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.
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).
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).
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 asteroid hitting the Earth (Buckley,1997).
BIOLOGICAL AND GEOLOGICAL EVIDENCE
There were both gradual extinctions leading up to and sudden extinctions at the K-T Boundary. Dinosaurs and ammonites show both gradual and sudden extinction. The number of dinosaur genera decreased from thirty to seven during the Cretaceous period. The remaining seven genera became extinct at the K-T Boundary (Smith, 1997a).
Other species that became extinct gradually were the Inoceramids (hefty thick-shelled clams) which were abundant in the late Mesozoic but by the start of the Maastrichian epoch there were only four genera left and at the K-T Boundary all genera were extinct. The Rudists (molluscs) were decimated during the Maastrichian epoch and their reefs terminate well before the tertiary period. (Smith, 1997b).
Species that show sudden extinction patterns are the Globigerinids which were an extremely successful species during the Maastrichian epoch but showed an enormous decrease in diversity at the K-T Boundary. The Coccoliths were an extremely abundant species during the Cretaceous but barely survived the K-T extinction. At the Stevns Klint site chalk below the K-T Boundary (that is older material) showed a one hundred percent coccoliths content. There was no chalk containing coccoliths at the K-T boundary, only clay was found. Above the K-T boundary there were some coccoliths found (Smith, 1997b).
Ferns have the ability to rebound after a catastrophe and survived the K-T extinction. Before the K-T boundary ferns show a mixture of 30 % spores and 70% pollen. At the K-T Boundary ferns were found to have 99% spores and after the K-T Boundary ferns reverted a a mixture of 30% spores and 70% pollen (Smith, 1997d).
Iridium is a rare element on Earth but is in relative abundance at the K-T Boundary. The Iridium deposits at the K-T Boundary appear to be a unique event in the Earth’s history (Claeys, 1995). A distinct layer of sediment – the Magic Layer which is 3mm thick and the Ejecta Layer which is 20mm thick has been found in numerous places around the Earth at the K-T Boundary. It is these layers that contain the unusually large amounts of Iridium. Iridium in such quantities could only have come from the Earth’s core or deposited by the impact of a meteor or asteroid (Smith, 1997d).
The presence of large amounts of Iridium tells scientist that some enormous event occurred at the K-T Boundary but could support both the Intrinsic Gradualists and the Extrinsic Catastrophists.
Scientists who believe that the widespread distribution of the Iridium layer could only have been caused by a meteor impact cite the abundance of small droplets of basalt, called spherules, in the K-T Boundary layer as evidence that the basalt from the Earth’s crust was melted and flung into the air on impact. The presence of shocked quartz, tiny grains of quartz that show features diagnostic of the high pressures of impact, are found in the K-T Boundary layer and provide additional evidence of an extraterrestrial impact. Recent research suggests that impact site may have been the Chicxulub Crater in Mexico on the Yucatan Peninsula (“Speculated Causes of the End-Cretaceous Extinction”, accessed 2000).
The K-T Boundary layer contains soot that could be the result of large global fires that would have resulted from the impact of an extraterrestrial object. Huge tidal waves would have been generated by an asteroid or meteor impact. Evidence of such waves has been found around the Gulf of Mexico. Along with the tidal waves the impact would have started a chain reaction of earthquakes and volcanic explosions. High winds caused by the blast would have carried large amounts of soot and ash around the world causing darkness and an initial decrease in global temperature. Later global temperatures would have increased due to large amounts of carbon dioxide being released by the global fires. Eventually this would have caused a chemical reaction resulting in acid rains. The impact on flora and fauna alive at that time would have been devastating (Buckley, 1997).
The high concentrations of Iridium in the K-T Boundary layer has also been attributed to another source, the Earth’s mantle. Scientists speculate that the Iridium layer may be the result of a huge volcanic eruption, as evidenced by the Deccan Traps, extensive volcanic deposits laid down at the K-T Boundary, especially in India and Pakistan. These lava flows occurred when India moved over a “hot spot” in the Indian Ocean, producing lava flows that exceeded 100000 square kilometres in area and 150 metres in thickness. These flows would have produced enormous volumes of ash that altered global climate and changed ocean chemistry. The presence of spherules and shocked quartz also support volcanic activity Peninsula (“Speculated Causes of the End-Cretaceous Extinction”, accessed 2000).
PUNCTUATED EQUILIBRIUM AND THE K-T EXTINCTION
The K-T Extinction is one of several mass extinctions throughout the Earth’s history. Mass extinctions play an important role in the evolution of life. Without some species becoming extinct others would not have had an opportunity to evolve (Courtillott, 1990). After the K-T Extinction fossil records show that evolution accelerated (Alvarez and Asaro, 1990). The period of recovery after the K-T Extinction would have seen the surviving species moving into the ecological niches left vacant by the dead species. After a short period of time some of the plants that had been burnt down would have regrown from buried seeds or root stock. Commonly after mass extinctions a sudden evolutionary burst occurs as new species develop. The age of mammals began (Buckley, 1997).
The fossil record indicates that in normal times each species becomes increasingly well adapted to it’s environment and this makes it difficult for other species to evolve. This creates stasis in evolution. Mass extinctions provide the opportunity for surviving species to evolve and fill newly vacant ecological niches (Alvarez and Asaro, 1990).
The concept of punctuated equilibria believes that evolution is not a slow, gradual process but that it occurs in concentrated and rapid bursts (Lowood,1998). The theory of punctuated equilibria provides us with an explanation for the patterns found in fossil records. This includes the sudden appearance of new species, the stability in some widespread species, the distribution of transitional fossils, the differences between the ancestors and offspring in other species and the pattern of extinction in some species . Punctuated equilibria relies on the knowledge gained by studying modern species. The consideration of environmental factors, how species interact with each other and how those species are distributed, all play an important role (Elsberry,1996).
The Cretaceous – Tertiary Mass Extinction was a momentous event in the Earth’s History causing the disappearance of 85 per cent of all species (“The End-cretaceous (K-T) Extinction”, accessed 2000). Biological, botanical and geological evidence for this event can be found in a layer of sediment in numerous places around the world. Scientists ascribe to either of two main theories regarding the cause of this mass extinction. Intrinsic Gradualists believe the K-T Extinction was a slow, gradual process and caused by severe volcanic activity and the effect of plate tectonics. Extrinsic Catastrophists attribute the K-T Extinction to a sudden, catastrophic event such as the Earth being struck by a large meteor or asteroid.
The K-T Extinction allowed the surviving species to evolve rapidly and become abundant over time. These species may not have had the opportunity for evolution if the K-T Extinction had not occurred.
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