Ancient fish fossil provides missing link in evolution of land animals

The discovery of a 380 million-year-old fossil in a remote region of Western Australia has given scientists new insight into the process by which fish evolved into land animals.

The perfect skeleton of the Gogonasus fish was found preserved in limestone during an expedition organised by Melbourne's Museum Victoria.

"It looks like it died yesterday," said the expedition leader, John Long. "You can still open and close the mouth."

Dr Long said the fossil demonstrated that fish developed the anatomical features of four-legged land creatures, or tetrapods, much earlier than once thought. Other specimens from that era had been incomplete, or squashed flat. This one was complete and three-dimensional, enabling palaentologists to analyse it in greater detail.

Gogonasus, which swam in an ancient reef system before there was life on land, had a large hole in its skull and a fin strong enough to support its body weight. "It's definitely a fish," Dr Long told the Australian Broadcasting Corporation. "It's got gills, it swims in water, it's got fins. But it's a fish that is showing the beginnings of the tetrapod's advanced body plan that would eventually carry on to all living land animals."

The skeleton, which he called "the most perfect, complete, three-dimensional fish of its kind ever discovered in the whole world", was found in the rugged Kimberley region, in a fossil area known as the Gogo site. In 1985 Dr Long discovered a snout and skull fragment of the same species there. He named it Gogonasus, which means "snout from Gogo".

The latest specimen was found by Tim Senden, a member of the team that went on the expedition in July last year. Dr Senden, who was on his first field trip, told the Melbourne Herald-Sun that he did not initially appreciate its significance. "I was the lucky one that picked up the right rock," he said. "It was the thrill of a lifetime."

It took Dr Long four months to extract the fossil from the limestone. It was then analysed, using new software technology developed by Dr Senden, at the Australian National University in Canberra.

Dr Long, who reported the team's findings online yesterday in the journal Nature, said that analysis of the limited specimens previously available had suggested that Gogonasus had quite primitive features. But the new fossil - which has gone on display at Museum Victoria - demonstrated that it was "hiding a lot of deceptively advanced features that were not recognised before, until we had such a perfect specimen". Dr Long added: "This particular fish is a bit like a wolf in sheep's clothing."

The hole in the skull is believed to be an early version of the middle ear in land animals. Gogonasus's pectoral fin had the same bone pattern as tetrapod forelimbs. It also had a single pair of nostrils, like those of humans.

The transition from fish living and breathing in water to animals with arms and legs, living on land and breathing air, is a key stage in the history of evolution.

Dr Long believes that Gogonasus is more closely related to tetrapods than another fish, Eusthenopteron, which was considered the common ancestor of all land animals. "It's replaced Eusthenopteron as the best fish to use when studying the ancestry of the first tetrapods," he said.

Another ancient species, Tiktaalik, which was discovered this year, is the most amphibian-like fish identified so far. Dr Long said many questions remained , such as the manner in which fin rays evolved into digits.