Australia: The Land Where Time Began

A biography of the Australian continent 

Origin of Lungs

Prior to the arrival of the osteichthyans no fish had a swim bladder. Once fish had a swim bladder they were released from the necessity of living on or near the bottom of the sea, lake or river, and no longer needed broad pectoral fins for generating lift. This allowed them to evolve pectoral, and other fins, that were more suitable for high manoeuvrability and sudden braking. Because the shape of fish changed with the addition of the swim bladder, it is possible to see in the fossil record where the swim bladder became effective, without the need for fossils to be so finely detailed that internal organs could be discerned. In this way it is possible to see that the swim bladder dates back to the Late Silurian.

The evolutionary step from swim bladder to lung is not a great morphological leap. In its role as a flotation device it was designed to hold a gas. With the addition of folds and increased vascularisation it became suitable for gas exchange.

Oxygenated blood flows to the swim bladder in most osteichthyans, the blood coming by the arteries from the posterior efferent brachial arteries, or directly from the dorsal aorta. In Protopterus, the partially oxygenated blood to the lung is from is from the ventral and dorsal aortae and pulmonary arteries. Advanced development of musculature is also required to get air into the lung. Most fish swallow air that moves to the lung via a pneumatic duct.

The mudskippers are a type of modern fish that can use their swim bladder to take in air and hold it for a short time. Other fish use the swim bladder to send or receive sound.

It is believed that the change from swim bladder to lung occurred at least twice during osteichthyan evolution, once in the group of crossopterygians that led to the quadrupeds - early amphibians, the Osteolepiformes, and once in the lungfish. The evidence for this is found in the presence in fossil lungfish of cranial ribs. Based on these cranial ribs it seems the gulping of air, so the acquisition of lungs, occurred as the ancestral lungfish moved into freshwater from the sea.

All primitive lungfish apparently lack cranial ribs. The cranial ribs of modern lungfish anchor the pectoral girdle during air-gulping, showing how the development of cranial ribs of lungfish is tied to the development of air-gulping in the ancestral fish.

It is uncertain when crossopterygians first developed the ability to breath air. The presence of an internal palatal nostril (choana) is believed to be the first evidence of air breathing in fish and amphibians. If this is the case, then osteolepiforms began occasional air breathing at the beginning of the Middle Devonian, around the same time that lungfish acquired the capacity to gulp air. As with the lungfish, the crossopterygians began air breathing when the moved to freshwater.

Evidence of a possible marine origin and cause for the development of air-breathing

It has been found that the atmospheric oxygen dropped to about 12 % (now about 20 %) at around the time the air-gulping fish evolved, about 375 million years ago. In a paper published in Biology Letters it is proposed that the development of air breathing may have been the result of the oxygen drop forcing fish to adapt to low oxygen levels in the water. It also apparently occurred in a marine fish, Rhinodipterus, from the Gogo Formation in Western Australia. It has previously been believed that the adaptation to air gulping occurred in freshwater environments.

Rhinodipterus image 

Sources & Further reading

  1. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995
Author: M. H. Monroe
Last Updated 03/01/2011 



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