This taxon, as coined by Kuhn (1968) and used by Steel (1973), was a monotypic family redundant with the Late Cretaceous Sphagesaurus hueni. The type of Sphagesaurus huenei consists only of two teeth, but these are very derived in form with fluted crowns (Price 1950). Recent discovery of a partial cranium of the species (Pol 2003) shows that S. hueni is a derived notosuchian crocodyliform closely related to the Chinese species Chimerosuchus paradoxus (Wu and Sues 1996). The evidence is strong and consists of the presence of only two premaxillary teeth, the participation of the premaxilla in the dorsal margin of the external naris, and other features (Pol 2003).
Independently, Chimaerasuchidae was coined by Carvalho et al. (2004) and defined with a node-based definition to include Chimaerasuchus and Simosuchus. This group, in contrast, has not been supported by other analyses and is united on the basis of weak, homoplastic character evidence (Carvalho et al. 2004:988). The family Sphagesauridae Kuhn 1968 has precedence over Chimaerasuchidae Carvalho et al. 2004, and the existence of this derived clade is now very strong.
Nevertheless, these two taxa are not synonyms by definition; Chimaerasuchidae, defined using Simosuchus as an internal specifier (Carvalho et al. 2004), will circumscribe some as yet uncertain, larger clade (depending on the position of Simosuchus). The active definition Sphagesauridae is stem-based and cites as external specifiers the best known, nominotypic species of other notosuchians and extant Crocodylia, ensuring the inclusion only of forms most closely related to Sphagesaurus huenei.