Why are some animal groups difficult to place in a single taxonomic rank?

Some animal groups are difficult to place in a single taxonomic rank because there are no explicit, universally defined biological characters that dictate what constitutes a specific rank, such as a phylum, class, or order. Taxonomists must first identify natural groups based on shared characteristics or evolutionary history and then subjectively decide which rank to assign them.1

Several factors contribute to this difficulty:

• Subjectivity and Consensus: Because there is no quantitative measure for the "maximal difference" allowed at each level, an order in one authority's classification may be considered a superorder or class by another. Most established classifications rely on a general consensus among experts rather than rigid, objective rules.1
• Evolutionary Complexity: Features characteristic of one rank in one group may vary significantly among closely related members of another group. Furthermore, phenomena like convergent evolution—where unrelated animals evolve similar traits—can obscure true evolutionary relationships, making it challenging to determine if a group is a natural, monophyletic unit.123
• Incomplete Data: For many groups, especially those with poor fossil records, constructing a clear phylogenetic tree is impossible.4
• Shifting Methodologies: The field has moved from morphology-based systems to phylogenetic taxonomy, and some modern approaches even favor nonhierarchical systems over traditional Linnaean taxonomy, leading to ongoing debate regarding the utility of fixed ranks.5