is approximately 41per cent. It should be remembered that this compaction value is not a value for the interbedded clastic units that also occur within this formation. Separate analyses of fossils from these clastic units would be necessary in order to estimate their compaction ratio. Nevertheless, it is unlikely that the overall compaction ratio for the Fluminimaggiore Formation as a whole is less than 41per cent. Indeed, the last estimated thickness of the Fluminimaggiore Formation is 45-50 metres (Ferretti et al., 1998). Based on our new compaction results, it seems likely that the unit's original (pre-compaction) thickness was on the order of 70 metres.