This group also includes all extinct dinosaurs, pterosaurs, and extinct relatives of crocodilians. The broadest snouted modern crocodilians are alligators and larger caimans. Dinosaurs belong to a group known as archosaurs, which also includes modern crocodilians. Australia was characterized by marsupials, monotremes, crocodilians, testudines, monitors and numerous large flightless birds. Frogs, salamanders, turtles, crocodilians and champsosaurs also dwelled in the aquatic habitats. Crocodylomorpha is a group of archosaurs that includes the crocodilians and their extinct relatives. This was previously thought to only exist in the archosaurs (crocodilians and birds). Black caimans are relatively more robust than other crocodilians of comparable length. Most morphological attributes of Nile crocodiles are typical of crocodilians as a whole. Another trait that separates crocodiles from other crocodilians is their much higher levels of aggression. Phytosaurs would be considered closely related to the ancestors of both crocodilians and dinosaurs. Some species of crocodilians are known to have attacked humans. The snout shape of crocodilians varies between species. Various types of aquatic habitats are used by different crocodilians. The American crocodile rates somewhere in the middle of all crocodilians temperamentally.