Scientists have discovered that the coelacanth has a shrivelled, obsolete lung.
Present day specimens of the deep-sea fish, which was once thought to be extinct, use gills to extract oxygen from the water.
But scientists think that 400 million years ago, the ancestors of the coelacanth probably used the lung to breathe in low-oxygen, shallow waters and that the organ was rendered defunct by evolution.
Dr Paulo Brito of the Rio de Janeiro State University, who co-authored the research, said: "By the Mesozoic Era, adaptation of some coelacanths to deep marine water, an environment with very low variations of oxygen pressure, may have triggered the total loss of pulmonary respiration.
He added that this adaptation to deeper water could explain how it survived the extinction event 66 million years ago that wiped all non-avian dinosaurs and most other life from Earth — and probably also those gill- and lung-breathing coelacanths inhabiting shallow waters.
As the lung shrank in size and became useless, a fatty organ that the fish uses for buoyancy control in deep waters grew to take over the space once occupied by the lung.
Although researchers cannot say for sure whether the fatty organ ever existed in fossil forms, due to its unique soft-tissue constitution, it has a function in buoyancy control in present day lungfish.
Researchers used samples from infant and adult specimens of one of the two species of coelacanth, Latimeria chalumnae, and created 3D reconstructions of the developmental stages of the lung.
They discovered that the obsolete lung is proportionally much larger in the embryo than the adult, indicating that the growth of the organ slows as the fish gets older, eventually becoming a functionless organ.
They also noted the presence of small, flexible plates scattered around this vestigial lung in the adult specimens and suggest they are comparable to the 'calcified lung' of fossil coelacanths.
The evidence of 'calcified lungs' in the fossil record, as well as the developing lung early in the coelacanth’s embryonic development, means that the lung could possibly be a primitive characteristic in bony fish, Brito added.
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