A PHYLOGENETIC REASSESSMENT OF THE TADORNINE-ANATINE DIVERGENCE (AVES, ANSERIFORMES, ANATIDAE)

Phylogenetic analysis of eight problematic genera of waterfowl (Sticto netta Plectropterus, Sarkidiornis, Hymenolaimus, Tachyeres, Merganetta , Malacorhynchus, and Salvadorina) and representatives of other tadorn ine genera and anatine tribes was undertaken using 114 morphological c haracters (39 skeletal, four tracheal 11 natal, and 60 of the definiti ve integument) and a variety of weighting schemes, in an attempt to cl arify relationships in the most poorly understood segment of the phylo geny of Anseriformes (Livezey, 1986a). Under equal weighting of charac ters, 40 equally parsimonious trees were found that shared the followi ng findings: (1) a sister-group relationship of Stictonetta to a clade comprising the Tadorninae and Anatinae; (2) a sister-relationship bet ween Plectropterus and Sarkidiornis; (3) monophyly of the typical shel ducks and sheldgeese (Tadorna, Cyanochen, Alopochen, Neochen, and Chlo ephaga); (4) monophyly of Hymenolaimus, Tachyeres, and Merganetta; and (5) a sister-relationship between Salvadorina and Malacorhynchus. An analysis wherein skeletal characters were given twice the weight of ot her character groups resulted in four shortest trees which, in additio n to the findings under equal weighting, indicated that: (1) Plectropt erus + Sarkidiornis represent the sister-group of the typical shelduck s and sheldgeese; (2) a sister-relationship between the clade comprisi ng Hymenolaimus, Tachyeres, and Merganetta and the preceding clade, co llectively constituting the ''tadornine'' clade; and (3) a sister-rela tionship between Malacorhynchus + Salvadorina and an ''anatine'' clade comprising the remaining tribes and subtribes (Cairineae, Nettapodeae , Anateae, Aythyini, Mergini and Oxyurini). An analysis in which skele tal, tracheal, natal, and definitive characters were assigned weights of four, three, two, and one,respectively, produced identical results to the preceding weighting scheme. Successive weighting by rescaled co nsistency indices produced two shortest trees that resembled those fou nd under the two preceding protocols except: (1) the Mergini were plac ed as the sister-group of the ''tadornine'' clade defined above; and ( 2) the Cairineae (Cairina, Pteronetta, and Air) were placed as the sis ter-group to plectropterus + Sarkidiornis. Based on the comparative st rengths of hypothesized homologies of skeletal characters and assessme nts of topological support (including bootstrapping, decay indices, an d near-shortest trees) for all weighting schemes, a conservative class ification was based on the a priori weighting schemes favoring skeleta l characters. Maps of selected ecomorphological attributes on this phy logenetic tree revealed several distinct evolutionary trends, but inco mpletely resolved relationships among these diverse genera and within suprageneric taxa limited inferences. Additional promising areas for a natomically based systematic study are identified, and the need for co ntinued cladistic study of this poorly resolved but topologically crit ical segment of the phylogeny of Anseriformes is emphasized.