Mollusk shells can often be found in large numbers in marine sediment deposits. These shells that are all found together are considered an “assemblage”. The animals that produced these shells might all have lived together at the same time, or they might have lived at different times and left behind shells in the same location, but either way, the result over time is a group of shells that occur in the sediment core as one assemblage. These assemblages generally consist of multiple species, and the composition of the species can tell us a lot about what the environment was when these shells were alive, including the water depth. Water depth is an important environmental variable. Some species only live in shallow environments, while others prefer deeper environments. Some species have a narrow tolerance for depth variation (meaning they only occur at one depth), while other have a broader tolerance for depths (they can live at different depths).

By understanding how a specimen interacted with its environment, we can gain a lot of information about how the Po Plain changed over time in water depth and depositional environment.

This figure shows the distribution of shell species along the depositional profile of the Po Plain. The pie charts show species abundance over progressively deeper areas. You can see that the species assemblages look very different at shallow versus deep water.

figure by Jackie Wittmer