Why it evolved

The pressure of scarce oxygen, particularly in stagnant pockets of the Tethys Shallows' lagoons, selects for individuals that can extract more oxygen from the water or utilize alternative methods. This descendant has developed a larger gill surface area and the ability for cutaneous respiration (oxygen absorption through the skin), especially on its enlarged pectoral fins. This allows it to survive periods of low oxygen but comes at the cost of increased energy expenditure for maintaining these larger respiratory surfaces and potential vulnerability to osmotic stress in the brackish water.