What are Dinosaurs?

Dinosaurs are a diverse group of reptiles that were the dominant terrestrial life form on Earth during the Mesozoic Era, about 251.9 million years ago. So far more than 900 distinct genera and more than 1,000 different species of non-avian dinosaurs have been identified by paleontologists, using fossil evidence. Most, if not all dinosaurs, were egg layers. Some were probably warm-blooded.

Classification

Dinosaurs are classified as either ornithischians (bird hipped) or saurischians (lizard hipped), based on pelvic girdle structure.

Extinction

Dinosaurs became extinct near the end of the Cretaceous Period, about 66 million years ago. One of the theories for the dead of the dinosaurs is that a meteor the size of a mountain slammed into Earth, filling the atmosphere with gas, dust, and debris that drastically altered the climate. Another theory is that volcanism and mountain-building cycles altered the habitat and changed the climate.