MASS EXTINCTION EVENTS DURING THE PHANEROZOIC EON
The Phanerozoic Eon is the name given to the span of time from the first appearance of life in the fossil record to the present, about 570 million years, or about 15% of the 4.6 billion years that Earth as a planet has existed. The term refers to the age of visible life, that is, life that has left a discernable fossil imprint in the stratigraphic record of sedimentary rocks. Phanerozoic as a name is derived from the Greek word, phanes, for ‘light’ or visibility, prefixed to the Greek word zoos, meaning life in general and animal life in particular.
Preceding the Phanerozoic were the Archaen and the Proterozoic eons, a time known collectively as the Precambrian.
The Phanerozoic is subdivided into three great eras, the Paleozoic, or ‘old life’ era; the Mesozoic, or era of ‘middle life’; and finally the era of ‘recent life’ up to and including the present, the Cenozoic. Broadly speaking, the Paleozoic era is characterized by the rise of complex marine organisms and the emergence of diverse vertebrate forms, both marine and terrestrial; the Mesozoic by further diversification of marine invertebrates, the rise of the dinosaurs, and the first appearance of flowering plants, birds and small mammals; the Cenozoic by the ascent and dominance of the mammals, further diversification of birds and flowering plants, and, in the late Cenozoic, the appearance of hominids, and finally, of course Man.
By definition then, the Phanerozoic Eon is the span of time during which the fossil record contained within the rock column accumulated. A pattern associated with this testimony of the rocks became apparent to investigators very early in the formative stages of geological science. From the time of Georges Cuvier in the early 19th Century it was evident that the sequence of fossils did not reveal a smooth, continuous record of evolving life forms, but instead contained numerous abrupt transitions, or discontinuities, from one ordered state to another. The transition episodes were characterized by major species loss that showed up as gaps in the fossil sequences. These zones of depletion were enclosed by older rocks below and younger rocks above, each having distinct fossil assemblages, the older periods showing extinction of existing species at their conclusion and the younger periods showing an eventual replenishment with newly evolved forms, indicating the emergence of a new geological period.
By the early years of the Twentieth Century a strict Uniformitarianism had come to dominate geological and paleontological thinking and attention shifted away from the transition episodes to focus upon the unvarying periods of fossil accumulation. It was assumed that discontinuities were the result of strictly local dislocations of normal sedimentary processes and of no relevance to understanding the majestic sweep of the normal evolutionary continuum. Attention focused almost exclusively upon the zones of fossil accumulation where, it was assumed, “nothing happened” to the neglect of the zones where ‘something happened.’ That this attitude became enshrined as dogma is evident by the recommendations of the International Sub-Commission on Stratigraphic Classifications, in its Circular No. 25 from 1969, wherein it is stated “Boundary-stratotypes should always be chosen within sequences of continuous sedimentation. The boundary of a chronostratigraphic unit should never be placed at an unconformity. Abrupt and drastic changes in lithology or fossil content should be looked at with suspicion as possibly indicating gaps in the sequence…and should be used only if there is adequate evidence of essential continuity of deposition. The marker for a boundary-stratotype may often best be placed within a certain bed to minimize the possibility that it may fall at a time-gap.” In other words, if it appeared that anything out of the ordinary, anything contrary to the normal orderly course of deposition and sedimentation occurred, it was to be assiduously avoided in defining stratigraphic units, which were to be taken from well within the beds, away from those awkward unconformities. Obviously, this attitude resulted in an almost total neglect of those events whose action was inconsistent with assumptions about rates and magnitudes derived from exclusive reference to modern processes.
Yet, even during this time of near total domination of the gradualistic paradigm, some geological transitions were recognized as revolutionary by the geological establishment, simply because they were so obviously extreme and far-reaching that they could not easily be ignored. One such alteration in the global balance of nature was the end-Cretaceous event of 66 million years ago― involving the total disappearance of the dinosaurs and many other species― but even this revolution was viewed as something playing out over many millions of years and assumed in most cases to result from a concatenation of converging causes acting slowly and incrementally over time to eliminate existing species while simultaneously replacing them with new species.
However, by the second half of the Twentieth century geologists and paleontologists had comprehended the fossil record in enough detail to realize that some transition events were relatively abrupt and apparently worldwide in scope. Paleontologist Norman D. Newell with the American Museum of Natural History made extensive studies of the distribution of marine fossils in the 1960’s. In a series of papers he proposed six mass extinction episodes that interrupted the normal orderly succession of marine life. In a 1963 paper Newell referred to these events as ‘Crises in the History of Life.’
Criticizing the conceptual methodology for stratigraphic classification as advocated by the International Commission in the quote above, Digby McLaren, in his Presidential Address to the Paleontological Society in 1970, argued that some markers were in fact worldwide and can only be effectively understood by allowing a catastrophic interpretation. This address by McLaren, and the paper which followed, signify a milestone in the slowly emerging recognition of the important role of abrupt, large scale events in Earth history. In the 1980’s, working from a more extensive data base than available in the 1960s, Jack Sepkowski and Dave Raup confirmed five of Newell’s nominees for major mass extinctions; the sixth, a late Cambrian event, was recognized as a less severe, but still significant crisis in the history of life.
While there have been dozens of mass extinction events since the first appearance of life on Earth, these ‘great five’ are the most profound. The purpose of this section is to give an overview of these events with the objective of providing a larger framework from which to understand the catastrophic history of Earth and the relevance of abrupt change in the balance of nature and terrestrial life. The great revolutionary upheavals and crises are an integral part, if not the dominant part, of the evolutionary story of terrestrial biology. This big picture of past catastrophic change provides the backdrop against which we must evaluate the full range of environmental variations potentially occurring in the future. We have supplemented discussion of the Great Five extinctions with several other events, not so extreme in terms of taxonomic losses, but noteworthy nonetheless because of their closer proximity to our own time.
It should be obvious to anyone who peruses the material of this web site that we have a bias towards the interpretation that great changes in Earth and life, unfolding over time, are largely governed by forces whose origin is cosmic. We will predict that eventually most, if not all, of the great upheavals and transitions preserved in the record of life on Earth will be associated with cosmic events― phenomena involving alterations in the astronomical environment. The effects on the terrestrial biosphere resulting from these cosmic alterations include― in addition to mass extinctions― episodes of large scale, intense volcanism; major climate changes, including cycles of glaciation and interglaciation; prominent changes in sea level; alterations in Earths electromagnetic field; and major periods of tectonism and orogenesis. All of these phenomenons could be induced, controlled or influenced by forces of exogenic origin.