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