Darter - Systematics and Evolution

Systematics and Evolution

This family is very closely related to the other families in the suborder Sulae, i.e. the Phalacrocoracidae (cormorants and shags) and the Sulidae (gannets and boobies). Cormorants and anhingas are extremely similar as regards their body and leg skeletons and may be sister taxa. In fact, several anhinga fossils were initially believed to be cormorants or shags (see below). Some earlier authors included the darters in the Phalacrocoracidae as subfamily Anhinginae, but this is nowadays generally considered overlumping. However, as this agrees quite well with the fossil evidence some unite the Anhingidae and Phalacrocoracidae in a superfamily Phalacrocoracoidea.

The Sulae are also united by their characteristic display behavior, which agrees with the phylogeny as laid out by anatomical and DNA sequence data. While the darters' lack of many display behaviours is shared with gannets (and that of a few with cormorants), these are all symplesiomorphies that are absent in frigatebirds, tropicbirds and pelicans also. Like cormorants but unlike other birds, darters use their hyoid bone to stretch the gular sac in display. Whether the pointing display of mates is another synapomorphy of darters and cormorants that was dropped again in some of the latter, or whether it evolved independently in darters and those cormorants that do it, is not clear. The male raised-wing display seems to be a synapomorphy of the Sulae; like almost all cormorants and shags but unlike almost all gannets and boobies, darters keep their wrists bent as they lift the wings in display, but their alternating wing-waving, which they also show before take-off, is unique. That they often balance with their outstretched wings during walking is probably an autapomorphy of darters, necessitated by their being plumper than the other Sulae.

The Sulae were traditionally included in the Pelecaniformes, then a paraphyletic group of "higher waterbirds". The supposed traits uniting them, like all-webbed toes and a bare gular sac, are now known to be convergent, and pelicans are apparently closer relatives of storks than of the Sulae. Hence, the Sulae and the frigatebirds – and some prehistoric relatives – are increasingly separated as the Suliformes, which is sometimes dubbed "Phalacrocoraciformes".