Problemas relativos às extinções de organismos

Abstract

Extinctions of taxonomic categories are so important in the geological record that they have to be viewed as an intrinsic constituent of life. Terminal extinctions, meaning extinctions which leave no descendants, may occur at slow or fast rates, geologically speaking. Often in the latter case they are inappropriately referred to as The terminal extinctions at the Cretaceous/Tertiary boundary are the most studied of the so-called mass extinctions. A paper published by ALVAREZ et al. (1980), revealing the existence of an anomaly in iridium concentration in clays deposited at this boundary in many parts of the world, started a boom of publications that attempt to explain this phenomenon and its possible relation to The worldwide distribution of this anomaly means that something unusual happened at the K/T limit that might have affected life, but detailed study of the biostratigraphical distribution of taxonomic categories in the beds laid down before, during and after the iridium anomaly clays demonstrates that the deterioration processes of these taxa started before the iridium anomaly. Thus, the phenomenon responsible for this anomaly affected already impoverished faunas. The study of extinctions during the Quaternary Period is particularly elucidative with respect to this problem. The Pleistocene extinctions were probably due to climatic fluctuations eventually enhanced by the action of predatory animais, mainly man. The encroachment of civilization has contributed to Holocene extinctions in spite of the efforts of conservationists. Viewed from a geological perspective, such extinctions would probably be packed together as a single mass extinetion and a possible catastrophe sought as its unique cause despite the fact that at least two events with distinctly different causes were involbed.