Species survivorship in the Cenozoic planktonic foraminifera: A test of exponential and Weibull models

The species survivorship curve for Cenozoic planktonic foraminifera was fitted to exponential (age-independent) and Weibull (age-dependent) models. Model fits that account for censorship bias have substantially lower slopes (extinction probabilities) than suggested by censored data. The best-fit Weibull model has a shape parameter significantly greater than one (beta = 1.19), which normally indicates an age-dependent increasing probability of extinction. Deviation of Weibull beta from 1.0 (exponential case) seems closely related to temporal variation in records of very short-lived taxa (less than or equal to 1 my). The shape of the curves are not sensitive to Corrected Species Survivorship adjustment of longevities, but may reflect taxonomic procedure or temporal variability in. extinction probability.