MadSci Network: Evolution

Re: When did the Sarcopterygii separate from Actinopterygii?

Date: Thu Feb 15 13:59:33 2001
Posted By: David Kopaska-Merkel, Staff Hydrogeology Division, Geological Survey of Alabama
Area of science: Evolution
ID: 982259042.Ev


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


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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