|MadSci Network: Evolution|
Douglas: Actually, the Sarcopterygii are fish, just like the Actinopterygii. These are the two major groups of bony fishes, the fleshy-finned fish and the ray-finned fish. It is true that tetrapods (amphibians, reptiles, mammals, birds) descended from members of the Sarcopterygii, but accepted classification schemes do not put these highly derived descendants in with the fleshy-finned fish. The oldest known fish are Cambrian, but these are primitive forms not assigned to either Sarcopterygii or Actinopterygii. According to Romer (1966), the split probably occurred in the early Silurian or late Ordovician, though the oldest known fossils of these two groups are early Devonian. McAlester (1968) placed the split in the Silurian. The oldest known members of these two groups are not particularly primitive, so it is clear that the two groups diverged some considerable time before the early Devonian. I am afraid that, at least for the moment, I can't be more precise. It is possible that new discoveries of primitive fish have recently, or will in the future, allow us to be more precise. It is also possible that molecular-clock studies can provide a relatively precise estimate of that latest common ancestor of these two groups, but to my knowledge this has not yet been done. David Kopaska-Merkel Geological Survey of Alabama PO Box 869999 Tuscaloosa AL 35486-6999 (205) 349-2852 fax (205) 349-2861 www.gsa.state.al.us REFERENCES McAlester, A. L., 1968, The History of Life: Englewood Cliffs, NJ, Prentice-Hall, 152 p. Romer, A. S., 1966, Vertebrate Paleontology, 3rd edition: Chicago, The University of Chicago Press, 468 p. ISBN 0-226-72488-3
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