Intraorganismal homology, character construction, and the phylogeny of Aetosaurian archosaurs (reptilia, diapsida)

Character construction, the methods by which characters and character states are produced from observations of variation, is a crucial but poorly understood step in phylogenetic analysis. Alternative approaches are used in practice, but there has been relatively little investigation of their theoretical bases and analytical consequences. We reviewed three published numerical analyses of the phylogenetic relationships within the Triassic Aetosauria. Combined data from these studies were used to explore the impact of alternative approaches to character construction. Some previous aetosaurian characters represent parallel variations in the morphology of osteoderms from different body regions, and their independence is questionable, leading us to propose more composite alternative constructions. Phylogenetic analyses revealed that inferred relationships within the Aetosauria are in general poorly resolved and weakly supported by the available data and are sensitive to alternative approaches to character construction. Thus, the results from this and previous studies should not, for the most part, be accepted as robust hypotheses of aetosaurian interrelationships. The treatment of systems of intraorganismal (e.g. serial, antimeric) homologues, such as osteoderms, in character construction is discussed. Applied to parallel variations in systems of intraorganismal homologues, previous advice on choosing among alternative character constructions and Hennig's auxiliary principle agree in favoring a more composite approach, in accordance with common practice.