The term 'nothosaur' has had several different meanings over the years. It was originally used to describe a diverse group of plesiomorphic basal sauropterygians including more-or-less all sauropyerygians except for plesiosaurs and placodonts. This included the groups known today as pachypleurosaurs, cymatosaurids, and pistosaurids, which are all now recognised as phylogenetically distinct. The traditional concept of nothosauria is therefore no longer used because it included this polyphyletic assemblage of distinct clades (Storrs, 1993).
The modern concept of nothosaurs is much more restricted, but still contains a wide variety of genera (Rieppel, 2000, 2002). The broadest group to which the name 'nothosaur' can be applied is the Nothosauroidea. The Nothosauroidea contains two groups, the Simosauria and the Nothosauria. The Nothosauria is a monophyletic clade of basal sauropterygians that include Germanosaurus, Nothosaurus and Lariosaurus.