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Revealed: How the fish got its fingers By Steve Connor, Science Editor 02 April 2004 A two-lane highway in America has helped scientists to explain one of life's most enduring mysteries: how fish grew the fingers that enabled them to crawl out on to land. The road in Pennsylvania happened to be cut out of 365 million-year-old rock in which the researchers found the oldest known fossilised arm bone of one of the world's first four-legged creatures, or tetrapods. Specialists said that the bone - a prototype humerus - can help to explain how ancient fish with primitive lungs grew rudimentary limbs from their fins which allowed them to make the first tentative steps towards a terrestrial life. The discovery, in the journal Science , suggests the first tetrapod limbs were used to prop up the heads of air-breathing fish and only afterwards did the fish begin to use these legs to clamber out of water. The fossilised bone is thick and flat and would not have allowed much movement between the limb and the shoulder, indicating that it was of little use other than to prop up the creature's front half. Michael Coates, of the University of Chicago, said: "When this humerus is compared to those of closely related fish, it becomes clear that the ability to prop the body is more ancient than we previously thought. This bone is a lot more robust than a humerus from any of the ancient species. Relative to other tetrapods, this is almost over-engineered. There's a massive space for the attachment of substantial muscle going to the chest. "This means that many of the features we thought evolved to enable life on land originally evolved in fish living in aquatic ecosystems." The tetrapod creature probably lived in shallow, muddy water during the late, or Upper Devonian period between 360 million and 370 million years ago when the exploration of dry land by vertebrates had yet to start. At that time, the land was largely the domain of insects, rather than animals with backbones, and the water was occupied by fish with bony skeletons and fins with hand-like bone structures that were to evolve into vertebrate limbs. Dr Coates and his colleagues Neil Shubin and Edward Daeschler believe the fossilised bone found in Pennsylvania helped the forelimb fulfil an intermediate function between the braking and steering of a fish's fin and the walking movements of an early amphibian. Drs Daeschler and Shubin found the fossil in 1993 when they were excavating near the highway but it took nearly eight years to discover it was important. "We found a number of interesting fossils at the site," Dr Daeschler said. "But the significance of this specimen went unnoticed for several years because only a small portion of the bone was exposed and most of it lay encased in a brick of red sandstone." After the bone was fully excavated, its true significance quickly became apparent, Dr Shubin said. "We knew it was a humerus, but it was an entirely different kind. We had never seen one like it before. It's a mosaic of primitive fish and derived amphibian." Jenny Clack, a vertebrate palaeontologist at Cambridge University, said the primitive forelimb could have propped the creature's head out of water to allow it to breathe air, or it could have been used to anchor the animal in fast-moving water, Dr Clack said. "It begins to fill in the picture about what we think about the transition. The difference between fish and these early tetrapods is becoming increasingly blurred." The same palaeontology site in Pennsylvania has yielded two other types of tetrapod living in the Devonian period, Dr Clack said. "If this is really a third form, it hints at a wide diversity of tetrapods existing in close proximity, in what is emerging as one of the richest and most varied of any late Devonian vertebrate site," she added. The scientists who have excavated the Pennsylvania site said it contains fossils of other plants and animals that suggest the area was "teeming" with life more than 360 million years ago. _______________________________________________ http://www.mccmedia.com/mailman/listinfo/brin-l
