What characteristic separates Class Reptilia from Class Mammalia, concerning body temperature regulation?

Reptiles are generally ectothermic, relying on external sources for thermal regulation

The classification into Class Reptilia is defined by several key physiological and external characteristics that differentiate them from other vertebrate classes like Mammalia or Aves. A fundamental distinction lies in thermoregulation. Reptiles are predominantly ectothermic, meaning they must actively bask in the sun or seek shade to raise or lower their body temperature to optimal levels for activity, digestion, and survival. In contrast, mammals are endothermic, maintaining a stable internal body temperature primarily through internal metabolic processes. Furthermore, reptiles are characterized by having scales or scutes and breathing air using lungs, traits that distinguish them clearly from amphibians, which often have permeable skin, and from endotherms.