About Florida Alligator:
The most recent evidence indicates that crocodilians (which includes alligators) and dinosaurs evolved from a common ancestor that existed subsequent to the common ancestor that they share with other reptiles. So, even though alligators are classified as reptiles along with lizards, snakes, and turtles, they are actually more closely related to birds, whose direct ancestors were dinosaurs!
THE HEART OF AN ALLIGATOR
While most reptiles have 3-chambered hearts, the heart of alligators, and all crocodilians, has 4 chambers, a trait shared with mammals and birds. The advantage of a 4-chambered heart is that oxygenated blood and deoxygenated blood are separated, which results in more efficient respiration needed for the high metabolism of endothermic (warm-blooded) animals, and enables different pulmonary (lung) and systemic blood pressures, but is seemly over-complex for ectothermic (cold-blooded) crocodilians. The single ventricle of the 3-chambered reptile heart allows some mixing of oxygenated blood with deoxygenated blood, which may help regulate their metabolic state. Crocodilians have evolved a shunt between the left and right aorta (immediately above the ventricles) to facilitate the mixing of oxygenated and deoxygenated blood. Crocodilians also have a valve in the pulmonary artery that, when closed, forces deoxygenated blood to recirculate through the left aorta, which increases mixing. This increased mixing helps crocodilians transition to a lower metabolic state, and enables them to dive for extended periods.
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