BOOK REVIEW : Scientist’s Tale Is No Fish Story : LIVING FOSSIL: The Story of the Coelacanth, <i> by Keith Stewart Thomson</i> , W. W. Norton, $19.95, 252 pages

In December, 1938, young Marjorie Courtney-Latimer received a call at the science museum office in the coastal town of New London, South Africa, from a trawler captain reporting an interesting catch.

At the dock was the most extraordinary fish she’d ever seen. Almost 5 feet long and weighing 127 pounds, it was pale mauve-blue with iridescent silver markings. Boasting two dorsal fins and a lobed tail, it had paired fins that “flopped in every direction, for all the world like legs.”

Courtney-Latimer sent a quick sketch off to J. L. B. Smith, Africa’s foremost ichthyologist. Smith thought he recognized a coelacanth (pronounced SEEL-uh-kanth), a fish believed to have been extinct for about 65 million years.

So begins Keith Stewart Thomson’s “Living Fossil,” an engrossing tale of obsession, adventure and scientific reasoning. Head of the Academy of Natural Sciences in Philadelphia, Thomson is, happily, an expert on coelacanths and a storyteller.

We learn a lot about Smith, his wife and his examination of the coelacanth. Appetite whetted, Smith wanted very much to find another, and so he distributed posters with a picture offering a bounty of 100 pounds.

Smith had to wait until 1952 before a second coelacanth was hauled out of the Indian Ocean 1,500 miles away, off the Comoro Islands.

Smith managed to take it back to South Africa, but the Comoros were not British and opportunities for research were now wide open. With independence, the islanders increased their catch and their capital by selling dead coelacanths to museums all over the world.

It is not until halfway through his book that Thomson introduces himself, explaining that in 1965, with a new Ph.D. from Harvard in paleontology and a postdoctoral fellowship working with fossil fish at London’s University College, Yale offered him a job.

He was at Yale’s Peabody Museum when the first coelacanth arrived to such publicity that the crowds reminded Thomson of the faithful passing Lenin’s bier. They came to see a “living fossil,” perhaps the missing link between fish and land-dwelling amphibians.

In a lucid chapter, Thomson explains the apparent oxymoron. A “living fossil” is the representative of an ancient group of organisms that have somehow survived. Other examples Thomson gives are the horseshoe crab and the redwood tree. Thomson puzzles over the very existence of these species: “Does the survival of living fossils represent the operation of some common factor, or does each species survive for a unique reason?”

Whatever the answer, living fossils have enormous scientific value. Thomson explains how “biology is the quintessential comparative science. Because of the evolutionary thread that links all organisms in one great network of genetic relationships.” Those floppy fins that reminded Courtney-Latimer of legs may reveal how tetrapods--four-legged creatures--arose from fishes to conquer land.

Thomson explains how evolutionary biologists reason from hypothesis and comparison. For example, he explored the eyes of a dead coelacanth, but because its eyes are so like those in some living fish, and those living fish live in a special light zone, he can argue that coelacanth are also used to living in a certain kind of light.

During the last 20 years, researchers in a submersible ship have filmed a live coelacanth swimming, and other researchers have explored the function of the mysterious rostral organ in the brain.

The riddle of its reproductive life was solved after the specimen acquired by the American Museum of Natural History was dissected, revealing five fetuses with yolk sacs attached, indicating live birth and a gestation of at least a year.

In the light of this long gestation period and the increased number caught by fisherman, Thomson suspects that the coelacanth is endangered.

Are we even responsible, Thomson asks, for a species we didn’t know existed until 60 years ago, and are we obliged to save them? Of course, he replies.

It is rare to find such a satisfying book. Thomson knows how to spin a yarn and better yet how to engage the reader when it comes to solving puzzles by presenting possible solutions, and as if thinking aloud, working them through.

Next: Jonathan Kirsch reviews “Bob Dylan: Behind the Shades” by Clinton Heylin (Summit Books).