I examined the phylogenetic relationships of suckers of tribe Moxostomatini (Cypriniformes, Catostomidae) using mitochondrial cytochrome-b gene sequences and nuclear intron sequences of the Growth Hormone gene. Analyses were performed with maximum parsimony and Bayesian maximum likelihood methodologies and topologies were compared to alternative hypotheses to assess congruence. The phylogenies recovered from these analyses were significantly different from the most recent estimates of relationships based primarily on morphology (Smith, 1992) and cytochrome-b sequences (Harris et al., 2002). Overall, there was little support for many basal nodes in phylogenies derived from these data as most trees were characterized by numerous polytomies. Lack of resolution among basal nodes could be associated with rapid cladogenesis or lack of suitable and informative variation in the analysis. However it was clear from the analyses that Genus Scartomyzon was not monophyletic, despite morphological evidence for this provided Robins and Raney (1956, 1957), Jenkins (1970), and Smith (1992). The analyses could not reject the possibility that there are two, independent lineages of Scartomyzon-like taxa in North America. Growth Hormone sequences provided good support for a monophyletic Western Scartomyzon lineage and thus suggested a single ancestral invasion into drainages of Texas and Mexico. However, total evidence analysis suggested that the evolutionary history and phylogenetic resolution of these groups could be more complex. The evolution of Western Scartomyzon is intimately linked to the paleohydrological history of Texas and Mexico. Phylogenetic relationships of Western taxa are structured geographically and do not conform to current taxonomy. The analyses also revealed cryptic endemism within this clade. Thus further taxonomic analysis of this group is warranted
