Land animals require more energy since they have to overcome greater inertia during locomotion. Land mammals have adopted many evolutionary mechanisms to support high energy demand.
Heart is four chambered instead of the usual two in aquatic animals like fishes. More number of divisions in heart increases the efficiency of blood circulation process which in turn increases the oxygen delivery rate to cells for producing more energy.
Land mammals also have developed lungs which increase the efficiency of gas exchange process which increases the output of respiration process.
In addition, land mammals also have a strong and developed musculoskeletal system which helps in locomotion.
scientific conclusions are reliable as they are helpful for many things however it is true that after the evolution of new ideas the old theory have some changes or may change fully or may be proved wrong but yeah the old theories are still helpful for many reasons and things.
