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While reading a book on dinosaurs, you notice that they existed in a time span of about 230 to 65 million years ago in the Mesozoic Era, and you wonder what the first dinosaurs looked like. When you find an artist's rendering of these creatures, you notice that they look similar to modern monitor lizards (such as Komodo dragons), crocodiles, or alligators. Moreover, you are surprised to find out that flying reptiles, marine reptiles, and many other dinosaur contemporaries were not actually dinosaurs. As you read about dinosaurs toward the end of the Mesozoic Era, you also see the phrase "birds are dinosaurs".

Are dinosaurs, monitor lizards, crocodiles, and alligators related to one another? If so, what common ancestors did they have? What is the basis of the phrase "birds are dinosaurs"? If this premise is acceptable to you, then how are birds related to monitor lizards and crocodiles?

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