Herbivores have longer small intestines than carnivores. The 4-chambered heart evolved twice independently in 2 lineages of reptiles. This is called convergent evolution . The birds retained the right aorta and mammals retained the left aorta . We see more connections with reptiles all the time.
Lets look at the unique blood flow in a fetus as compared to an adult mammal.
The lungs are highly vascularized with much gas exchange surface area. Temperature Regulation Mammals are endotherms. Because of this they are able to live in more diverse environments than ectotherms. Also because of the higher metabolism to maintain a higher temperature an insulation is required to conserve the heat. Mammals generate the internal heat via metabolism.