When did the dinosaurs first appear on Earth?
The oldest dinosaur types are known from rocks in Argentina and Brazil and are about 230 million years old. The most primitive of these types, Eoraptor, was a small meat-eating dinosaur. Because Eoraptor's skeleton shows some advanced skeletal features, older dinosaurs may yet be found.
Are all fossil animals dinosaurs?
No. Dinosaurs are a group of ancient reptiles that had a set of particular skeletal features. The hips, hind legs, and ankles were specialized and allowed the legs to move directly under the body, rather than extending out from the side of the body as in modern lizards. This arrangement enabled dinosaurs to bring their knees and ankles directly below their hips and provided the necessary attachments for very strong leg muscles. Dinosaur skeletons were well designed for supporting a large body, for standing erect (upright), and for running. The front legs were adapted for grasping prey, for supporting weight, or for walking and running. The skulls of dinosaurs were designed for maximum strength, for minimum weight, and (in some cases) for grasping, holding, or tearing at prey.
These skeletal features separated dinosaurs from other ancient reptiles such as Dimetrodon, the plesiosaurs, and pterosaurs. Fossil mammals, like mammoths and "saber-toothed tigers" (e.g., Smilodon), are also often incorrectly called dinosaurs.
Did people and dinosaurs live at the same time?
No! After the dinosaurs died out, nearly 65 million years passed before people appeared on Earth. However, small mammals (including shrew-sized primates) were alive at the time of the dinosaurs. Many scientists who study dinosaurs (vertebrate paleontologists) now think that birds are direct descendants of one line of carnivorous dinosaurs, and some consider that they in fact represent modern living dinosaurs. This theory remains under discussion and shows that there is still much we don't know about dinosaurs.
Where did dinosaurs live?
Paleontologists now have evidence that dinosaurs lived on all of the continents. At the beginning of the age of dinosaurs (during the Triassic Period, about 230 million years ago) the continents we now know were arranged together as a single supercontinent called Pangea. During the 165 million years of dinosaur existence this supercontinent slowly broke apart. Its pieces then spread across the globe into a nearly modern arrangement by a process called plate tectonics. Volcanoes, earthquakes, mountain building, and sea-floor spreading are all part of plate tectonics, and this process is still changing our modern Earth.
Did all the dinosaurs live together, and at the same time?
Dinosaur communities were separated by both time and geography. The "age of dinosaurs" (the Mesozoic Era) included three consecutive geologic time periods (the Triassic, Jurassic, and Cretaceous Periods). Different dinosaur species lived during each of these three periods. For example, the Jurassic dinosaur Stegosaurus already had been extinct for approximately 80 million years before the appearance of the Cretaceous dinosaur Tyrannosaurus. In fact, the time separating Stegosaurus and Tyrannosaurus is greater than the time separating Tyrannosaurus and you. At the beginning of dinosaur history (the Triassic Period), there was one supercontinent on Earth (Pangea). Many dinosaur types were widespread across it. However, as Pangea broke apart, dinosaurs became scattered across the globe on separate continents, and new types of dinosaurs evolved separately in each geographic area.
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Source: United States Geological Survey
