Bowfin

The bowfin ("Amia calva") is a primitive bony fish. They live in North America. 
It is a relic species, last of a group known as the Halecomorphi. That group appeared the Lower Triassic, about 250 million years ago. 
The bowfin is described as "primitive" because they have characteristics of their early ancestors. The closest living relatives of bowfins are gars: the two groups make up the clade Holostei.
Their physiology is quite interesting. Like gars, bowfin are "bimodal breathers": they can to breathe both water and air. Their gills exchange gases in the water, and they also have a gas bladder. The gas bladder is the secret: it keeps buoyancy, and lets the fish breathe air by a small pneumatic duct which goes from the foregut to the gas bladder. They break the surface to gulp air. This lets them survive conditions of hypoxia which would be lethal to most species.