Though there were some dinosaurs with mammal-like qualities, dinosaurs were actually reptiles; however, though the word "dinosaur" roughly translates to "terrible lizard" in Greek, dinosaurs were not actually lizards, either. Dinosaurs represent a distinct group of reptiles that is now extinct, though there may be some living animals, including some birds, that are distant evolutionary relatives of the dinosaurs. The animals described as being mammal-like dinosaurs were actually early relatives of the dinosaur, and they were the dominant land animal about 260 million years ago, as opposed to the dinosaur, which was the dominant land animal about 231 million years ago.

The dinosaurs themselves are not known to have exhibited mammal-like qualities; there is no evidence to support the idea that they had mammalian characteristics such as warm blood and hair or fur, and there is evidence to suggest that they laid eggs rather than giving live birth as mammals do. However, some of the early mammal-like dinosaurs may have had some mammalian features, such as hair, fur or whiskers. There is not any evidence to suggest that these features carried over into dinosaurs. Dinosaurs themselves had physical characteristics that separate them from modern lizards, indicating how different life on earth was during prehistoric eras.