The evolution of fishes into tetrapods has been mysterious because there were only a couple of fossils from this transition, with a gap of 20 million years (Romer’s Gap) in the known fossils. But in the last few years more fossils have come to light and we are beginning to make sense of how and why.
December’s issue of “Scientific American” has a good article by Jennifer Clack about tetrapod evolution. New fossils have been discovered and old ones from dusty museum drawers have been recognized as transitional tetrapods. It seems that the fish who became tetrapods inhabited shallow and occasionally stagnant water. They developed the ability to raise their heads out of the water to gulp air. The forelimbs strengthened, the vertebrae intelocked, the head disengaged from the shoulder, and the ribs lengthened to support the body cavity.
All these changes were immediately useful for an air-gulping fish. The forelimbs developed bones and paddle-like digits in place of rayed fins. The fish could still swim but it could also raise itself up. The head could be raised above the body to gain more air with less body movement. The shoulders had a hinge-like joint, suitable for levering the body out of the water but not useful for walking. It all makes sense. The characteristics, developed for one use, are now poised to be modified again for new uses.
Jennifer Clack is Reader in Vertebrate Palaeontology and Senior Assistant Curator, University Museum of Zoology, Cambridge, U.K. She has published a book about tetrapod evolution: Gaining Ground, The Origin and Evolution of Tetrapods, from Indiana University Press, 2002, ISBN 0-253-34054-3. You can see her reconstruction of Acanthostega on the Tree of Life Web. And you can read her summary of Acanthostega characteristics on her own Web site. For a discussion of fish and tetrapod evolution and of Jennifer Clack’s contributions, see Highlights in the Evolution of Vertebrates.