A detailed study of the benthic foraminiferal assemblages at the Bajocian Global Stratotype Section and Point (GSSP—Murtinheira section, Lusitanian Basin, western Portugal) presented herein increases the correlation potential of this relevant global geological time reference. The recorded assemblages are relatively abundant and diverse, and are composed of well-preserved taxa typical of the Boreal Realm. The most abundant suborders are Lagenina and Spirillinina. Representatives of the Suborder Textulariina are also abundant in some assemblages, while those of the suborders Robertinina and Rotaliina are scarce. Lenticulina specimens are the most abundant in all assemblages, but at the specific level, there are differences between Aalenian and Bajocian assemblages, with Lenticulina muensteri being most abundant in the former and Spirillina orbicula in the latter.
Two biozones, based on calcareous foraminifers, have been established and calibrated to the standard ammonite zonation: 1) the Lenticulina quenstedti Zone, ranging from the upper part of the Gigantea Subzone, Bradfordensis Biozone, upper Aalenian, to the lower part of the Discites Biozone, lower Bajocian, and 2) the Ramulina spandeli Zone, beginning at the top of the preceeding zone and having an undefined upper boundary. In addition, 11 bioevents are defined on first occurrences, last occurrences, changes in abundances of some taxa, or changes in assemblage diversity. After a severe impoverishment at the Aalenian–Bajocian transition, an important foraminiferal turnover takes place in the early Bajocian, where some Lower Jurassic and Aalenian forms are replaced by typical Middle Jurassic taxa.