ESS 2.2 AS91188

• Dinosaurs are a group of reptiles that have lived on Earth for about 245 million years. 

• In 1842, the English naturalist Sir Richard Owen coined the term Dinosauria, derived from the Greek deinos, meaning “fearfully great,” and sauros, meaning “lizard.”

• Dinosaur fossils have been found on all seven continents.

• All non-avian dinosaurs went extinct about 66 million years ago.

• There are roughly 700 known species of extinct dinosaurs.

• Modern birds are a kind of dinosaur because they share a common ancestor with non-avian dinosaurs.

The extinction of the dinosaurs, a pivotal event in Earth's history, has captivated scientists and the public alike for centuries. For over 150 million years, dinosaurs were the dominant animals on the planet. Then, about 65 million years ago, toward the end of the Cretaceous period, they became extinct. There are many theories about why dinosaurs became extinct. Climate change, geological events, change in vegetation and diseases are some of the factors may have contributed the extinction of the dinosaurs.

While the exact cause remains a subject of debate, two main theories have emerged: the gradualist and the catastrophist perspectives. 

• The gradualist theory posits that the dinosaurs' demise was a slow and drawn-out process, driven by internal Earthly forces. This theory suggests that massive volcanic eruptions, like those that formed the Deccan Traps in India, released vast amounts of carbon dioxide into the atmosphere, leading to a gradual warming of the planet and a decline in dinosaur populations.

• The catastrophist theory, on the other hand, proposes that the dinosaurs were wiped out by a sudden and catastrophic event. The most widely accepted theory within this camp is the Alvarez hypothesis, which suggests that a large asteroid impacted Earth approximately 66 million years ago, causing widespread devastation and triggering a mass extinction. This hypothesis gained traction after the discovery of a high concentration of iridium, a rare element found in meteorites, in rock layers dating back to the time of the extinction.

Asteroid impact

The most accepted theory talks about the impact of a large asteroid or comet on earth at Chicxulub in the Yucatan Peninsula in Mexico that might have caused the dinosaurs to become extinct. The impact could have caused massive damage, including a giant cloud of dust spreading iridium throughout the land. This led to major changes in climate destroying several forms of plant life. Herbivorous dinosaurs might have died of hunger and carnivores were left with nothing to eat as well.

This theory stems from the discovery that a layer of rock which dates precisely to the time of extinction is rich in deadly iridium. This theory makes sense but has a problem: the scientists and palaeontologists have not yet managed to find skeletons and dinosaur fossils that date from the time of impact.

www.nhm.ac.uk/discover/how-an-asteroid-caused-extinction-of-dinosaurs.html

Competition from Mammals

Some scientists suggest that the dinosaurs became extinct because mammals stole and ate all the eggs of dinosaurs. But this theory is unsupported because marine mammals were non-existent at the end of the Cretaceous period.

Theory of Ice Age

Throughout its history Earth has witnessed Ice Ages. Since dinosaurs were thought to be cold-blooded, it would have been very difficult for them to come in terms with such cold conditions. However there has been no evidence of any occurrence of ice age during the life of the dinosaurs.

Theory of Volcanoes

It is said that 65 million years ago the Deccan Plateau in western India witnessed severe volcanic eruptions which cast a huge amount of dust and debris into the atmosphere, enough to prevent the sun’s rays coming to earth for a long period of time. This caused huge change in climate and the dinosaurs found it very difficult to adapt to this situation.

The Cretaceous-Paleogene (K-Pg) extinction event, also known as the K-T extinction, was a catastrophic event that occurred approximately 66 million years ago, wiping out three-quarters of plant and animal species on Earth. This event marked the end of the reign of the dinosaurs, with the exception of avian dinosaurs, which evolved into modern birds. The K-Pg extinction also led to the demise of most tetrapods weighing over 25 kilograms, with the exception of certain ectothermic species like sea turtles and crocodiles. This event marked the end of the Mesozoic era and ushered in the Cenozoic era, the current geological era.

The K-Pg event is clearly visible in the geological record as a thin layer of sediment called the K-Pg boundary, also known as the Fatkito boundary or K-T boundary. This boundary layer can be found globally in both marine and terrestrial rocks. The boundary clay exhibits unusually high levels of iridium, a metal more abundant in asteroids than in Earth's crust.

A theory, proposed by Luis Alvarez and his son Walter in 1980, posits that the K-Pg extinction was triggered by the impact of a massive asteroid, measuring 10 to 15 kilometers in diameter, 66 million years ago. This impact created the Chicxulub crater, causing widespread devastation to the global environment. The impact hypothesis, also known as the Alvarez hypothesis, gained significant support with the discovery of the 180-kilometer Chicxulub crater in the Yucatán Peninsula of the Gulf of Mexico in the early 1990s. This discovery provided conclusive evidence that the K-Pg boundary clay represented debris from an asteroid impact.

The K-Pg extinction event was a pivotal moment in Earth's history, marking the end of an era dominated by dinosaurs and paving the way for the rise of mammals. While the impact of the asteroid is widely accepted as the primary cause, other factors, such as increased volcanic activity, may have contributed to the extinction event. The K-Pg extinction event serves as a stark reminder of the fragility of life on Earth and the potential for catastrophic events to reshape the planet's biodiversity.