Dinosaurs are mainly divided into two orders: Saurischia and Ornithischia.

Dinosaurs were prehistoric reptiles that lived on Earth during the Mesozoic Era, flourishing at one time in the Triassic, Jurassic and Cretaceous periods. Based on fossil research, scientists usually divide dinosaurs into two major groups according to their pelvic structure: Saurischia and Ornithischia, which is also the most widely circulated classification method at present.

The pubis of saurischian dinosaurs extends forward and downward, and their pelvic structure is similar to that of lizards; they are further divided into theropods and sauropods. Most theropods were bipedal carnivorous dinosaurs, such as Tyrannosaurus rex and Velociraptor; sauropods were mainly huge herbivorous dinosaurs, such as Brachiosaurus and Diplodocus, which often used their long necks to feed on plants high up.

The pelvic structure of ornithischian dinosaurs is similar to that of birds, and their members were mostly herbivorous, including stegosaurs, ankylosaurs, ceratopsians and hadrosaurs. Triceratops, Stegosaurus and Ankylosaurus all belong to this major group, and they often used defensive structures such as head horns, bony plates or tail clubs to fend off natural enemies.

It should be noted that although pterosaurs, ichthyosaurs and plesiosaurs are often called dinosaurs, strictly speaking they do not belong to the dinosaur family. In addition, scientists generally believe that birds evolved from theropod dinosaurs; from this perspective, the descendants of dinosaurs are in fact still living on Earth today.