The family Threskiornithidae includes 35 extant species of large wading birds, and one more that became extinct in historical times. The family has been traditionally classified into two subfamilies, the ibises and the spoonbills; however, recent genetic studies have cast doubt on this arrangement, and have found the spoonbills to be nested within the Old World ibises, and the New World ibises as an early offshoot. These studies found two major clades within the family, a widespread clade that includes the type genus Threskiornis and seven other genera (including the spoonbills) occurring in both the Old and New Worlds, and a smaller group of entirely New World species, with five genera.
Taxonomy
The spoonbills and ibises were once thought to be related to other groups of long-legged wading birds in the order Ciconiiformes, until genetic studies found that they are members of the order Pelecaniformes. In response to these findings, the International Ornithological Congress (IOC) reclassified Threskiornithidae and their sister taxon Ardeidae under the order Pelecaniformes, instead of the previous order of Ciconiiformes, in 2010.
A study of mitochondrial DNA of the spoonbills plus the sacred and scarlet ibises found that the spoonbills formed a clade with old world genus Threskiornis, with Nipponia nippon and Eudocimus as progressively earlier offshoots and more distant relatives, and hence casts doubt on the arrangement of the family into ibis and spoonbill subfamilies. Subsequent studies have supported these findings, the spoonbills forming a monophyletic clade within the "widespread" clade of ibises, including Plegadis and Threskiornis, while the "new World Endemic" clade is formed by the genera restricted to the Americas such as Eudocimus and Theristicus.
Ibises are a rather old group, with well-preserved definitive fossils known as far back as the Eocene (Rhynchaeites), and potential members dating as far back as the Late Paleocene (Dakotornis).
