The phylogenetic trunk: maximal inclusion of taxa with missing data in an analysis of the lepospondyli (Vertebrata, Tetrapoda).

The importance of fossils to phylogenetic reconstruction is well established. However, analyses of fossil data sets are confounded by problems related to the less complete nature of the specimens. Taxa that are incompletely known are problematic because of the uncertainty of their placement within a tree, leading to a proliferation of most-parsimonious solutions and "wild card" behavior. Problematic taxa are commonly deleted based on a priori criteria of completeness. Paradoxically, a taxon's problematic behavior is tree dependent, and levels of completeness are not directly associated with problematic behavior. Exclusion of taxa on the basis of completeness eliminates real character conflict and, by not allowing incomplete taxa to determine tree topology, diminishes the phylogenetic hypothesis. Here, the phylogenetic trunk approach is proposed to allow optimization of taxonomic inclusion and tree stability. The use of this method in an analysis of the Paleozoic Lepospondyli finds a single most-parsimonious tree, or trunk, after the removal of one taxon identified as being problematic. Moreover, the 38 trees found at one additional step from this primary trunk were reduced to 2 by removal of one additional taxon. These trunks are compared with the trees that were found by excluding taxa with various degrees of completeness, and the effects of incomplete taxa are explored with regard to use of the trunk. Correlated characters associated with limblessness are discussed regarding the assumption of character independence; however, inclusion of intermediate taxa is found to be the single best method for breaking down long branches.