In addition to the points raised by Gideon Butler regarding the behavior of recent marine reptiles, it is worth considering their evolution and the characteristics of prehistoric marine reptiles.

While present-day marine reptiles exhibit a preference for four limbs due to their specific lifestyle, it is important to note that there were prehistoric marine reptiles that were open ocean predators and swimmers. The Plesiosauria, for instance, were believed to be top marine predators during a certain period, and they possessed four limbs.

Therefore, the reason for the prevalence of four limbs in marine reptiles might be attributed to evolutionary constraints. Four-flippered reptiles offered certain advantages, especially for those with well-developed flippers. Transitioning to a two-flipper morphology would require a reduction in the rear flippers, resulting in a decrease in speed, agility, and overall reproductive fitness. To maintain their fitness, the development of a fluke would have been necessary before the reduction of rear flippers, but this transition would have required substantial energy expenditure without providing any advantage during the intermediate stages. In short, such a change without evolutionary advantage would not occur. On the other hand, in marine mammals, the development of flippers and flukes began with the emergence of a fluke and subsequent reduction of the rear flippers. They never reached the point of irreversibly having rear flippers, which is why flukes became the prominent feature in their movement behavior.

Therefore, the observed differences between marine reptiles and marine mammals can be attributed to the unique evolutionary paths they followed, driven by selective pressures and advantageous adaptations within their respective lineages.