Dinosaur Extinction

Aproximately 65 million years ago, non-avian dinosaurs suddenly became extinct, along with many other groups of animals, such as mosasaurs, ammonites (nautilus-like mollusks), plesiosaurs, herbivorous crocodiles and turtles, pterosaurs, most birds, and numerous groups of mammals. This mass extinction is referred to as the Cretaceous-Tertiary extinction event.

Possible Causes of Extinction

Though the general consensus of paleontologists is that an impact event was the primary cause of the Cretaceous-Tertiary extinction event and the disappearance of dinosaurs from the fossil record, some scientists suggest other causes, or support the idea that a convergence of a number of factors was responsible.

1. Impact Event

The asteroid collision theory links the extinction event at the end of the Cretaceous period to a bolide impact that took place approximately 65.5 million years ago. This theory, which was introduced in 1980 by Walter Alvarez and his colleagues, proposed that a sudden increase in iridium levels recorded in the period's rock stratum around the world was direct evidence of the impact. They suggested that a 3-9 mile (5-15 km) wide bolide created the 110 mile (170 km) wide Chicxulub Crater in Mexico's Yucatan peninsula, and triggered a mass extinction. Various models suggest that this extinction was extremely rapid.

Most scientists who support this theory believe that the impact caused extinctions both directly (by heat from the meteorite impact) and also indirectly (via a worldwide cooling brought about when matter ejected from the impact crater reflected thermal radiation from the sun).

2. The Deccan Traps

The Deccan Traps are a large igneous province located on the Deccan Plateau of west-central India and consist of multiple layers of solidified flood basalt that together are more than 2 km thick and cover an area of 500,000 sq km. They are one of the largest volcanic features on Earth. They were formed between 60 and 68 million years ago,  at the end of the Cretaceous period, with the bulk of the volcanic eruption occurring some 66 million years ago. It is thought that the gases released in the process may have played a role in the Cretaceous-Tertiary extinction event through several mechanisms, including:

• the release of sulphuric aerosols and dust into the air, which may have blocked sunlight and thereby reduced photosynthesis in plants
• the release of carbon dioxide emissions, which would have increased the greenhouse effect when the aerosols and dust cleared from the atmosphere, contributing to an apparently massive global warming

The Deccan Traps theory was initially linked to the slow extinction of the dinosaurs, but eventually, most palaeontologists began to accept the idea that the mass extinctions at the end of the Cretaceous were largely (or at least partly) due to a massive Earth impact. However, most acknowledge that the major changes produced the Indian Deccan Traps before the impact may have contributed to the extinctions.

3. Climate Change

At the peak of the Mesozoic period, the planet's temperature was much more uniform, with only 25°C (45°F) separating temperatures at the equator from those average polar temperatures. The poles were 50°C (90°F) warmer than today. There were no polar ice caps, and sea levels are thought to have been 100-250 metres (300-800 ft) higher than they are in modern times.

In addition, the atmosphere's composition during this period was vastly different; oxygen formed 32 to 35% of the atmosphere (compared to 21% today) and carbon dioxide levels were up to 12 times higher than today's levels. However, by the late Cretaceous era, the environment was changing dramatically. Volcanic activity was decreasing, which led to a cooling of temperatures as levels of atmospheric carbon dioxide dropped. Oxygen levels also started to fall considerably. Some scientists suggest that lower oxygen levels combined with climate change, might have led directly to the demise of many species. Given the enormous oxygen demands of their very large bodies, it may have been particularly difficult for dinosaurs to cope with reduced respiratory efficiency.

4. Failure to Diversify

In the Mid Cretaceous period, flowering angiosperm plants became a major part of terrestrial ecosystems, which had previously been dominated by gymnosperms such as conifers. Fossilized dung (dinosaur coprolites) indicate that, whilst some ate angiosperms, the majority of herbivorous dinosaurs mainly ate gymnosperms. It has been suggested that the dinosaurs' failure to diversify in reaction to changing ecosystems doomed them to extinction.

Possible Dinosaur Survivors

In 2002, the discovery of a single hadrosaur leg bone fossil in the San Juan Basin, New Mexico dated to the early Paleocene epoch was reported by paleontologists Zielinski and Budahn. The discovery of this bone suggested that that some dinosaur populations may have survived at least a half million years into the Cenozoic Era. However, many scientist dismiss the Paleocene dinosaurs, suggesting that they have been washed out of their original locations and then re-buried in much later sediments.