Les sciences sociales

Disponible uniquement sur Etudier
  • Pages : 12 (2763 mots )
  • Téléchargement(s) : 0
  • Publié le : 25 novembre 2010
Lire le document complet
Aperçu du document
Proc. Nati. Acad. Sci. USA Vol. 81, pp. 801-805, February 1984 Evolution

Periodicity of extinctions in the geologic past
(evolution/time series/paleontology)

Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637

Contributed by David M. Raup, October 11, 1983

The temporal distribution of the major exABSTRACT tinctionsover the past 250 million years has been investigated statistically using various forms of time series analysis. The analyzed record is based on variation in extinction intensity for fossil families of marine vertebrates, invertebrates, and protozoans and contains 12 extinction events. The 12 events show a statistically significant periodicity (P < 0.01) with a mean interval between events of 26million years. Two of the events coincide with extinctions that have been previously linked to meteorite impacts (terminal Cretaceous and Late Eocene). Although the causes of the periodicity are unknown, it is possible that they are related to extraterrestrial forces (solar, solar system, or galactic).

Virtually all species of animals and plants that have ever lived are now extinct, and the knownfossil record documents some 200,000 such extinctions. It has been generally assumed that extinction is a continuous process in the sense that species are always at risk and that mass extinctions simply reflect relatively short-term increases in that risk. Following this view, the extinction process is often described mathematically as a time homogeneous process using standard birth-death models(1-3). There is increasing evidence, however, that many extinctions are actually short-lived events of special stress, separated by periods of much lower, or even negligible, risk. Fischer and Arthur (4) departed from convention by arguing that the major extinction events of the past 250 million years (ma) occurred periodically at nearly constant intervals of 32 ma (see also ref. 5). Their studyused a limited data base, and no statistical testing was done. The purpose of this paper, therefore, is to test the proposition of periodicity in the record of marine extinctions over the past 250 ma (Late Permian to Recent) by using as rigorous a methodology as present data permit.

comparatively accurate time scale, and is divided into relatively short stratigraphic stages. The interval comprises39 international stages ranging in age from 253 ma B.P. (base of the Dzhulfian Stage of the Late Permian) to 11.3 ma B.P. (top of the Middle Miocene), using the Harland time scale (10). The mean duration of these 39 stages is 6.2 x 106 years, which makes it impossible to resolve extinction events separated by less than about 12 x 106 years (the Nyquist rate). Although finer resolution is possiblein some parts of the geologic column and with some biologic groups, the resolution used in the present study is the best that can be achieved for comprehensive analysis in the present state of synoptic work. One can debate the quality of the individual data. The family is a rather large and arbitrary taxonomic unit and as such tends to damp variation at the species level (11): even if 99% of themembers of a family become extinct at one time, the persistence of a single species will prevent that family from reflecting the extinction event. However, in spite of this problem, patterns of familial diversification and extinction do seem to correlate well with data for fossil species (12), and the available data for families represent a far more uniform and less biased sampling of the fossilrecord than any species-level data set. Age designations in the data set are also open to question, especially because geologic time scales are continually being revised. A given time scale represents a merger of a chronostratigraphic sequence (based primarily on fossils) and a scattering of radiometric dates (13). The present analysis uses the Harland and Odin time scales (10, 14), which have...
tracking img