The Cretaceous - Tertiary Boundary (K-T) Extinction

ABSTRACT

Several mass extinctions have occurred during the Earth's history. The Cretaceous - Tertiary Boundary (K-T) Extinction caused the loss of at least three-quarters of all species known at that time including the dinosaurs. The cause of this mass extinction is a controversial subject among scientists but the fossil evidence of it's occurrence is abundant.

INTRODUCTION

The K-T Extinction occurred 65 million years ago. Many species perished in that extinction. Today evidence for this extinction can be seen in the fossil record. Biological, botanical and geological evidence at the Cretaceous - Tertiary Boundary show that some enormous event occurred that caused mass extinction of life on the Earth. Controversy about the cause of the K-T extinction exists with two main theories currently being in favour. One theory is called Intrinsic Gradualism and believes the cause of the K-T Extinction was a slow and gradual Earth generated event, caused by intense volcanic activity and the effect of plate tectonics. The second theory is known as Extrinsic Catastrophism and proposes that the K-T Extinction was caused by a sudden and violent catastrophic event such as the Earth being struck by a meteor or asteroid. The K-T Extinction supports the concept of Punctuated Equilibrium in evolution because surviving species evolved and others were exterminated. This creates the stepladder effect of evolution seen in the fossil record .

THE K-T EXTINCTION

The Cretaceous period occurred between 144 and 65 million years ago. The K-T Extinction is an event that happened at the end of this period 65 million years ago. By the beginning of the Tertiary period eighty-five percent of all species disappeared, making it the second largest mass extinction event in geological history ("The End-Cretaceous (K-T) Extinction", accessed 2000).

Among the species that perished were the dinosaurs, pterosaurs, belemnoids, many species of plants, except ferns and seed-producing plants, ammonoids, marine reptiles and rudist bivalves. Severely affected organisms included planktic foraminifera, calcareous nannnoplankton, diatoms, dinoflagellates, brachiopods, mollusca, echinoids and fish. Mammals, birds, turtles, crocodiles, lizards, snakes and amphibians fared much better and were mostly unaffected by the End-Cretaceous mass extinction ("The End-cretaceous (K-T) Extinction", accessed 2000).

CAUSE OF THE K-T EXTINCTION

The cause of the K-T extinction has been the subject of much debate among scientists and the public alike. Public imagination about what killed the dinosaurs has sometimes been intense. Serious scientists take note of the fact that dinosaurs were only some of the many species that became extinct at the end of the Cretaceous period (JRH, 1995a).

Scientists considering the cause of the K-T extinction tend to subscribe to either of two main points of view "intrinsic gradualism" or "extrinsic catastrophism" (JRH, 1995b). The two views do have some common ground. Both schools of thought agree that:

1. There was a very rapid global climate change with the environment changing from a mild, warm one to a cooler one (JRH, 1995a).

2. As well as permanent global climatic change, there is evidence that less lasting changes occurred at the end of the Cretaceous period. These changes may have been the result of a massive terrestrial disturbance that threw up soot, caused short term acid rain, emission of poisonous gases and global cooling. Long term effects of this event would have been a global greenhouse effect with warming and reduced sunlight (JRH, 1995a).

3. Many organisms, both marine and terrestrial, vertebrate and invertebrate became extinct. The reason for this extinction was probably this climate change (JRH, 1995a).

4. At or near the K-T boundary in several places around the world, a thin layer of clay with unusually high iridium levels has been found. This may be evidence for the dust cloud as mentioned in point 2 (JRH, 1995a).

Intrinsic Gradualism

Scientists subscribing to this view believe the K-T extinction cause took some time to occur, maybe millions of years and was intrinsic, that is earth generated (JRH, 1995b).

Two main hypotheses exist for the "intrinsic gradualists" regarding the cause of the K-T extinction:

1. Volcanism - there is evidence of increased volcanic activity at the end of the Cretaceous period. Over a period of several million years this increase could have created enough dust and soot to block out sunlight and produce a climate change. In India during the late Cretaceous period huge volcanic eruptions caused massive lava flows that can be noted at the K-T boundary. These ruptures in the Earth's surface are called Deccan Traps. The earth's molten mantle is rich in Iridium so this would explain the high presence of iridium found (JRH, 1995b).

2. Plate Tectonics - major changes in the arrangement of the continental plates (continental drift) occurred at the K-T boundary. The oceans were experiencing a regression and receding from the land. A cooler climate would have resulted over a long period of time (JRH, 1995b).

The two intrinsic gradualist hypotheses are interdependent, volcanism does not occur without the action of plate tectonics and vice versa. If the K-T extinction was intrinsic and gradual both processes probably played a role (JRH, 1995b).

Extrinsic Catastrophism

This view believes that the cause of the K-t extinction was extrinsic, of an extraterrestrial nature, catastrophic and sudden. The main hypothesis was proposed by Luis and Walter Alvarez of the University of California at Berkley and claims that a large extraterrestrial object collided with the Earth with the impact causing large amounts of dust which lead to a climatic change (JRH,1995b).

A team of scientists including Luis and Walter Alvarez were making a study of the rocks around the K-T boundary in Gubbio, Italy. They were studying an unusual layer of clay that contained abnormally high levels of the rare element Iridium. This indicated a something serious must have happened at the K-T boundary. Iridium on Earth comes from two sources, the main one coming from outer space in the form of cosmic dust which is constantly showering our planet. The second source is from the Earth's core when certain types of volcanoes erupt. The presence of high levels of Iridium led the scientist to believe that the K-T Extinction may have been caused by the impact of a large