Are all fossil animals dinosaurs?

No. Dinosaurs are a group of ancient reptiles that had a set of meticulous skeletal features. The hips, hind legs, and ankles were specialized and allowed the legs to move straight under the body, rather than extending out from the side of the body as in contemporary lizards. This deal 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 big body, for standing erect (upright), and for running.

The front legs were tailored for grasping prey, for supporting weight, or for walking and running. The skulls of dinosaurs were designed for maximum force, for minimum weight, and (in some cases) for avaricious, holding, or tearing at prey. These skeletal features separated dinosaurs from other olden reptiles such as Dimetrodon, the plesiosaurs, and pterosaurs. Fossil mammals, like mammoths and saber-toothed tigers (e.g., Smilodon), are also often wrongly called dinosaurs.

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