The writers, Erik Ahlberg of Uppsala University in Sweden and Jennifer A. Clack of the University of Cambridge in England, are often viewed as rivals to Dr. Shubin's team in the search for intermediate species in the evolution from fish to the first animals to colonize land.

In a statement by the Science Museum of London, where casts of the fossils will be on view, Dr. Clack said the fish "confirms everything we thought and also tells us about the order in which certain changes were made."

H. Richard Lane, director of paleobiology at the National Science Foundation, said in a statement, "These exciting discoveries are providing fossil 'Rosetta Stones' for a deeper understanding of this evolutionary milestone Â fish to land-roaming tetrapods."

The science foundation and the National Geographic Society were among the financial supporters of the research. Besides Dr. Shubin, the principal discoverers were Edward B. Daeschler of the Academy of Natural Sciences in Philadelphia and Farish A. Jenkins Jr., a Harvard evolutionary biologist.

Michael J. Novacek, a paleontologist at the American Museum of Natural History in Manhattan, who was not involved in the research, said: "Based on what we already know, we have a very strong reason to think tetrapods evolved from lineages of fishes. This may be a critical phase in that transition that we haven't had before. A good fossil cuts through a lot of scientific argument."

While Dr. Shubin's team played down the fossil's significance in the raging debate over Darwinian theory, which is opposed mainly by some conservative Christians in the United States, other scientists were not so reticent. They said this should undercut the creationists' argument that there is no evidence in the fossil record of one kind of creature becoming another kind.

One creationist Web site (emporium.turnpike.net/C/cs/evid1.htm) declares that "there are no transitional forms," adding: "For example, not a single fossil with part fins part feet has been found. And this is true between every major plant and animal kind."