What is a Dinosaur?

What makes a dinosaur a dinosaur? Dinosaurs lived on land. They all had a backbone and a tail. No dinosaurs had fur. All dinosaurs laid eggs.

Marine reptiles and flying creatures cannot be dinosaurs, except for birds. Birds are actually really close dinosaur relatives.

Flying pterosaurs like Quetzalcoatlus, Pteradon, and Hatzegopteryx are not dinosaurs.