Final answer:
Birds are the only extant direct descendants of dinosaurs, having evolved from theropod dinosaurs. Other animals like crocodiles, alligators, lizards, snakes, and mammals are relatives but not direct descendants.
Step-by-step explanation:
The Only Extant Animals Descended from Dinosaurs
The question asks, "Which of the following are the only extant animals that descended directly from dinosaurs?" The answer is: birds. While crocodiles and alligators are close living relatives of birds and other reptiles may share a common ancestry with dinosaurs, it is only birds that have descended directly from dinosaurs, specifically from theropod dinosaurs. Lizards, crocodiles, snakes, and mammals all evolved from different lineages of ancient reptiles.
Birds are highly modified diapsids that belong to the group called the archosaurs, with their evolution stemming from maniraptoran theropod dinosaurs. This evolutionary journey is supported by fossil evidence, with the Archaeopteryx being one of the oldest known fossils to show the transition from dinosaurs to birds. Therefore, out of the options provided, only birds fit the profile as direct descendants of dinosaurs.