London, Dec 17 (IANS): Our jawed ancestors were not responsible for the demise of their jawless cousins as had been assumed, says a research.

The blame could be shifted to rising sea levels, the researchers said.

"When our jawed vertebrate ancestors overtook their jawless relatives 400 million years ago, it seems that it might not have been through direct competition but instead the inability of our jawless cousins to adapt to changing environmental conditions," said Robert Sansom from University of Manchester.

The researchers studied the patterns of diversity of fossil jawless fish.

These boney fish with a tank like construction (ostracoderms) were dominant and diverse in ancient seas.

The team found that patterns of ostracoderm diversity were correlated with changing environmental and geological conditions, the fish were strongly reliant on the availability of shallow water seas and ecosystems.

"Our research suggests the dependence of these armoured fish on shallow environments is likely to be a factor behind their demise and eventual extinction in the Devonian period when sea levels rose," Sansom added.

The findings also suggest that the jawless fish could have existed earlier than previously thought.

The study was published in the journal Proceedings of the Royal Society B.