Answer:
In the modern classification system birds are in a separate class from reptiles, but fossil evidence shows they have a common ancestor.
Explanation:
Zoologists traditionally divide vertebrates into classes: major divisions with names such as mammals, birds, reptiles, and amphibians. Some zoologists, the so-called “cladists,” claim that a class proper must consist of animals that all have a common ancestor that belonged to this class and has no descendants outside that group. Birds would be a good example of class. All birds are descended from a single ancestor that would also have been called a bird and would have in common with modern birds the fundamental identifying characteristics: feathers, wings, beak, etc. In this sense, animals commonly referred to as reptiles do not form a good class.
This is because, at least in conventional taxonomies, this category explicitly excludes birds (they form a class of their own), and yet some “reptiles”, so conventionally recognized (eg, crocodiles and dinosaurs), are closer relatives of birds of the world. than other “reptiles” (eg lizards and turtles). In fact, some dinosaurs are closer to birds than other dinosaurs. Therefore, "reptiles" is an artificial class, as birds are artificially excluded. In a strict sense, if we were to consider reptiles a truly natural class, we would have to include birds in it. But what is special about birds that tempts us to separate them from reptiles? What seems to justify giving birds the honor of “class” when, evolutionarily speaking, they are just a branch of reptiles? It is the fact that the reptiles immediately around them, their close neighbors in the tree of life, happen to be extinct, and only the birds, the only ones in their group, remained alive. The closest relatives of the birds are all among the long-extinct dinosaurs. If a wide variety of dinosaur strains had survived, the birds would not stand out: they would not have been raised to the condition of a specific class of vertebrates, and no one would be asking questions such as "where are the missing links between reptiles and birds?"
