EXTINCTION OF INOCERAMID BIVALVES IN MAASTRICHTIAN STRATA OF THE BAY OF BISCAY REGION OF FRANCE AND SPAIN

Maastrichtian strata of the Zumaya-Algorta Formation of southwestern F rance and northeastern Spain record a major pulse of extinction among inoceramid bivalves well before the Cretaceous-Tertiary (K-T) boundary . Inoceramids are the most abundant macrofossils preserved in the stud y sections; at least six species of Inoceramus [I. (Endocostea) aff. I . (En.) balticus Giers, I. (En.) pteroides Giers, I. (Platyceramus) af f. I. (PI.) cycloides Wegner, I. (Trochoceramus) nahorianensis Kociuby nskij, I. (Tr.) morgani Sornay, and I.(?) goldfussianus d'Orbigny] are common to abundant in lower Maastrichtian strata. However, all six sp ecies disappear over a few tens of meters of section near the base of the upper Maastrichtian, as defined by the first appearance of the pla nktonic foraminifer Abathomphalus mayaroensis. Tenuipteria argentea (C onrad), which has not been recovered from the lower Maastrichtian port ions of the sections, occurs at low abundances through the upper Maast richtian, disappearing within 10 cm of the K-T boundary. The mid-Maast richtian extinction inter-val among inoceramids occurs within the uppe r Globotruncana gansseri to lower Abathomphalus mayaroensis planktonic foraminiferal zone, in nannofossil zone 24 to 2 5A, in the Anapachydi scus fresvillensis ammonite zone, in magnetochron 31N, and near the ba se of a change in slope of the seawater strontium curve, all as recogn ized by previous studies in one or more of the study sections. Whereas the new data presented here are not global in extent, the observed di stribution of inoceramids may be the local manifestation of global oce anic changes during the mid-Maastrichtian.