Lower and Middle Jurassic foraminifera from the Iberian and Basque-Cantabrian Basins exhibit distinct features due to different biostratinomic and diagenetic processes. The main mechanisms of taphonomic alteration identified in our material are bioerosion, abrasion, fragmentation, dissolution, sedimentary infilling, cementation, mineralization, recrystallization sensu lato, pressure-dissolution processes, distortion and removal. Sample preparation prevents the detection of mechanism such as reorientation, regrouping or geopetal infillings. Bioerosion is represented by rare and isolated holes. Pitted surfaces associated with wall pores are the most important corrasion features. Abrasion signs such as polished longitudinal and sutural ribs have been recognized. Fragmentation is common and it is controlled by the shape of the test rather than by the shell size or thickness. Different types and patterns of dissolution have been detected. Micritic infillings have been observed in most of the specimens; nevertheless, sparitic and pyrite cementations are also abundant. Neomorphism includes features such as inversions and recrystallization; pyrite and possibly silica replacements have been recognized. A great number of the specimens show calcitic crusts that obscure wall surfaces and may prevent identification. Mineralization is represented by sylvite crystals on the surface of some uppermost Toarcian and Aalenian foraminiferal shells. Fossil diagenetic distortion is frequent and affects the specimens independently of their morphology. Signals of reworking (“resedimentation”) are common in the specimens. Two direct criteria for the recognition of reworking (“reelaboration”) are shown for the first time on the foraminiferal shells. More research must be done to discern the history of the foraminiferal assemblages in order to achieve accurate biostratigraphical, paleoecological and paleobiogeographical interpretations.