Interpreting bioevents: What exactly did happen to planktonic foraminifers across the Cretaceous-Tertiary boundary?

The fossil record is the only evidence for the history of life on earth, but suffers from serious biases. The Signor-Lipps and Jaanusson effects are universal in the fossil record and distort patterns of biotic turnover more seriously the more incomplete the record. Hiatuses may also distort the record. The Cretaceous-Tertiary boundary (KTB) is the best global time plane in the entire rock record. The distinctive boundary clay corresponds to a major biotic turnover and occurs globally. Its base is a fall-out deposit that may be the same age worldwide to within a year. Thus the KTB is the best place in the fossil record to test the reliability of patterns of biotic turnover. The record of planktonic foraminifers across the KTB is controversial as regards the timing (sudden versus gradual) and severity (proportion of surviving species) of their extinction. The timing is affected by the Signor-Lipps Effect (SLE). A new technique is presented to compensate quantitatively for the SLE, which shows that the decline in diversity of planktonic foraminifers across the KTB at El Kef started before the boundary, but the SLE accounts for half the observed decline. As regards severity, probabilistic stratigraphy is the best technique to distinguish Cretaceous survivors from reworked specimens and suggests that at maximum 22% of Cretaceous planktonic foraminifers survived the boundary, 11% survived into zone P1a. The 'Hiatus Effect' is more imaginary than real, but can be investigated by computer simulation.