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SILURAN PERIOD

444-416 million years ago

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The atrium of the heart was partially divided in ancestral sarcopterygians with the vena cava bringing blood from the body back to the right half of the atrium and the pulmonary vein bringing blood from the lungs to the left half of the atrium. This same pattern exists in tetrapods.

Ancestral sarcopterygians evolved a partial interatrial septa (and although a partial interventricular septum may be present in modern sarcopterygians, it may not be homologous with that of amniotes) (Kardong 465). The pulmonary circulation was modified so that the sinus venosus empties into right atrium and the pulmonary vein into left (Kardong p. 465). The pulomary vein transported blood from the lungs to the left atrium. The pulmonary artery developed as a branch of aortic arch VI which also serviced the gills before emptying into the dorsal aorta through the ductus arteriosus (an anatomical arrangement which amphibians maintain) (Kardong).
Sarcopterygians also increased the amount of cardiac muscle in both the atria and ventricles (Webster, 1974) and developed a posterior vena cava (Romer 475). The pericardial cavity no longer opened into the coelom around digestive organs (the early pericardial cavities were continuous with the general coelom and even in cartilaginous fish and primitive actinopterygians a connection remains) (Romer, p. 317)