Classically, bird relationships were based solely on
morphological characteristics. The Pelecaniformes were traditionally, but
erroneously, defined as birds that have feet with all four
toes webbed (totipalmate), as opposed to all other birds with webbed feet where only three of four were webbed. Hence, they were formerly also known by such names as
totipalmates or
steganopodes. The group included
frigatebirds,
gannets,
cormorants,
anhingas, and
tropicbirds. Research from the beginning of the 21st century strongly suggested that the similarities between this traditional definition of Pelecaniformes are the result of
convergent evolution rather than
common descent, being adaptations that were converged upon by birds living in and near aquatic environments; the traditional definition of the group would thus be
polyphyletic.
Sibley and Ahlquist's landmark
DNA–DNA hybridisation studies (now known as the
Sibley–Ahlquist taxonomy) led to them placing the families traditionally contained within the Pelecaniformes together with the
grebes,
cormorants,
ibises and spoonbills,
New World vultures,
storks, penguins,
albatrosses,
petrels, and
loons together as a subgroup within a greatly expanded order
Ciconiiformes, a radical move which by now has been all but rejected: their "Ciconiiformes" merely assembled all early advanced land- and seabirds for which their research technique delivered insufficient phylogenetic resolution. Morphological study had suggested pelicans are sister to a gannet-cormorant clade, yet
genetic analysis groups them with the hamerkop and shoebill, though the exact relationship between the three is unclear. Mounting evidence pointed to the
shoebill as a close relative of pelicans. Reviewing genetic evidence to date, Cracraft and colleagues surmised that pelicans were sister to the shoebill with the hamerkop as the next earlier offshoot. Ericson and colleagues sampled five
nuclear genes in a 2006 study spanning the breadth of bird lineages, and came up with pelicans, shoebill and hamerkop in a clade. Hackett and colleagues sampled 32 kilobases of
nuclear DNA and recovered shoebill and hamerkop as
sister taxa, pelicans sister to them, and herons and ibises as sister groups to each other, with the heron and ibis group a sister to the pelican/shoebill/hamerkop clade. Another hypothesis is that Threskiornithidae is sister to the rest of Pelecaniformes, and Ardeidae and Pelecani form a clade, as can be seen below: }} These controversies have been explained by analyses of rare genomic changes, which suggested that after the divergence of the pelican (Pelecanidae), heron (Ardeidae), and ibis (Threskiornithidae) lineages, interspecies hybridization led to gene flow causing the heron lineage to exhibit genetic signatures indicating affinity to both the pelican and ibis lineages.
Fossils All families in the traditional Pelecaniformes (except the Phalacrocoracidae) have only a few handfuls of
extant species at most, but many were more speciose in the Early
Neogene. The pelecaniform lineages appear to have originated around the end of the
Cretaceous, appearing to belong to a close-knit group of "higher waterbirds" which also includes groups such as penguins and
Procellariiformes. Quite a lot of fossil bones from around the
Cretaceous–Paleogene boundary cannot be firmly placed with any of these orders and rather combine traits of several of them. This is, of course, only to be expected, if the theory that most if not all of these "higher waterbird" lineages originated around that time is correct. Of those apparently
basal taxa, the following show some similarities to the traditional Pelecaniformes: •
Lonchodytes (Lance Creek Late Cretaceous of Wyoming, US) •
Torotix (Late Cretaceous) •
Tytthostonyx (Late Cretaceous/Early Palaeocene) •
Cladornis (Deseado Early Oligocene of Patagonia, Argentina) •
"Liptornis"—a
nomen dubium Fossil genera and species are discussed in the respective family or genus accounts; one little-known prehistoric Pelecaniforme, however, cannot be classified accurately enough to assign them to a family;
"Sula" ronzoni from Early
Oligocene rocks at
Ronzon, France, which was initially believed to be a
sea-duck, is a possible ancestral Pelecaniform. The proposed
Elopterygidae—supposedly a family of Cretaceous Pelecaniformes—are neither
monophyletic nor does
Elopteryx appear to be a modern bird. == References ==