AGES after some adventurous (or misadventurous) fish left the sea and planted the flag of vertebrate animal life on land, their descendants had it both ways as amphibians and then completed the epic transition, evolving into terrestrial reptiles, mammals and birds. But something about the water must have kept beckoning, until a few irredentists among the mammals did eventually reclaim a place in the sea.

Most prominent of these mammals are the whales. Although they may swim the oceans with power and grace, these leviathans are more closely related to the camel and cow than any fish in their wake. Their anatomies retain vestiges of the four-legged land animals in their ancestry, the ones that began the bold return to the sea more than 50 million years ago.

As early as Aristotle, people recognized that more than size set whales apart from other marine life. Aristotle noted that whales as well as dolphins, the other members of the cetacean group, bear their young live, unlike fish. This made them, by definition, mammals. (It is now known that some fish do bear live young.) But even Darwin much later could not fathom the evolutionary steps by which some land mammals had become whales.

"It was a big evolutionary plunge, one that has eluded documentation for many decades," said Dr. Michael J. Novacek, a paleontologist who is dean of science at the American Museum of Natural History in New York City.

New fossil discoveries have now revealed several of the critical evolutionary steps in the earliest history of whales. Scientists have identified some intermediate species as land mammals steadily changed physical form while adapting to swimming, diving, feeding and otherwise thriving in their new habitat. One surprise is that the transformation of four-legged land mammals into an animal completely adapted to marine life took only 10 million years -- hardly any time at all in evolutionary terms.

"I am absolutely delighted to report that our usually recalcitrant fossil record has come through in exemplary fashion," Dr. Stephen Jay Gould, a Harvard University paleontologist and essayist, wrote in this month's issue of Natural History magazine. "The embarrassment of past absence has been replaced by a bounty of new evidence -- and by the sweetest series of transitional fossils an evolutionist could ever hope to find."

One discovery, which Dr. Gould called "a remarkable smoking gun," introduced scientists to an animal that lived 50 million years ago. It was about the size of a modern sea lion, weighing 600 to 700 pounds and measuring about 10 feet from snout to tail. Judging by fossil remains, the animal was an amphibious species clearly intermediate between a terrestrial ancestor of whales and aquatic modern whales.

This fossil animal, named Ambulocetus natans, which means swimming walking-whale, was excavated from sediments of an ancient seabed in Pakistan. Dr. J. G. M. Thewissen, an anatomist and paleobiologist at Northeastern Ohio Universities College of Medicine in Rootstown, Ohio, reported the discovery in January.

Ambulocetus was just what scientists would have expected to find. The animal still had four limbs for walking on land, though probably with diminished agility. It could also hunt in the sea, probably swimming by kicking its big feet. Find in Pakistan

Another discovery in Pakistan has now advanced the transition story three or four million years and out into deeper waters. Scientists reported last week that they had found fossils of an animal that had obviously made a more resolute commitment to a life at sea.

Writing in the current issue of the journal Nature, Dr. Philip D. Gingerich, a paleontologist at the University of Michigan at Ann Arbor, said the new species, Rodhocetus kasrani, was the earliest known transitional whale with an anatomy adapted for swimming like a whale. It had a more streamlined body and a fully flexible rear spinal column, which could have produced the motions for the powerful beat of a horizontal tail fluke that propels modern whales.

Whether Rodhocetus had indeed made this important advance cannot be determined until more complete tail fossils are uncovered. But the evidence, Dr. Gingerich and colleagues concluded, "shows that tail swimming evolved early in the history of cetaceans."

In a commentary accompanying the Nature report, Dr. Novacek said the Ambulocetus and Rhodocetus findings provided a "fascinating mosaic of early adaptive experiments in the transition from land to water."

Besides whales and dolphins, two other major types of mammals have made this dramatic transition: Sirenia, which includes dugongs and manatees, and the group composed of seals, sea lions and walruses.

The migration of whales to the sea is a classic example of transcendental evolutionary change, along with the first fishlike animals establishing their beachhead 370 million years ago, birds becoming flying animals and early human ancestors developing upright walking. But why would animals like whales, with all the apparatus for successful living on the land and ages of experience doing it, seek such a different habitat? Food as Lure Back to Sea