The bony lung of the coelacanth Macropoma
Date
2011-05-13
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Abstract
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.
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Keywords
Coelacanth, Macropoma, Axelrodichthys