Reptiles, birds, and mammals are all amniotes. Dinosaurs were also amniotes! All amniote embryos develop in a fluid-filled sac, some inside the mother's body and some inside a tough, semipermeable shell.
All amniotes have two circuits of blood vessels, the pulmonary circuit moves blood from the heart to the lungs, and the systemic circuit moves blood from the heart to the rest of the body.
Reptiles have a 3-chambered heart, while birds and mammals have 4-chambered hearts. Amniotes manage their body heat in different ways. Ectotherms ar organisms whose body temperature is determined by their surroundings. Endotherms, have the ability to use their own metabolic activity to maintain body temperature.
This cladogram shows a simplified version of the amniote family tree.