The bony lung of the coelacanth Macropoma
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In 1833, Louis Agassiz described a large flask-shaped structure inside the body of the Cretaceous coelacanth Macropoma. Its microscopic structure was analyzed by W.C. Williamson in 1849. Coelacanths have been intensively studied ever since a living coelacanth, Latimeria, was discovered in the Indian Ocean in 1938. It became the most famous living fossil, because it was thought to be the closest living relative of the creatures that came on land to become the first amphibians. Latimeria is very closely related to Macropoma, but it does not have a bony lung, and bony lungs are not a part of the story of the origin of terrestrial vertebrates. My study of the bony lung of Macropoma is the first since 1849. I compare its microscopic structure to the much thicker ossified lung of another Cretaceous coelacanth, Axelrodichthys, and analogous bony structures in birds and teleost fishes. The bony lungs in Macropoma and Axelrodichthys share features such as being composed of plates that overlap and decrease in diameter posteriorly, but are strikingly different in microscopic structure like the possession of osteocytes within the bone matrix. I discuss the formation and possible functions of this curious organ, in the light of some ideas that have come along since 1849: evolution, classification based on genealogy, developmental biology, and biomechanics.