444-416 million years ago
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).