To be a dinosaur, an animal must have lived during the Mesozoic Era, a time period that stretched from about 248 million years ago to about 65 million years ago, over 180 million years. The Mesozoic Era can be further broken down into the Triassic Period (248 to 208 million years ago), the Jurassic Period (208 to 144 million years ago, this is the period that the book and movie "Jurassic Park" were named after), and the Cretaceous Period (144 to 65 million years ago). So, if an animal did not live during the Mesozoic Era, it cannot be properly called a dinosaur.
The first dinosaurs appeared in the Triassic Period, they were able to spread all over the Earth during the Jurassic Period, and they ruled the planet until they became extinct at the end of the Cretaceous Period. No-one knows for certain why the dinosaurs went extinct after dominating the planet for such a long time, but many scientists have some interesting theories. We'll look at some of these theories in an upcoming article.
Another important characteristic for an animal to be called a dinosaur is that it must have lived on land. So aquatic animals that lived in the ocean, like Mosasaurs, Pleisiosaurs, and Ichthyosaurs are not dinosaurs. Instead, they are called Marine Reptiles, and are related to modern extant (still living) reptiles.
Animals that could fly are also not dinosaurs. These animals are called Pterosaurs, and include animals like Pteranodon. The name "Pterosaur" means "winged lizard". See my article "Pterosaurs: The First Flyers" for more information about Pterosaurs.
Yet another important characteristic an animal must have to be called a dinosaur is an upright stance. This means that a bipedal or two-legged dinosaur's feet would be directly under its hips when it stood up, like an ostrich, and a quadrupedal or four-legged dinosaur's feet would be directly under its shoulders and hips, like your pet dog or cat. In contrast, reptiles have a sprawling gait, meaning that it would look like they were doing push-ups.
Dimetrodon is a reptile that is often confused with the dinosaurs. You can see that it seems to be crouching. Dimetrodon was a Pelycosaur, a carnivorous (meat-eating) reptile that lived during the Permian Period (286 to 245 million years ago), before the time of the dinosaurs. Dimetrodon is also different from the dinosaurs in the shape of its skull, and in the
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