When fish crossed the bridge to become land animals

Washington, June 26 : Swedish researchers have used new exquisitely preserved fossils from Latvia to cast light on when our ancestors left the water and ventured onto land, which is a key event in our evolutionary history.

The research was done by Per Ahlberg and Henning Blom from Uppsala University, who reconstructed parts of the fossilized animal and explain the transformation.

It has long been known that the first backboned land animals or "tetrapods" - the ancestors of amphibians, reptiles, birds and mammals, including ourselves - evolved from a group of fishes about 370 million years ago during the Devonian period.

However, even though scientists had discovered fossils of tetrapod-like fishes and fish-like tetrapods from this period, these were still rather different from each other and did not give a complete picture of the intermediate steps in the transition.

In 2006, the situation changed dramatically with the discovery of an almost perfectly intermediate fish-tetrapod, Tiktaalik, but even so a gap remained between this animal and the earliest true tetrapods (animals with limbs rather than paired fins).

Now, new fossils of the extremely primitive tetrapod Ventastega from the Devonian of Latvia cast light on this key phase of the transition.

"Ventastega was first described from fragmentary material in 1994; since then, excavations have produced lots of new superbly preserved fossils, allowing us to reconstruct the whole head, shoulder girdle and part of the pelvis", said Professor Per Ahlberg at the Department of Physiology and Developmental Biology, Uppsala University.

The recontructions made by Professor Ahlberg and Assistant Professor Henning Blom together with British and Latvian colleagues show that Ventastega was more fish-like than any of its contemporaries, such as Acanthostega.

The shape of its skull, and the pattern of teeth in its jaws, are neatly intermediate between those of Tiktaalik and Acanthostega.

"However, the shoulder girdle and pelvis are almost identical to those of Acanthostega, and the shoulder girdle is quite different from that of Tiktaalik, suggesting that the transformation from paired fins to limbs had already occurred," said Per Ahlberg.

"It appears that different parts of the body evolved at different speeds during the transition from water to land", he added.

ANI