Author: Jan Scholten
Suliformes was traditionally placed in Pelecaniformes. Recent evidence shows that the traditional Pelecaniformes is polyphyletic. Only Pelecanidae, Balaenicipitidae, and Scopidae remain in Pelecaniformes.
Phaethontidae has since been moved to their own order Phaethontiformes. Genetic analysis seems to show that the Pelecaniformes is actually closely related to the Ardeidae and Threskiornithidae. According to Hackett loons, penguins, storks, and as well as Suliformes and Pelecaniformes, all seem to have evolved from a common ancestor.
Hackett, S.J. & others; A Phylogenomic Study of Birds Reveals Their Evolutionary History; Science 320, 1763; 2008.