Rigorous ground plans as terminal taxa: a demonstration in Gastropoda (Mollusca), focusing on Caenogastropoda

Despite being widely discussed, the use of ground plans (or ground patterns) as terminals in phylogenetic analyses often faces bias during peer review, as evidenced by criticisms of phylogenetic studies. A ground plan can be defined as the sum of all character states associated with an internal node, representing the synapomorphies and plesiomorphies of a cladogram branch. It is, in fact, a critical outcome of cladistic analysis. Ground plans allow us to theorize about the ancestral species from which a branch evolved and can be effectively employed in subsequent phylogenetic analyses, offering advantages over using sets of actual species. This paper provides a practical example using samples of Gastropoda representatives (Mollusca). The study revisits a large dataset from a cladistic analysis previously published in 2011, which was based on actual species representing the main branches of Mollusca. Here, the main branches of the original cladogram are replaced with their respective ground plans as terminals. The resulting cladogram is presented and compared to the original analysis. All terminals are represented by ground plans, which are discussed in detail. This paper advocates for the broader use of ground plans as terminals in phylogenetic analyses, provided they are derived from a prior, rigorous phylogenetic framework. One major advantage of this approach is the reduced influence of homoplasies, which may be present in actual species but are absent in the ground plan of the taxon it represents, thereby minimizing noise in the final results. The main objective is to mitigate criticisms of using ground plans as terminals in phylogenetic analyses, using a practical and real example as a basis.