It looked like an otter, spent part of its time on land and part of it in the water, and lived 24 million years ago.

That's the short dossier for Puijila darwini, a fossil discovered in the far northern reaches of Canada by Mary Dawson, curator emeritus of the Carnegie Museum of Natural History, and Natalia Rybczynski, a research scientist at the Canadian Museum of Nature in Ottawa.

The creature, first described last month in the journal Nature, has been called a "missing link" between earlier land mammals and their modern-day aquatic descendants, the seals, sea lions and walruses.

That makes Puijila an important addition to the story of life. It also puts it squarely in the middle of one of the longest-running debates between proponents and opponents of Darwinian evolution.

A basic tenet of evolution is that different species have descended from each other -- birds from dinosaurs; dogs from wolves; and, most controversially for some, humans from apes.

Much of the evidence for this has come from fossils, the hardened remains of animals and plants that have lived over the past several million years. But the fossil record of one species changing into another is not complete, and opponents of evolution have argued that those missing links are signs of a fatal flaw in evolutionary theory.

Most of those opponents are creationists or their latter-day cousins, the intelligent-design advocates, who base many of their ideas on biblical accounts of the origin of life or the idea that some supernatural being has been involved in the organization and diversity of life.

"Creation scientists state that evolutionists have had over 140 years to find a transitional fossil and nothing approaching a conclusive transitional form has ever been found and that only a handful of highly doubtful examples of transitional fossils exist," says an entry in the CreationWiki, a Web site devoted to creationist positions.

The trouble with that argument is that it's simply not true, says David Lampe, an associate professor of biological sciences at Duquesne University and one of the organizers of "Darwin at 200," a lecture series that celebrated the 200th anniversary of Charles Darwin's birth.

"The fossil record is remarkably complete," Dr. Lampe said, "and there are many transitional fossils that link later life forms to earlier ones." There are fossils of human ancestors, horse progenitors, creatures that bridged reptiles and mammals, and mammals that were predecessors of whales, among others, he said.

"It's just willful ignorance that people don't know about this," Dr. Lampe said.

Dr. Dawson, who helped uncover Puijila on Devon Island in Canada, said there are plenty of geological and environmental reasons why the fossil record has gaps in it.

First, she said, "in order to become a fossil you have to be buried," and if an animal died in an area undergoing erosion, it likely would be eaten or would decompose before it could become fossilized. Animals that died in tropical regions not only decomposed more quickly, she added, but the acid in the surrounding vegetation tended to dissolve the bones.

Another misconception people have is that paleontologists dig through the earth like archaeologists do, she said, but "we don't dig, we look." And that means parts of the globe that are covered with glacial deposits, for instance, may contain fossils that no one has ever seen.

She also isn't claiming that Puijila itself is a direct ancestor of seals, seal lions and walruses, because an early member of that Pinniped family called Enaliarctos seems to have lived around the same time as Puijila.

Instead, she said it was probably a descendant of the creatures that preceded the Pinnipeds, a so-called relict, similar to today's opossum, which "would be perfectly comfortable living 40 million years ago."

Puijila's skeleton and the cool, forested environment it would have lived in suggest it spent most of its time on land, she said, but its flattened paw bones and its muscular attachments showed it probably paddled through the water with webbed feet hunting for food.

The fossilized remains of its last meal showed a repast of either duck or swan, she said.

One reason it's often difficult to find transitional fossils such as Puijila, she added, is that "evolutionary pressures would be fairly high" for such creatures to turn into one type of animal or another, "and that means when you consider the whole fossil record, these things are not around for a long length of time, so the chance of finding transitional creatures like this one are therefore reduced."

Dr. Dawson and her colleagues found the fossil after their field vehicle ran out of gas and they decided to poke around in an ancient crater lake while they were stranded.

Not all such finds are so coincidental, though.

Neil Shubin, an organismal biology professor at the University of Chicago, is one of the people who in 2006 discovered a famous "missing link" between fish and four-legged amphibians that they called Tiktaalik.

Because the basic theory of evolutionary descent is reliable, Dr. Shubin said, his team was able to find this fossil "because we deliberately sought out 375-million-year-old rocks in the Canadian Rockies to find a flat-headed fish."

Yet there are still some important missing links in the fossil record, and geology and climate may not be the only culprits, says Jeffrey Schwartz, an anthropologist at the University of Pittsburgh and an expert in humanoid fossils.

Dr. Schwartz believes it's possible that many new species have formed without prior transitional creatures because of the way genetic changes can take place.

When environmental stresses such as changes in temperature or moisture occurred in past eons, he said, some creatures may have undergone recessive genetic mutations that did not show up for many generations, until there were enough creatures with the mutation to breed with each other and produce a brand new species.

In those cases, he said, the new species would not have a transitional form that linked it to its ancestors.

Standard Darwinian thinking does not embrace this idea, he said, and that ends up giving creationist critics ammunition to make non-scientific arguments. "I believe that a lot of the visibility and attacks of the creationists get momentum from the fact that the modern synthesis of evolution has gotten rid of all other scientific voices."

Duquesne's Dr. Lampe said, however, that modern adherents of evolution are willing to acknowledge that new species can form "at a much more rapid rate than Darwin believed" and thus might never leave behind a transitional fossil.

One good example of that, he said, is the way that cichlid fish in the large lakes of Africa have evolved rapidly in recent times into many different sizes, colors and dietary preferences.

The problem he has with most creationist or intelligent-design arguments, Dr. Lampe said, is that "they're really good at negative nitpicking, but they're not good at producing a positive hypothesis, except to argue that there has been some kind of direct intervention by God" in the creation of life's diversity.