The evolution of the terrestrial vertebrates: environmental and physiological considerations.

Physiological evidence has long been used to suggest that the gnathostomous vertebrates (those possessing jaws) were primitively fresh water. The same was also the case for the Osteichthyes (bony fish) and the Tetrapoda (Amphibia, Reptilia, Aves, Mammalia). However, the geological evidence favours a marine origin for the vertebrates as a whole, and, for the gnathostomes and the osteichthyans in particular. Some of the earliest amphibian remains may be associated with tidally influenced sediments. Furthermore, during the early part of the Devonian, fresh water chemistry may well have been different from that of today, lessening the divide between marine and non-marine environments. Urea formation via the ornithine cycle, and urea retention in the body fluids, are useful adaptations for terrestrial life. They prevent excessive water loss associated with the elimination of nitrogenous waste. These abilities may have been primitive for the gnathostomes, and were developed in the marine environment to reduce osmotic dehydration. In the aqueous medium, gaseous exchange is effected by the gills. These organs are, on the whole, useless in air. For vertebrates, air-breathing is effected by an inflatable sac, with moist linings, and an internal location. Some form of air-breathing sac was primitive for the osteichthyans, and may have been primitive for the gnathostomes. Again, this adaptation for terrestrial life developed in response to conditions experienced in the marine, aquatic environment. A new model of tetrapod evolution is proposed in the light of the basic marine origin and character of the ancestors of the tetrapods.