Mass Extinctions over Earth History

Corresponding Readings in Primack, Richard B. Essentials of Conservation Biology.
Chapter 7: pages 147-150

Introduction

Extinction is a natural event � perhaps 95% of all the species that have ever lived have gone extinct. By late 1700�s, as more fossil species were unearthed, catastrophism became the dominant explanation. In 1830�s, Lyell sought to explain geological phenomena as the result of the long, gradual action of natural forces � uniformitarianism.

In 1858, Darwin published his famous theory of evolution, also based on gradual processes of evolution and extinction. Whether evolution happens gradually, or is better described by periods of stasis interrupted by rapid episodes of change (punctuated equilibrium) is debated today. There is no argument that extinctions occur both gradually and episodically, in mass extinctions. Life has radiated following these mass extinctions, as new forms have arisen, radiated, and given the world a new biota.

How does the present, human-induced wave of extinctions compare to historical events? If life has always recovered, indeed progressed, what is the fuss about? Extinction, even mass extinction happens, and life goes on.

The Geological Time Scale

History of the earth is divided into four eons, eons into eras, and eras into periods.

Fossil record becomes extensive after the Pre-Cambrian/Cambrian boundary, some 600 million years ago. A period is defined by the fossils of organisms that dominated in that period; the end of a period is marked by a transition in fossils.

Clearly, many ancient forms no longer exist. As species have appeared and others have disappeared, the overall trend in diversity has been strongly upward. Note also that this trend has periodic interruptions, called mass extinctions.

The average rate over the past 200 my is 1-2 species per year, and 3-4 families per my. The average duration of a species is 2-10 million years (based on last 200 million years).

Mass extinctions

Roughly 20 are recognized, of which 5 are major and the largest is the Permean, in which 50% of all marine families and 80+% of all marine species went extinct. How do we recognize a mass extinction?

  1. includes many different kinds of taxa

  2. includes considerable numbers of higher taxa (eg, dinosaurs)

  3. species� existence is truncated relative to expected duration
  4. followed by appearance and rapid (5-10 my) radiation of previously unknown or previously minor taxa

What causes mass extinctions?

  1. explanations include climate change (usually a cooling, perhaps with glaciation), meteor showers, biologically superior life forms, cosmic radiation

  2. no reason to assume the same cause for all

  3. cooling seems plausible in a number of cases
  4. a meteor impact is very strongly indicated for the K-T event

    5. The K-T Extinction event

    1. Dinosaurs, many land plants, ammonites, much marine life perished. Mammals, which had long co-existed with reptiles as small, undifferentiated scavengers, radiated into all the main mammals lineages within ~ 10 my.
    2. The explanation: a huge meteor, perhaps 150 km in diameter, collided with Earth. A dust cloud caused darkness for months or years, cooling the earth and drastically curtailing plant photosynthesis. Subsequent volcanic activity produced noxious gases, and acid rain.
    3. Alternative explanations: dual meteors, or massive volcanic activity triggered by a meteor.
    4. The evidence: thin layer of iridium, rare in the earth�s crust but common in meteors, throughout the world and dating to 63 my; ratio of other minerals including gold and platinum was characteristic of meteors, shocked (compressed) quartz crystals suggestive of impact; crater off the coast of the Yucatan.

 Replacement by Descent

 One species splits into two -- the ancestor is lost, but two descendent species replace it; or a species simply evolves gradually over time into something different from its ancestor.

 Invasion of the Biologically Superior Life forms

Evolution occurs independently in different parts of the world, driven by chance appearance of new life forms, and environmental change. Unique mutations appear, unusual genetic combinations arise, and environmental pressures that drive natural selection differ from place to place. Speaking metaphorically, life continually experiments with new body plans and new adaptations. Many are failures, but others are better than anything else on the playing field at that point in time. The North American � South American faunal interchange illustrates what happens when two faunas collide.

  1. prior to about 5 my ago, NA and SA were isolated.

  2. SA historically was joined with Antarctica and Australia, and had a fauna of marsupial mammals (acting as rodents and carnivores) and primitive herbivores (notoungulates).

  3. NA historically had exchanged its fauna with Eurasia across the Bering land bridge, and had a fauna of placental mammals that included very large-bodied taxa

  4. By 3 my ago the faunal exchange was in full swing. Many more NA species established themselves in SA than the converse (possums, armadillos, porcupines and the now-extinct ground sloth are SA�s gift to NA). Today, nearly half of the genera of SA are of NA origin.
  5. Why were NA species "biologically superior"?

 How Do Historic and the Present-day mass Extinction Compare?

  1. Probably similar in magnitude based on total species

  2. Uncertain whether higher taxa will be eliminated

  3. Impossible to predict which groups will "inherit the earth", as human dominance tends to surpress evolutionary opportunity

  4. five million years is along time to wait!
  5. Not healthy for us!

 

Transparencies: 1. Geological time scale 2. Marine families since Cambrian 3. the six (?five?) mass extinctions 4. NA-SA interchange

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