Lack of oxygen forced fishes' 1st breathCBC: A global drop in oxygen levels may have been the driver that led ancient fish to leave the water and evolve into the first air-breathing animals on land... Doctoral student Alice Clement, from the Australian National University's Research School of Earth Sciences, and Museum Victoria researcher Prof. John Long... make the claim based on the fossilized remains of a fish that lived about 375 million years ago... known as Rhinodipterus...
A number of features found in modern lungfish that are important to its air-gulping behaviour were found in the fossil. These included a long mouth cavity and cranial ribs attached to the base of the skull... The longer mouth cavity enables them to hold a bubble of air in their mouths, while the cranial ribs anchor the pectoral girdle during air gulping.
Yet while modern lungfish exist in freshwater environments, The Rhinodipterus lived in the ocean. 'This runs counter to the standard theory that fish evolved the ability to breathe air once they moved to freshwater habitats,'... said Clement.
The researchers suggest low global oxygen levels during this period, known as the Devonian, may explain the evolution of air-gulping characteristics. Previous studies have shown oxygen levels fell as low as 12 per cent of the total atmosphere. Today, global oxygen levels are about 20 per cent...
'This makes us believe that breathing air arose twice at this early time in vertebrate evolution: once in lungfishes and once in the fish lineage leading to land animals, and ultimately to us.' [Long] said.
