Final answer:
The traditional classification separated birds (Aves) from reptiles (Reptilia), but modern classification includes birds as avian reptiles within the reptile clade, acknowledging their descent from dinosaurs. Differences include reptiles being ectothermic and often having scales, while birds are endothermic with feathers and a sophisticated respiratory system.
Step-by-step explanation:
The traditional view categorized amniotes into Mammalia, Reptilia, and Aves (birds), treating birds as a separate class from reptiles. However, modern classification recognizes birds as avian reptiles, descended directly from theropod dinosaurs, effectively classifying them within the reptile clade. This shift is based on physical and genetic evidence including the discovery of Archaeopteryx, which exhibits characteristics of both birds and reptiles.
Reptiles, such as turtles, crocodiles, lizards, and snakes, are generally ectothermic, which means they rely on external sources to regulate their body temperature. Birds, on the other hand, are endothermic, generating their own body heat and maintaining it with adaptations such as feathers. While reptiles traditionally have scales containing keratin, birds have evolved feathers, which are unique to them and were first present in their dinosaur ancestors.
Regarding physical structure, reptiles are characterized by their tetrapod lineage, though some have evolved to lose limbs as seen in snakes. Birds are also tetrapods but have adapted their forelimbs into wings, which aids in flight, a feature not seen in modern reptiles. The respiratory system of birds has evolved for hundreds of millions of years and is distinct from that of reptiles, contributing to their high metabolic rate and endothermy.
It is worth noting that not all ancient "reptiles" share the same characteristics as present-day reptiles, indicating a diverse evolutionary path. Contemporary understanding of reptile evolution is under constant revision, with DNA analysis and research continuously updating the phylogenetic tree that depicts their relationships.