Crocodiles have important scientific value in evolutionary history. It is the only living clue of dinosaurs in early history and a close relative of birds. Many crocodile fossils have been found. Three of the four suborders of crocodiles are extinct.

The oldest protoodont in Upper Triassic is very similar to the tooth. In Jurassic and Cretaceous, the supratemporal fossa of Chinese alligator was very large, and the internal nostril moved forward between the flap bone and pterygoid bone. Jurassic and Cretaceous marine crocodiles have paddle-like short forelimbs and a finned and pointed tail. The crocodile Siebel was discovered in Eocene in South America. There were real crocodiles in the early Cretaceous, with the most species. Most crocodiles in Tertiary and Quaternary belong to the suborder True Crocodiles, and there are three existing families of true crocodiles: ① Crocodiles (alligators), with short and wide snout and no obvious boundary with the back of skull. When you shut up, the mandibular teeth are in the mandibular teeth. The fourth mandibular tooth is enlarged, bite? The cavity of the maxilla. There are two typical species of catfish, one is catfish in North America, and the other is catfish (alligator sinensis) in the Yangtze River. ② Crocodile family, the snout is short and wide, the upper and lower dentition are staggered, and the fourth tooth of the mandible is enlarged. A depression in the upper jaw. Typical crocodiles are 1 1, which are widely distributed in tropical areas. ③ Fish-eating crocodiles have a long and narrow snout, which is obviously different from the back of skull. There are 1 species of Ganges fish-eating crocodiles in India alone. Crocodiles have not changed much since Triassic, so the existing crocodiles can be called living fossils. Crocodile skin has high economic value. It can be tanned and made into expensive bags and shoes. Meat and eggs can be eaten. It is said that meat is delicious.