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Phylogenetic analyses of sequences of the 18S rDNA and MT‐CO2 gene fragments indicated that the oviparous and viviparous gyrodactylid‐like monogenoids formed independent monophyletic clades within the Order Gyrodactylidea, supporting the reinstatement of the Oogyrodactylidae and limiting the Gyrodactylidae to the viviparous species. Analyses further indicated that the clade comprising the two families shared a common ancestor with the Udonellidae. Two clades, that of Aglaiogyrodactylus and that of Phanerothecium, were identified within the Oogyrodactylidae, while Onychogyrodactylus was shown to be polyphyletic and Oogyrodactylus basal within the family. One putative synapomorphy was identified for the Oogyrodactylidae, that is presence of a massive Mehlis’ gland. The Gyrodactylidae was limited to species having a viviparous mode of reproduction, although relationships within the family were generally poorly resolved. Several putative synapomorphies were found for the Gyrodactylidae, including viviparity and protogyny, a bulbous and armed MCO, absence of a vitellarium, and presence of a knob‐like deep anchor root (Fig. 3e). Ultrametric analyses suggested that the initial divergence of the clade of the gyrodactylid‐like monogenoids and Udonellidae occurred about 335 mya (based on the 18S rDNA fragment) and about 400 mya (based on the MT‐CO2 gene fragment). Using the 18S rDNA fragment and three calibration points, ultrametric analyses indicated that the Gyrodactylidae and Oogyrodactylidae diverged at approximately 278 mya, with initial diversification within the Gyrodactylidae (about 211 mya) occurring earlier than that of the Oogyrodactylidae (about 133 mya), the latter coinciding with the breakup of Gondwana and the initial diversification of the armoured catfishes (Loricariidae). Finally, diagnoses were provided for the Gyrodactylidae and Oogyrodactylidae along with a list of genera assigned to each family.

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