All three organisms are found on coral reefs. Durania and Prorichthofenia have converged in form to that of the cup coral. If one wants a system of classification which truly represents the evolutionary relationships, then we must look closely to find true homologies. Monophyly A taxonomic group is monophyletic if a single species is the ancestor of all the other species in the group. A polyphyletic group includes members who have characteristics of analogous similarity.
Major branches give rise to less major ones.
And, these give rise to even smaller clusters of branches. The recognition of this heirarchy of different levels of branching gave rise to the Linnaean system of classification. The species is a very special unit of classification because species seems to be a natural unit of classification. Species act as individual evolutionary units. One would suspect that there might be a continuous range of sub-species, say, as one species began to give rise to another.