Abstract
Within the class Monogenea, three clades are currently recognized : the Polyonchoinea, the Oligonchoinea and the Polystomatoinea. While most monogeneans parasitize chondrichthyan and teleost fishes, the Polystomatoinea re the only group parasiting terrestrial vertebrates, i.e. freshwater tetrapods and the Dipnoi. They are represented by two families of different sizes, the Polystomatidae, with about 200 described species found in the Australian lungfish, amphibians and freshwater turtles, and the Sphyranuridae, with only three extant species from north American salamanders. From a phylogeny based on the analysis of partial 18S sequences, we investigated the phylogenetic relationships of the main genera of the sPolystomatidae and one species of the Sphyranuridae to examine their origin. The basal position fo the polystome from the lungfish within the Polystomatoinea suggests that the clade originated during the evolutioinary transition between aquatic and terrestrial vertebrates, ca. 425 million years ago. The monophyly of both amphibian and turtle parasites, together wih molecular dating estimates, indicate that the two lineages probably diverged from each other ca 360 million years agon. The polystomes of turtles radiated ca 190 million years ago, following a switch from an aquatic amniote to turtles. Parasites of Amphibia would have originated during the setting-up of the major amphibian lineages in the Lower Triassic, and neobatrachain polystomes could have diverged during the breaking-up of Gondwana. Finally, the phylogenetic position of the Sphyranuridae, which is nested wihin the Polystomatidae, combined with its peculiar morphology suggests that representatives of this family could be neotenic parasites infesting neotenic hosts. We proposed to invalidate this family and to include it in the Polystomatidae