Reverend AtheiStar Posted August 8, 2007 Share Posted August 8, 2007 http://www.world-science.net/othernews/070801_coelacanth.htm 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. 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 clarifies the evolution of this crucial structure, they say, which in fish descendants evolved into walking limbs and then arms. 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 often see coelacanths as “living fossils,” but that’s not quite accurate, 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 vertebrates that 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. Link to comment Share on other sites More sharing options...
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