Synapsozoa

Synapsozoa is a historical term that appeared in some 19th- and early 20th-century classifications to designate a proposed clade of unicellular eukaryotes, commonly referred to as protozoa. The motivation behind the name was to group various single-celled organisms believed to share a common origin based on morphological and cellular characteristics observed before the routine use of molecular data.

The composition of Synapsozoa varied between authors. Different researchers included different lineages of flagellates and amoeboid forms, selecting organisms on the basis of putative similarities in cellular organization, feeding mode, or other cytological traits. Because there was no fixed definition or universally agreed diagnostic character, the membership of Synapsozoa was inconsistent and often subjective.

With the advent of molecular phylogenetics and broader genetic sampling, Synapsozoa was not supported as a monophyletic or natural group. In contemporary taxonomy, the term is not recognized as a valid clade, and many organisms once placed in Synapsozoa are now assigned to diverse major eukaryotic lineages, such as Amoebozoa, Excavata, SAR, or opisthokonts. The term persists mainly as a historical note illustrating the era when classifications relied primarily on morphology rather than DNA-based evidence.

Synapsozoa thus serves as an example of how early classification efforts can be rendered obsolete by advances in phylogenomics, highlighting the ongoing evolution of our understanding of protist diversity.