Bidart is considered one of the most complete sections in South Eastern Europe, which contains the transit K/T, and for that reason, has been the object of many stratigraphic y paleontologic studies. However, in this section no detailed study of planktic foraminifera has been carried out. A high-resolution analysis has demostrated the excellent conditions shown in the Bidart section.
A total of 33 samples were collected, the richness in the upper Maastrichtian ranges from 86.000 to 155.000 per gram of dry residue. These results continue up until the last 5 cm of the Cretaceous in the Bidart section, though, in the last centimeters the planktic foraminifera show a clear mark of dissolution. The richness in the Tertiary samples increase progressively as the age of the rocks decreases, ranging from 4.000 to 72.000 specimens.
A total of 80 planktic foraminifera species has been determined in the Bidart section, 62 from the Cretaceous and 18 from the Paleocene. 28 Cretaceous species remain at the Early Paleocene together with the new Tertiary species. In the Upper Maastrichtian, the proportion of planktic foraminifera versus the total foraminifera remains more or less stable, with an average of over 95%, falling drastically at the K/T boundary.
In the final meters of the Maastrichtian, the number of genus and species shown a small variation of the remarkable relative abundance, from a quantitative point of view. Nevertheless, in the last Cretaceous samples a small decrease is shown in the species richness, again drastically falling at the K/T boundary.
Four Biozones have been differentiated, the Abathomphalus mayaroensis Biozone in the Upper Maastrichtian and the Guembelitria cretacea, the Civisina euskalherriensis and the Parasubbotina pseudobulloides Biozones at the basal Tertiary.
According to previous studies, the index fossil, A. mayaroensis, does not appear in the last 8 metres in the Maastrichtian of the Bidart section. However, in the present study, A. mayaroensis has been found up until the boundary. Also, specimens of the index taxa, Ps. pseudobulloides, have been found, thus permitting the limit of the homonimus Biozon to be set lower.
The planktic foraminifera assaciations before and after the K/T boundary, change within a short period of time, showing clearly a catastrophic destruction of the various ecological niches. The extinction of 50% of the planktic foraminifera has been observed coinciding with the K/T boundary, without noticing a gradual extinction.
The surviving taxon, become extinct in the lower part of the Danian, coinciding with the appears and evolution of the new Terciary species, giving way to a post-límite speciation.
The evidences of abrupt planktic foraminifera extinction would be in line with the theory of a meteoric body impact, perhaps associated with others events.
