Cretaceous Period

, EdScientists refer to the periods in Earth's history that lasted between 248 million years ago, and 65 million years ago, as the Mesozoic era. It was during this era that dinosaurs were the dominant land animals, the firsts birds evolved (probably from dinosaurs), pterosaurs (flying reptiles were found in the sky), a variety of large marine reptiles inhabited the sea, and the first mammals (tiny mouse-like creatures) and angiosperms (flowering plants) also appeared. The Mesozoic era is subdivided into three periods - the Triassic period (248 million to 213 million years ago), the Jurassic period (213 million to 144 million years ago), and the Cretaceous period (144 to 65 million years ago).

It was during the Cretaceous period that many of the best known types of dinosaurs lived, including Ankylosaurus, Iguanodon, Pachycephalosaurus, Protoceratops, Spinosaurus, Styracosaurus, Triceratops, Tyrannosaurus rex, and Velociraptor.

The biggest mystery of the Cretaceous is what happened at the end of it - suddenly, it seems that all the dinosaurs and other large animals, as well as a wide variety of terrestrial and marine life forms, became extinct. We probably should be grateful for this - as the extinction of the dinosaurs provided the opportunity for mammals to evolve into larger animals, and eventually humans (if the dinosaurs had not gone extinct, we would not be here), but of course we would love to know what was the reason that dinosaurs went extinct.

Many explanations have been proposed for dinosaur extinction, some more plausible than others. When considering, we need to bear in mind that we need to find explanation not only for the extinction of dinosaurs, but also for all the other creatures that became extinct at the same time. Currently, one of the most popular theories is that an asteroid hit the Earth blotting our sunlight for several years, and thus killing most plant life, and therefore depriving animals of food. Evidence for this theory, includes a layer of iridium (which comes from asteroids) found in rocks of the right age, and a crater in southern Mexico which may be the result of this asteroid's impact.

Reference:

Cretaceous Period, S. Tanna, http://www.goarticles.com/cgi-bin/showa.cgi?C=1329734