The difference between birds and lizards is mainly the shape of pubic bone in pelvis. Viewed from the side, sauropod's belt is three-shot, with pubic bone extending forward under intestinal bone and sciatic bone extending backward, similar to lizard. In birds, the pubic bone protrudes forward and is parallel to the ischium backward. Viewed from the side, the belt is a four-beam structure, which is the same as that of a bird.

Sauropods are divided into two suborders, sauropods and theropods.

The difference between them is not based on body size and whether they walk on all fours, but mainly on the classification of bone structure. Sauropods also have many small dinosaurs.