Sulids are related to a number of other aquatic birds, which all lack external nostrils and a brood patch, but have all four toes webbed and a gular sac. The closest living relatives of the Sulidae are the Phalacrocoracidae (cormorants and shags) and the Anhingidae (darters). The latter are somewhat intermediate between sulids and cormorants, but (like many cormorants) they are freshwater birds in a clade containing otherwise seabirds, and also symplesiomorphic with sulids but synapomorphic with cormorants in some other respects. Thus, the Sulidae seem to be the oldest and most distinct lineage of those three, which are united in a suborder Sulae. Therein, the Sulidae are typically placed simply as a family; sometimes, a superfamily Suloidea is recognized, wherein some of the primitive prehistoric forms (e.g. "Empheresula", "Eostega", and "Masillastega") are placed as basal lineages distinct from the living Sulidae. However, the proposed family Pseudosulidae (or Enkurosulidae) is almost certainly invalid.