The Basque coastal sections of the K/T boundary – A key to understanding “mass extinction” in the fossil record

Abstract

lnvestigations at the K/T boundary section al Zumaya, northern Spain, underline the importance of this section in (1) establishing a Maastrichtian ammonite zonation and (2) in understanding "mass extinctions" in the fossil record. "Mass extinction" at the K/T boundary can be restricted to two groups of oceanic surface plankton, i.e. foraminifera and calcareous nannofloras. Ammonites and inoceramids exhibit a gradual decline through the late Cretaceous, as probably also do the Temperate belemnites and the bulk of involved vertebrales. Most of these groups disappear a long time before the boundary. Compared with the available environmental factors, the course of ammonite diversity through time perfectly parallels the course of global sea leve! changes. Times of high extinction rates (e.g. the era and system boundaries) coincide with times of maximum regressions, while the subsequent evolutionary radiations match with the new sea leve! rise. There is no need for any cosmic catastrophy to explain the decline of most invertebrate and vertebrate groups. oceanic surface at the boundary the following and changes were observed: minor (positive as well as negative) changes in boundary iridium and and changes in and strontium isotopes. these used to the Alvarez hypothesis of a cosmic impact at the boundary. they are easier explosive volvanicity which, on the other hand, might correlate also with the time-equivalen! magnetic reversals. of a series of comet showers. While most of the earlier Mesozoic major fauna! breaks can likewise be related with marine regressions, the KIT boundary extinctions as a whole were caused by a complex scenario: the late Cretaceous changes in sea leve! as well as the increase of volcanic activity towards the boundary.