An international team of researchers found that tetrapods, the earliest known limbed vertebrates, primarily fed underwater and not on land. The four-legged limbed vertebrates, Acanthostega, evolved from fish and include today's amphibians, reptiles, birds and mammals.

Scientists from the University of Lincoln, University of Zurich, University of Cambridge and University of Bristol, studied the feeding mechanism of Acanthostega, one of the earliest and most ancient tetrapod.

The fossilized fragments of this species including its jaws were discovered in Greenland in 1987. Acanthostega belonged to the late Devonian period (416-360 Million Years Ago) and has often been regarded as a fish-tetrapod intermediate.

When the researchers closely examined the movement and structure of the lower jaws of Acanthostega and several other early tetrapods and tetrapod-like fish, they found that the jaws of the Acanthostega were more suited for hunting food underwater. This suggests that this tetrapod primarily maintained an aquatic lifestyle.

Acanthostega displayed both primitive and fish-like characteristics and tetrapod features like fingers and toes.

"Its broad snout appears to be consistent with aquatic feeding habits (suction feeding) but its complex cranial joints appear to be similar to those of terrestrial vertebrates and would suggest direct biting on land environments as a means of prey capture. This paradox prompted our stud," Dr Marcello Ruta from the University of Lincoln's School of Life Sciences, said in a statement.

Ruta also said that that the lower jaw of Acanthostega was anatomically and functionally similar to the jaws of early fish and 'contemporary fish-tetrapod intermediates.'

Dr Emily Rayfield from the University of Bristol's School of Earth Sciences said that although these animals were the earliest to evolve limbs with digits, they preferred feeding underwater rather than moving to the land to catch their prey.

The study was documented in the Proceedings of the Royal Society B.