Importantly, they also had a pair of ventral paired lungs, a feature lacking in sharks and rays.
The lung/swim bladder originated as an outgrowth of the gut, forming a gas-filled bladder above the digestive system. The heavy scale armour of the early bony fishes would certainly weigh the animals down.
The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear, and are areas of active research and debate among palaeontologists at present.
Most amphibians today remain semiaquatic, living the first stage of their lives as fish-like tadpoles.
They did, however, have certain traits separating them from cartilaginous fishes, traits that would become pivotal in the evolution of terrestrial forms.
With the exception of a pair of spiracles, the gills did not open singly to the exterior as they do in sharks; rather, they were encased in a gill chamber stiffened by membrane bones and covered by a bony operculum, with a single opening to the exterior.
The third mechanism for a breath is the actual motion of the breath.
The internal nares could be one set of the external ones (usually presumed to be the posterior pair) that have migrated into the mouth, or the internal pair could be a newly evolved structure.In its primitive form, the air bladder was open to the alimentary canal, a condition called physostome and still found in many fish. In cartilaginous fishes, lacking a swim bladder, the open sea sharks need to swim constantly to avoid sinking into the depths, the pectoral fins providing lift.