Since the beginning of geological time, the earth has undergone five mass extinctions. The "Big 5" caused the extinction of about 99% of the species that once inhabited the earth. The last major extinction occurred 65 million years ago, somewhere near the end of the Cretaceous and beginning of the Tertiary periods, commonly referred to as the K-T Boundary. This is when the dinosaurs mysteriously disappeared. The K-T extinction, though not the largest, has become the most baffling to scientists and fascinated the general public for years.
The inability to explain what really happened has lead to a multitude of extinction theories. Many of the theories were discarded because they did not account for the fact that some species were completely wiped out and others survived. Some only explained the demise of the larger species, but did not address the smaller animals or marine life that was also affected. Only a few of these theories have been proven to be scientifically viable but even with today's advanced technology, the exact cause of the large invertebrate's demise still remains unknown.
Mass extinctions disrupt the co-dependence of living organisms on Earth and are very complicated processes. They can be triggered by evolutionary issues, environmental and climactic changes, or catastrophic events. All extinction theories have been divided into two main categories:
1. Intrinsic Gradualists = the dinosaur's extinction happened gradually over time, and was earthly in nature.
2. Extrinsic Catastrophists = the extinction was due to a sudden, punctuated event, and was extraterrestrial in nature.
Instinsic gradualist theories:
A. Massive Volcanic Activity:
A huge increase in volcanic activity occurred around the same time of the dinosaur's extinction. Excessive volcanic activity would have spewed ash into the atmosphere, blocking the sun's rays, and coated the surrounding habitat with molten lava. Temperature shifts, and decreased or contaminated food and water supplies, would have had a major effect on the dinosaur's ability to survive.
B. Continental Drift/Plate Tectonics:
During the K-T Boundary period, the oceans were regressing from the landmasses. This would have impacted the climate, the food supply, and potentially could have created "land bridges" that allowed predators to invade the dinosaur's territory.
C. Infectious Disease:
Historically, deadly viruses and bacterium have been associated with the decrease in population numbers or the death of certain species. Some scientists believe that a dangerous pathogen must have evolved and the dinosaur's immune systems were not able to fight it off, causing the extinction.
* The dinosaur eggs were being eaten by rapidly evolving smaller mammals. Without the survival of their offspring, the dinosaur population dramatically declined and eventually just died out.
* The dinosaurs evolved to a point where they became too large and awkward to breed. Without the ability to produce another generation, the dinosaurs eventually ceased to exist.
Extrinsic Catastrophists theories
A. Asteroid/Meteorite/Comet Collision:
This has been a popular theory since the late 1970s and 1980s when Walter and Luis Alvarez linked the impact of a bolide with the Cretaceous Period's extinction of the dinosaurs. An asteroid/meteorite/comet struck an area near the tip of the Yucatan Peninsula and created a 65 mile-wide (170 km.) depression known as the Chicxulub Crater.
The collision would have caused an increase in iridium levels, generated intense heat, and produced shock waves that set off massive explosions and ignited firestorms. Huge clouds of ash and debris were ejected into the atmosphere and acted like reflectors for the sun's thermal radiation. This would have triggered a rapid, unnatural, climatic change, acid rain, water and food supply poisoning, and lack of heat from the sun. Any dinosaurs that managed to survive the initial impact were unable to tolerate the resulting conditions.
B. The Oort Cloud/Simultaneous, Multiple Collisions:
This theory builds upon the single collision theory, suggesting that a cluster of comets/meteors collided with the Earth. It is possible that a stream of comets from the Oort Cloud were dislodged by a passing star's gravitational pull and were drawn towards the Earth. If several celestial bodies hit the Earth simultaneously, it would explain a global extinction event like the K-T. This theory would multiply the disastrous effects cited in the single collision theory and better explain the global effects that resulted from the K-T extinction.
C. Explosion of a Supernova:
This theory proposes that a huge star, close to the Earth, either exploded or imploded, and released a phenomenal amount of electromagnetic radiation exposing the Earth to excessive amounts of X-rays and Gamma rays. Supernova explosions disperse radioactive matter into space and any life forms within about 50 light years will feel its devastating effects. Radiation sickness and a breach in the Earth's protective ozone layer would have resulted in the collapse of the food chain and an increased cloud cover that altered the climate. Most of the plants and animals would have either succumbed to the radiation, starved to death, or been unable to survive the drastic climatic change.
* A change in the Earth's Orbit resulted in an Ice Age. Temperatures on Earth are directly related to our distance from the sun. Any change in the Earth's circular orbit would bring us closer to, or further from, the sun. This would create dramatic temperature changes. The climate would become either too hot or too cold for most life forms to survive. If the Earth's orbit was disrupted by a passing celestial body's gravitational path, or something caused the Earth to "tilt or wobble" on its axis, dire consequences would result. Any intense changes in climate during the Cretaceous period could explain the dinosaur's extinction.
Both the Gradualists and Catastrophists have managed to find some "common ground" in their K-T extinction theories. Whether it was from the impact of an asteroid, comet, or other celestial entity, a disruption in the Earth's orbital pattern, or massive volcanic eruptions, fossils and geologic data have proven that a global climatic change did occur.
Eventually, science will discover the truth about what happened 65 million years ago, but for now, the extinction of the dinosaurs remains an unexplained mystery.
The following websites offer more detailed information on the extinction theories: