What is one method or line of reasoning that scientists could NOT use when testing the hypothesis that dolphins and Ichthyosaurs form a monophyletic group?

They cannot use the reasoning, "Dolphins and Ichthyosaurs form a paraphyletic group."

Explanation:

This is because they always form a monophyletic goup. Both of the species have specific morphological similarities that are enough to consider them in monophyletic group instead of a divergent paraphyletic.