Evolution of the aortic arches

This diagram shows a side view of the organisms, with the head facing left and heart and lungs to the right. The highly derived patterns of birds and mammals were formed by loss or specialization of the various arches. The order of events leading to each lineage has been reconstructed in some detail. As the caption says, "One of the hallmarks in comparative anatomy is the discovery and broadly based explanation of the evolutionary pattern of the heart and great vessels of vertebrates. It still has profound and pervasive implications in comparative biology."

From pp. 620-621 of: Liem, Karel F. and Walker, Warren F. (2001). Functional anatomy of the vertebrates: an evolutionary perspective. Fort Worth, Harcourt College Publishers. online source