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.
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