The aim of this work is to describe the benthic foraminiferal assemblages recorded across the Aalenian–Bajocian boundary (Middle Jurassic) in two sections of the Iberian and Basque-Cantabrian basins (North Spain), well calibrated with the standard ammonite zonation, in order to establish a biostratigraphic scale based on the foraminiferal record, and to infer the palaeoecological conditions that affected their development.A total of 29 samples were collected in the studied sections, and the composition of the foraminiferal assemblages recorded throughout the studied stratigraphic interval was analysed using the stratigraphic distribution of the identified species, the quantitative data obtained through the estimation of relative abundances and the calculation of several diversity indexes.The studied assemblages are moderately abundant and diverse, and they are constituted by a total of 5435 specimens, corresponding to 5 suborders, 13 families, 25 genera and 51 species. Their composition displays close similarities to those already described for the Jurassic carbonate platforms of the Boreal Realm (Type-A), Boreal Atlantic subdivision. For the studied stratigraphic interval, it was recognised the Lenticulina quenstedti Foraminiferal Zone, in which the recorded assemblages developed in intermediate part of a marine platform environmental conditions, well oxygenated and with normal salinity. In the Aalenian–Bajocian transition, it was possible to identify a sharp decrease in the abundance and diversity of the foraminiferal assemblages that has also been recognised in other coeval basins of the Iberian Plate palaeomargin, thus representing a bioevent of the Northern Hemisphere.

中文翻译：

---

伊比利亚盆地和巴斯克-坎塔布连盆地（西班牙北部）的阿累尼西亚-巴乔纪边界的底栖有孔虫组合

它们由5435个标本组成，分别对应5个亚纲，13科，25属和51种。它们的组成与已经描述的北方大西洋分支的北方地区（A型）的侏罗纪碳酸盐岩平台描述的相似。对于研究的地层间隔，认识到Lenticulina quenstedti有孔虫区，记录的组合在海洋平台环境条件的中间部分发育，充氧良好，盐度正常。在阿莱尼亚－巴乔纪过渡期，有可能发现有孔虫组合的丰度和多样性急剧下降，这在伊比利亚板块古边缘的其他中世纪盆地也得到了认识，因此代表了北半球的生物事件。