Provisional or Incomplete? Two Centuries of Systematic Inquiry into the Totipalmate Birds

The traditional Pelecaniformes, defined by the totipalmate foot, persisted as an apparently stable assemblage from the nineteenth century to the early 2000s, its coherence resting on a single diagnostic character whose phylogenetic interpretation was rarely tested. This essay traces the history of the group’s classification, from the comparative anatomy of the nineteenth century, through the cladistic analyses of 1985–2015, to the molecular redefinition of the early 2020s, in which the traditional order was found polyphyletic, with pelicans placed among herons, ibises, and the shoebill; cormorants and allies transferred to Suliformes; and tropicbirds to Phaethontiformes. The position of Phaethontiformes within Aequornithes remains the principal point of instability, recovered as sister to Eurypygiformes in molecular analyses but variously placed in morphological ones. The redefinition is supported mainly by molecular evidence, and the candidate morphological synapomorphies proposed for some groupings remain untested in a comprehensive analysis incorporating extant and fossil terminals and evaluated against the molecular topology. In such an analysis, the principal remaining task would be to test whether the molecular groupings are recognisable in phenotype, including in fossil taxa accessible only to morphology. The classification is incomplete in this sense rather than provisional, not because it is likely wrong but because that test has not been performed.