To add to this, it is not uncommon (in fact kind of the rule) for clades to be differentiated on the basis of a single differing trait. Or what appears to be. After all, the split after nephrozoa between the protostomes and deuterostomes is whether the mouth or anus develop first due to different placement of cells in gastrulation (cell differentiation). Perhaps a simplistic explanation but I'm no essay writer. Phylogeny sofar as I know largely keeps subdividing along shared core ancestral divergences near as we can identify them. The specifics of course are a lot more complicated, since why the cells end up doing opposite things first is more involved.