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Regarding the discovery of the dinosaur heart with the single aorta, your readers should note that this morphology is more likely to be related to high blood pressure than metabolic rate per se. The typical reptilian heart, with its incompletely divided ventricle and double aorta, is quite functional at separating oxygenated and deoxygenated blood. The reptilian heart also has the advantage of being able to shunt blood so that equal volumes needn’t be sent to the lungs and the body. However, with the higher blood pressure of birds and mammals, the lungs must be protected against high systemic pressures. This necessitates a completely divided heart and the accompanying loss of the accessory aorta. While high blood pressure is correlated with endothermy in birds and mammals, it would also be essential in a large animal with a long neck (as in many dinosaurs) for blood to be delivered to the brain, whatever the metabolism of the animal.

Christine Janis
Brown University
Providence, R.I.