Vertebrates have been modified cardiac morphologies and abilities to adapt terrestrial life with the use of lung for breathing in the evolution. The hearts morphologies have been more sophisticated and complicated from fish to mammals; fish have simple two-chambered heart (one atrium and one ventricle), amphibians have three-chambered (two atria and one ventricle), and crocodilian, birds, and mammals have four-chambered (two atria and two ventricles) heart. Interestingly at early stages mammalian heart forms a turning tube like as fish heart. Then we thought that to know the molecular mechanism of heart evolution leads to elucidation of developmental mechanism of cardiac septal formation and of congenital disease occurred in cardiac septa. Non-crocodilian reptiles hold a unique place in the evolution of the heart, as their ventricle chambers are apparent intermediates between three-chambered heart and four-chambered heart. The previous study indicated that a steep and correctly positioned Tbx5 gradient is important for ventricular septum formation by using reptiles and the transgenic mice in which Tbx5 expressions like as reptiles. Sarcopterygians, including lungfish and coelacanth, also sit on the unique position between two-chambered heart (fish) and three-chambered heart (amphibian), suggesting that sarcopterygian is quite useful for studying about atrial septum formation. We constructed 3D images of lungfish and coelacanth hearts to observe the inside structure in detail. As a result, coelacanth heart is still similar to the fish, but lungfish has very sophisticated septated heart both in atria and ventricles. In this session we want to present when and how atrial septum is formed in vertebrate evolution via lungfish heart development comparing fish and amphibians.