Rudist and Chondrodonta sp. shells present in Ereño limestones show different responses to the influencece
of diagenetic fluids. The different microstructures of shells could be responsible for different ways followed
by penetrating fluids. It seems to be a clear correlation between chemical analyses and shell microstructure
types. In this way, whereas Chondrodonta sp. and requienid shells show the more resistant behaviour for
influence of diagenetic fluids, the monopleurid and caprotinid ones are more intensity affected by diagenesis.
The SEM studies reflect that less mechanical altered and more structurally hermetic shells, the less
diagenetically affected