A lung and pulmonary circulation is also seen in surviving primitive ray-finned fishes, such as the bichir (Polypterus), and was likely a primitive bony fish feature, with the lung turned into a swim bladder in more derived ray-finned fishes such as teleosts. The most primitive lobe-finned fish, the coelacanth, also has a swim bladder and lacks a pulmonary situation, likely also a derived condition. The fish here does not represent a modern lungfish, where not all modern species retain the second aortic arch, but rather the likely condition in the common ancestor of lungfishes and tetrapods. A condition in both (as shown here) that is derived over the condition of fish such as Polypterus is that the atrium is divided into left and right chambers, and the pulmonary vein enters the right atrium (rather than the sinus venosus as in Polypterus). A spiral valve in the conus arteriosus is also a new feature in sarcopterygians: this separates out the flow of oxygenated and deoxygenated blood, entering the ventricle from left and right atria, respectively (when the lungs are in use), and sends the oxygenated blood preferentially to the head. Note that there is a new venous return to the heart, the posterior vena cava, first seen in lobe-finned fishes.
© 2007-2010 Christine Janis, Professor of Ecology and Evolutionary Biology
Colleen Brogan '10, Student Technology Assistant
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