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Ancestor of the “living fossil” sheds new light

Aug. 1, 2007
Special to  World Science  

Zo­ol­o­gists called it the find of the cen­tu­ry when in 1938, fish­er­men hoisted ashore a fish thought to have been ex­tinct since the di­no­saurs roamed. Called a coe­la­canth, it was a rel­a­tive of some of the first land-walk­ing crea­tures.

A present-day coelacanth. The fish live in caves. (Courtesy U. Chicago)

Yet the fos­sil fin is­n’t as si­m­i­lar to mod­ern fleshy-finned fish as it is to some prim­i­tive mem­bers of the oth­er great line­age of bony fish­es—the ray-finned fish­es, Fried­man and col­leagues said. These are the larg­est class of fish, and those whose fins are webs of skin sup­ported by spines. Some liv­ing ray-finned fish­es such as paddlefish­es and stur­geons have a branch­ing ar­range­ment of bones si­m­i­lar to that found in the coe­la­canth fos­sil, Fried­man and col­leagues said.

“This ends in­tense de­bate about the prim­i­tive pat­tern for lobed fins, which in­volves the an­ces­try of all limbs, in­clud­ing our own,” said the un­ivers­ity’s Mi­chael Coates, one the re­search­ers. “To un­der­stand the de­vel­op­men­tal ev­o­lu­tion of the limbs of tetrapods [four-limbed ver­te­brates], we should­n’t be look­ing at the fins of our near­est liv­ing fish rel­a­tives—lungfish­es and coe­la­canths—be­cause they’re far too spe­cial­ized.”

Sci­en­tists be­lieve anoth­er re­cently dis­cov­ered fos­sil is a true mis­sing link be­tween fish and tetrapods. It was a fierce pred­a­tor dubbed Tik­taa­lik roseae, which lived 385 mil­lion years ago.

The early coe­la­canth fin fos­sil shows that as far as limbs go, the key dif­fer­ence sep­a­rat­ing early fleshy-finned fish­es and Tik­taa­lik was in fin bones called ra­di­als, Fried­man and col­leagues wrote. These are widely thought to have evolved in­to fin­gers.

The fos­sil coe­la­canth is named Sho­sho­nia arc­topteryx af­ter the Sho­sho­ni peo­ple of Wy­o­ming and the Sho­sho­ne Na­tional For­est in north­ern Wy­o­ming, where the spec­i­men was found. “It was as­ton­ish­ing luck,” Fried­man said, adding that the fos­sil had fall­en off a cliff about 200 feet high on­to some rocks. The four-inch (10 cm) long spec­i­men de­tails the fin of the an­i­mal, which the sci­en­tists ap­prox­i­mate would have been about 18 to 24 inches (45 to 60 cm) long.

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Zoologists called it the find of the century when in 1938, fishermen hoisted ashore a fish thought to have been extinct since the dinosaurs roamed. Called a coelacanth, it was a relative of some of the first land-walking creatures. Now, scientists are reporting a fossil find that helps complete the story: the front fin of an early coelacanth, which is quite different from that of today’s coelacanths. It sheds light on the evolution of this crucial structure, which in the fish’s descendants evolved into walking limbs and later arms, scientists say. The fossil reveals connections to even more primitve fish, and shows that the fin bones still had to evolve a fair amount before the first walking creatures arose, according to the scientists. They described the finding in a paper in the July/August issue of the research journal Evolution & Development. People usually think of coelacanths as “living fossils,” but that’s something of a misconception, said Matt Friedman, a graduate student at the University of Chicago and lead author of the paper. “If you look deep in the fossil record to the first members of that group, they are really different and very diverse.” Same goes for some other so-called living fossils, he added. The 400 million-year-old coelacanth fossil is the first known of its kind, and fills a shrinking evolutionary gap between fins and limbs, the researchers said. Scientists are interested in early coelacanths because they’re close relatives of the first so-called fleshy-finned fishes. This is the lineage that, with their meaty fins, gave rise to limbed animals, which took the first steps onto land. Yet the fossil fin isn’t as similar to modern fleshy-finned fish as it is to some primitive members of the other great lineage of bony fishes—the ray-finned fishes, Friedman and colleagues said. These are the largest class of fish, and those whose fins are webs of skin supported by spines. Some living ray-finned fishes such as paddlefishes and sturgeons have a branching arrangement of bones similar to that found in the coelacanth fossil, Friedman and colleagues said. “This ends intense debate about the primitive pattern for lobed fins, which involves the ancestry of all limbs, including our own,” said the university’s Michael Coates, one the researchers. “To understand the developmental evolution of the limbs of tetrapods [four-limbed vertebrates], we shouldn’t be looking at the fins of our nearest living fish relatives—lungfishes and coelacanths—because they’re far too specialized.” Scientists believe another recently discovered fossil is a true missing link between fish and tetrapods. It was a fierce predator dubbed Tiktaalik roseae, which lived 385 million years ago. The early coelacanth fin fossil shows that as far as limbs go, the key difference separating early fleshy-finned fishes and Tiktaalik was in fin bones called radials, Friedman and colleagues wrote. These are widely thought to have evolved into fingers. The fossil coelacanth is named Shoshonia arctopteryx after the Shoshoni people of Wyoming and the Shoshone National Forest in northern Wyoming, where the specimen was found. “It was astonishing luck,” Friedman said, adding that the fossil had fallen off a cliff about 200 feet high onto some rocks. The four-inch (10 cm) long specimen details the fin of the animal, which the scientists approximate would have been about 18 to 24 inches (45 to 60 cm) long.