There is no biological classification that defines an animal as both a mammal and a reptile. Mammals and reptiles are two distinct classes within the animal kingdom, characterized by different physiological and reproductive traits. Mammals are warm-blooded, have fur or hair, and typically give live birth while also nursing their young with milk. Reptiles, on the other hand, are cold-blooded and lay eggs with leathery shells or give birth to live young but do not possess mammary glands. Although there are transitional species in evolutionary history, living animals cannot be classified simultaneously as both mammals and reptiles.