:::What is provided is a stem-based definition (sometimes called a branch-based definition). See the discussion at Cladistics#Phylogenetic definitions of a clade. Since statements like this do determine membership in a group, they are definitions, though they say nothing about the characteristics common to the group. For another example, see the Massopoda article: this group consists of the animals more closely related to Saltasaurus loricatus than to Plateosaurus engelhardti. The definition is silent as to what is common common to all Massapoda species. While the second paragraph of the Eutheria article does start with a sentence hinting, very vaguely, at the sorts of features that distinguish the eutherians, you will not find a precise list of distinguishing characteristics on Wikipedia.  As you point out, polydactyly is not common to all eutherians, but neither is live birth; marsupials are viviparous.