One of the best ways to identify dinosaur fossil bones is by size, as many of the bones are huge. For example, the upper leg bone, or femur, of an adult Apatosaurus often measures over 6 feet (1.8 meters) long.

But size is not everything, as many dinosaurs were the same size as a chicken or cat. The way scientists detect the differences between dinosaurs and other animal species is by the construction and orientation of their bones, including heads, tails, and hipbones. In addition, dinosaur fossils are often found in association with other dinosaurs at a site. Many times these fossils represent dinosaurs—from meateaters to plant-eaters—that gathered together along the shore of a lake or ocean. The dinosaurs were all searching for food along the banks of the water, a place that would attract many animals and plants.

Of course, not all dinosaurs are found in the conventional way. In 1998, an amateur fossil collector saw the movie Jurassic Park, and recognized that a fossil he had (which he thought was a bird) was actually a dinosaur. The specimen was found in Italy and measures only 9.5 inches (24 centimeters) long; scientists now know it was a young dinosaur (a theropod named Scipionyx samniticus) that had just hatched before it died. In this case, the dinosaur remains probably washed into oxygen-starved waters, where it was quickly buried. This may be one of the most important dinosaur fossils ever found, as many of its soft body parts were preserved, including the intestines, muscle fibers, and what appears to be the liver.