For example, pretend that all birds were extinct. If, as a scientist, you had never seen a living bird, you would rely on fossil remains to categorize them. When you discovered that there was a group of animals that all have feathers in their fossil remains, you might place them into one category and call them birds on this basis.
So, what links all the dinosaurs together? It’s probably not what you might think. All dinosaurs were reptiles – cold blooded, egg laying creatures. Some walked on two legs. Others walked on four legs. Some were very large. Others were quite small. Some ate leaves. Others preyed on other dinosaurs. What all dinosaurs had in common was actually their hips! At the place where a dinosaur’s thighbone (or femur) connects to its pelvis, there is a hole. Dinosaurs have open hip sockets!