Convergence is a process in which different species of organisms develop into one or more similar physical features in order to adapt to the same environment. The similarity between ichthyosaurs and fish is an example of convergence. Ichthyosaurs are marine reptiles and do not belong to the same class as fish. In order to adapt to the marine environment, the physical characteristics of ichthyosaurs tend to be consistent with those of fish. The most striking thing is that ichthyosaurs have fins like fish.
 A creature cannot be grouped just because it has similar physical features to its members is a reasonable corollary of the statement 'All members of a group must have something in common'.