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Ancient Fish Fossil May Rewrite Story of Animal Evolution

John Roach
for National Geographic News
October 18, 2006
 
A fish that swam on an ancient barrier reef in Australia 380 million years ago had fins and nostrils remarkably similar to the limbs and ears of the first four-limbed creatures to walk on land, according to a new study.

Four-limbed land animals, also known as tetrapods, such as modern amphibians, reptiles, birds, and mammals, evolved from lobed-finned fish.

That transition from water to land has long fascinated scientists, but the fossil record of how it occurred is still incomplete.

The new finding suggests that certain aspects of tetrapod ears and limbs can be traced much further back in "fishy looking" fish than had been previously known, says John Long, head of sciences at Museum Victoria in Melbourne, Australia.

"They were just cunningly disguised in the fossil record by their more fishlike overall features," he said in an email interview.

"They tell us that evolution progresses steadily but often hides the evidence until a really well preserved fossil like this turns up."

Long and colleagues report their findings in tomorrow's issue of the science journal Nature.

Fishy Fish

The team analyzed a remarkably well preserved fossil specimen of a fish called Gogonasus.

Previously the lobed-finned fish—which was about 12 inches (30 centimeters) long and covered in diamond-shaped scales—was known only from crushed or fragmented fossil remains.

But the new specimen, discovered last year in Western Australia, is nearly complete, with an almost intact skull, body, and fin bones (map of Australia).

Among the most remarkable features are slits on the roof of the skull called spiracles that scientists had previously seen only in creatures more closely related to tetrapods, such as the "missing link" fish Tiktaalik. (Related story: "Fossil Fish With 'Limbs' Is Missing Link" [April 5, 2006].)

Tiktaalik, the closest known fish ancestor to land tetrapods, had fins with armlike skeletal structures, a head that moved independently of its body, and spiracles on the top of its head.

"This Gogonasus fish shows similar adaptation for air breathing that we see in creatures much closer to tetrapods," said Jennifer Clack, a paleontologist and tetrapod expert at the University of Cambridge in England. Clack was not involved in the new study.

In more advanced tetrapods, spiracle structures became ears, Clack adds.

Long and colleagues also describe Gogonasus' fins in detail for the first time, showing they are stout and robust like those in early tetrapods.

"In simple terms, Gogonasus is a missing link between fishes that look like fish and the more amphibian-like elpistosteglians [tetrapod-like fishes such as Tiktaalik]," Long said.

Marine Dispersal

Prior to the discovery of Gogonasus, tetrapod-like fish were known mostly from the Northern Hemisphere, raising the question of how tetrapods got to the Southern Hemisphere.

"The marine environment of Gogonasus means that tetrapod-like fishes and tetrapods probably had more marine dispersal ability than previously thought," Long said.

The early tetrapod-like fish could have swum around the world, Cambridge's Clack adds.

However, Gogonasus may have evolved its tetrapod-like features independently of the first fishes to crawl out of water onto land, she says.

"It's possible the features we are seeing in Gogonasus—the air breathing and limb characters—are in fact evolved in parallel, in fact have nothing to do with those in tetrapods. They perhaps derived similar mechanisms quite independently," she said.

"We'll need a lot more work to sort that one out."

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