The caudal skeleton of teleost fishes of the order Cyprinodontiformes is described and compared on the basis of 394 extant and eight fossil species, supporting delimitation of 21 phylogenetic characters, of which 13 are firstly reported. The Cyprinodontiformes are unambiguously diagnosed by the presence of a single, blade-like epural, and by principal caudal-fin rays continuous on upper and lower hypural plates. Monophyly of the suborder Cyprinodontoidei is supported by the widened neural and hemal spines of the preural centrum 3 and presence of a spine-like process on the stegural, and monophyly of the Aplocheiloidei by the absence of radial caudal cartilages. A keel-shaped lateral process on the compound centrum supports monophyly of the Nothobranchiidae. Some characters of the caudal skeleton in combination to other osteological features indicate the cyprinodontiform fossil genus †Prolebias to be a paraphyletic assemblage; †P. aymardi, †P. delphinensis and †P. stenoura, the type species of the genus, all from the Lower Oligocene of Europe, possibly are closely related to recent valenciids; †“P.” meridionalis from the Upper Oligocene of France is an incertae sedis cyprinodontid; and, †“P”. cephalotes, †“P”. egeranus and †“P”. malzi from the Upper Oligocene-Lower Miocene of Europe are closely related to poeciliids, probably closely related to the recent African genus Pantanodon due to they sharing unique derived features of pelvic fin, branchial arches and jaws.

Cyprinodontiformes, killifishes, Miocene, morphology, Oligocene, osteology.