Categorizing universal phenotypes from George Dvorsky raises the interesting issue of ichthyosaurs as occupying the role of dolphin’s in today’s animal world.

Ichthyosaurs were reptiles; dolphins are mammals. They might look similar, but they were completely different creatures. Still, Dvorsky argues that were the same in terms of the role they played:

In its day [the ichthyosaur] occupied the same ecological niche as the dolphin does today. In fact, these organisms are practically identical despite the fact that one was a reptile and one a mammal. These unrelated organisms were shaped by the same environmental pressures, the same laws of physics and hydrodynamics, and likely fed upon the same type of prey. For all intents and purposes, they are the same creature. More importantly, they occupied the same ecological niche and arrived there due to virtually identical environmental pressures. Thus, what I’m arguing is that dolphins and ichthyosaurs require a common classification.