Revision and phylogenetic analysis of the Gyrocotylidea Poche, 1926 (Platyhelminthes: Cercomeria: Cercomeromorpha)

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Abstract
Phylogenetic analysis of 10 gyrocotylidean platyhelminth species, based on 24 character states comprising 13 homologous series, produced two similar phylogenetic trees of equal length. The consistency index value for each tree is 87.5%, indicating a low degree of parallel evolution in the morphology of gyrocotylidean platyhelminths. Slightly more than half of the observed associations between gyrocotylideans and holocephalan hosts can be attributed to coevolution. Remaining associations must be attributed to colonization, but these represent recolonizations of plesiomorphic hosts. The geographic distribution of gyrocotylideans remains enigmatic, as part of it seems to predate tectonic plate movements. Extensive dispersal has occurred; dispersal events are correlated with host transfers. It is hypothesized that the origin of gyrocotylideans predates separation of the continents, and that there were two phases in the evolution of the gyrocotylideans, an initial phase of coevolution and a later phase of colonization and dispersal.

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