444-416 million years ago


The most primitive bony fish possessed a gas bladder that could be used to breathe oxygen from the air. In the early sarcopterygians, this structure became paired to form lungs similar to those of amphibians. Sarcopterygian lungs evolved surfactant proteins which their descendants would use to keep their air sacs open.

In ancestral sarcopterygians the lungs became paired (Romer, p. 364) and the internal structure of the lungs was similar to that of tetrapods with internal divisions and alveoli-like chambers (Johansen, Kjell from Hoar, 1970; Romer ). Ridges of connective tissue increased respiratory surface area (Romer, p. 366). An internal naris evolved which connected the nostrils to the pharynx (Romer, p. 325). Nasal placodes from the stomodeum were incorporated into the mouth (Kardong, p. 490). Sarcopterygian breathing evolved aspects which would be retained in tetrapods such as the use of skeletal muscle and aspiratory forces help venous return (Hoar, 1970). Lining the surface of sarcopterygian lungs, both SP-A and SP-B surfactant proteins were present (Power, 1999) and there was an increase of DSP (disaturated phospholipids) in surfactant (Orgeig, 1995).