The extinction of the rudist bivalves.
The rudist bivalves were one of the many and varied groups of organisms to be extinguished at the end of the
Cretaceous Period. They were a group of bivalves which evolved during Late Jurassic times to dominate the
carbonate shelves on the margins of the Tethys Ocean during the Cretaceous Period. Through Late Cretaceous
times their diversity climbed to a peak and then entered a period of rapid decline, resulting eventually in the
complete extinction of the group.
Theories as to the cause of that extinction should clearly be based upon a knowledge of the detailed pattern of
the decline. Most important is the question of the timing of the extinction: How long did it take from the
peak of diversity to the elimination of the entire group? Is there one main extinction event - or are there
several - or is the pattern a gradual decline? In answering these questions this work adopts a new approach to
dating end Cretaceous strata by using strontium isotope stratigraphy. The method works by measuring the
87 Sr/86Sr of palaeo-seawater preserved in marine carbonate, such as the thick low-Mg calcite layer of rudist
shells. The 87Sr/86Sr of seawater was changing fairly rapidly through time in the latest Cretaceous. The
pattern of change has been established in detail using samples from Boreal sequences of which the ages are
known with respect to the belemnite stratigraphy. Using this as a standard graph, Tethyan rudist samples
have been dated by a comparison of the Sr isotope ratio.
The use of Sr isotope stratigraphy has enabled a time axis to be established, against which the ranges of
rudist bivalves and of their facies have been ploued. From these data it can be clearly seen that the ranges of
many of the established biostratigraphic markers are in error and that the stage boundaries, as defined by the
belemnite and planktonic foraminiferal stratigraphies, are offseL
When the pattern of rudist diversity is plotted against this time scale it can be seen that, at a specific level,
the rudists were at their most diverse at the Campanian/Maastrichtian boundary. Their decline lasted until
almost the end of the Maastrichtian after which the only survivors are Tertiary forms. The decline is related to
the disappearance of rudist facies with the end Cretaceous regression, though that graph is slightly displaced
from that of rudist diversity. This shows that the rudist extinction is not merely due to non-exposure of
terminal Maastrichtian strata.