The principles and methods of phylogenetic systematics and its application to the taxonomy of the Pronocephalidae Looss, 1902 (Platyhelminthes: Digenea)

Biological systematics has developed according to the Kuhnian model of science: there have been paradigm shifts in systematic practices, the consequence of changing perceptions of what is required of scientists, theories, and classifications. Under the current paradigm of phylogenetic systematics, there are two sub-disciplines which can be broadly categorised as Methodological Procedures and Modelling Tools. The former include the techniques of Parsimony and Compatibility, while the latter consists of the recently developed techniques of Maximum-Likelihood Estimation (MLE). Parsimony and Compatibility, while intuitively appealing, can lead to incorrect hypotheses of phylogeny when characters of taxa change at unequal rates. MLE takes account of unequal rates of change, but is mathematically demanding. In this thesis, a number of methods are derived which retains the simplicity of Parsimony and the efficiency of MLE. These techniques are applied to uncovering the relationships of the Pronocephalidae Looss, 1902 (Platyhelminthes:Digenea) a family of monostomatous parasites of reptiles. The analysis revealed that certain genera of the Pronocephalidae are polyphyletic. In revising the taxonomy of the group it was necessary to erect a paraphyletic genus because of the insufficiency of good character-taxon information. Under the revised classification, the Pronocephalidae consists of seven genera: Notocotyloides Dollfus, 1966, Pyelosomum Looss,. 1899, Charaxicephalus Looss, 1901, Pronocephalus Looss, 1899, Cetiosaccus Gilbert, 1938, Macravestibulum Mackim, 1930, and Neopronocephalus Mehra, 1932