By Matt McGrath
BBC science correspondent
Scientists say a fossil of a four-legged fish sheds new light on the process of evolution.
The creature had a fish-like body but the head of an animal more suited to land than water.
The researchers' study, published in the journal Nature, says Ventastega curonica would have looked similar to a small alligator.
Scientists say the 365-million-year-old species eventually became an evolutionary dead end.
Counting digits
About one hundred million years before dinosaurs began to roam the Earth, Ventastega was to be found in the shallow waters and tidal estuaries of modern day Latvia.
According to lead author, Professor Per Ahlberg, from Uppsala University, Sweden, this creature had the head of a tetrapod, an animal adapted to live on land. The body, though, was fish-like but with four primitive flippers.
"From a distance, it would have looked like an alligator. But closer up, you would have noticed a real tail fin at the back end, a gill flap at the side of the head; also lines of pores snaking across head and body.
"In terms of construction, it had already undergone most of the changes from fish towards land animal, but in terms of lifestyle you are still looking at an animal that is habitually aquatic."
Experts believe that Ventastega was an important staging post in the evolutionary journey that led creatures from the sea to the land. Scientists once believed that these early amphibious animals descended in a linear fashion, but this discovery instead confirms these creatures diversified into different branches along the way.
Professor Ahlberg points to the discovery of a fossil called Tiktaalik in Canada in 2004. It is believed to be the "missing link" in the gap between fish and land mammals. Ventastega is a later species but is a more primitive form of transition animal.
"Ventastega fills the gap between Tiktaalik and the earliest land based mammals. All these changes in these creatures are not going in lockstep; it's a mosaic with different parts of animal evolving at different rates. Ventastega has acquired some of land-animal characteristics, but has not yet got some of the other ones."
For instance, the creature had primitive feet - but with a high number of digits.
Superb sands
"I would draw the inference that Ventastega probably had limbs very much like Acanthostega (another transitional species). These were little things sticking out of the sides, with a strangely high number of digits. You would have seven, eight, maybe even nine toes per foot, rather than five or so which you would expect to find in modern day animals," the Uppsala scientist explained.
Unfortunately for Ventastega, a multitude of toes does not inevitably lead to evolutionary success. It eventually died out. Other creatures went on to become our very distant land-living ancestors.
Scientists are delighted with the quality of these Latvian fossils, saying they are really well preserved. Professor Ahlberg believes it is due to some of the geological characteristics of the area.
"This region has had a very quiet geological history since that time, and as a result the rocks have not been folded or squashed up to form mountains.
"We still find sediments not yet properly turned to rock. These fossils were found in compact, wet sand. It's not sandstone, it's sand; you dig it with a breadknife.
"Once you take it back to the lab very carefully, you can remove the remainder of the sand with brushes and needles. These fossils are fragile but superbly preserved. They are actually three dimensional, not flat. It makes it very easy to interpret the skeleton."
(BBC)
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