The five main vertebrate groups
Fish live in water, breathe through gills, and are ectothermic (cold-blooded). Amphibians (like frogs) breathe through both lungs and moist skin, live part of their life in water and part on land, and are also ectothermic. Reptiles have dry scaly skin, breathe with lungs, and are ectothermic. Birds have feathers, breathe with lungs connected to air sacs for highly efficient gas exchange, and are endothermic (warm-blooded). Mammals have hair/fur, breathe with lungs, feed their young with milk from mammary glands, and are endothermic.
Example 1: A student finds an animal with dry scaly skin that lays eggs on land and cannot regulate its own body temperature. Which vertebrate group is this most likely to belong to?
Reptile -- dry scaly skin and an inability to internally regulate body temperature (ectothermic) are both defining reptile features, distinguishing it from the moist skin of amphibians and the fur/feathers of endothermic mammals and birds.
Respiration adaptations
Fish use gills, which have a large surface area of thin membranes to extract the relatively small amount of oxygen dissolved in water. Amphibians can respire through their moist skin as well as simple lungs. Reptiles, birds, and mammals all use lungs, but bird lungs are uniquely efficient, using connected air sacs to keep air flowing through the lungs in one direction, extracting more oxygen per breath -- useful for the high energy demands of flight.
Circulatory adaptations
Fish have a two-chambered heart, sending blood through gills for oxygenation, then around the body, before returning -- a single circuit. Amphibians and most reptiles have a three-chambered heart, allowing separate pulmonary (to lungs) and systemic (to body) circuits, though the chambers allow some mixing of oxygenated and deoxygenated blood. Birds and mammals have a four-chambered heart, keeping oxygenated and deoxygenated blood completely separate -- supporting their higher, constant metabolic rate.