Marine fish 380 million years ago had developed arms and ears to conquer land

Oct 19, 2006 11:16 GMT  ·  By

Studying a spectacularly preserved fossil of a Devonian lobe finned fish (Sarcopterygii) named Gogonasus (image above), scientists discovered some unexpectedly advanced traits, common to early terrestrial vertebrates.

The evolutionary chain of transition from water to land was extensively studied, but many of the main fossils are incomplete. Gogonasus' remains are one of the most complete Devonian fishes yet discovered. "We've got a fish from the Devonian period ("The Age of Fishes") about 380 million years ago and preserved in three-dimensional stunning perfection," said John Long of Australia's Museum Victoria.

When the fossil was unearthed, its jaw could still be opened and closed. "It's like it died yesterday," Long said. "The perfectly preserved skeleton has revealed that fish developed features characteristic of land animals much earlier than once thought," he continued.

Gogonasus fossils were discovered last year in a limestone deposit in Kimberley Area, Western Australia, at a site of a former coral reef. With a computed tomography machine, three-dimensional images of the skeleton were created.

The middle ear and limbs of Gogonasus resemble those of land vertebrates and this fish shed light about transition between fish and four-legged land vertebrates. Its muscular front fins showed the beginnings of a wrist joint and well-formed humerus, ulna and radius, the same bones found in the arm of all terrestrial vertebrates, including human.

Gogonasus' skull had a large breathing hole through the top of the head. The cavity leads down into the gill chamber used for breathing and is thought to be the forerunner for nasal cavity. In the middle ear, Gogonasus possessed the same bones like amphibians on land. "The specimen, discovered in Western Australia last year, was fishlike in many respects, but features of its ear and limbs are surprisingly tetrapod-like," Long said.

"The degree to which these features resemble the earliest four-legged land animals makes Gogonasus a new model in the picture of how fishes evolved into land animals.

"Gogonasus is the world's first complete perfect skeleton of the kinds of fishes that gave rise to the first land animals."

"The transition from a fish living in water to an air-breathing land animal with arms and legs was one of the most dramatic transitions in the history of evolution and many unsolved questions remained," Long said.

This fish must have been an ambush predator, about 12 inches long, roaming those ancient reefs. Probably Gogonasus used the front fins to push off the sea bottom and dash at the prey. "So it could've rested on those fins and then just pushed off. I think rather than walking on land it was using this to push itself out of the reef to catch prey," Long said.

"The fins needed to be strong and muscular, because the thrust was coming from the front of the animal and not from the tail."

The Australian team thinks that Gogonasus is more related to tetrapods than Eusthenopteron, a fish with tetrapod features, a kind of ancestor, still in a marine stage (tetrapods are thought to have evolved from freshwater sarcopterygii). Gogonasus' front limbs are less-advanced than those of Tiktaalik, the most amphibian-like fish, discovered recently.

Tiktaalik roseae, a 375 million-year-old species, is seen as the missing link in passing from water to land animals. Tiktaalik's skull was identical to an amphibian, but Gogonasus had a very fish-like skull. "Gogonasus is more like an ordinary fish except its hiding very advanced features that you would not otherwise see unless you had a perfect three-dimensional specimen," Long said.

"I like to say it's a wolf in sheep's clothing. It's showing that evolution isn't as straightforward as we'd like to think."

The only living relatives of Gogonasus are Latimeria (photo bellow), two species of living fossils discovered late, during the XX century on the Indian Ocean. But Latimeria is not from the lineage that led to terrestrial vertebrates.

Image credit: Victoria Museum Photo credit: AFP

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