PHYLOGENETIC SIGNAL IN GEOGRAPHIC RANGE SIZE AND INTRASPECIFIC VARIATION BUT NOT IN SPECIES DURATION IN UPPER CAMBRIAN TRILOBITES OF THE GREAT BASIN, USA

Previous work has shown that in Upper Cambrian trilobites of the Great Basin, USA, geographic range size and duration are positively correlated but variation and duration are negatively associated. However, each species within this clade shared some evolutionary history with all of the other species. If two species attributes appear to covary, they may do so simply because they are both related to other variables that have not been controlled for, and phylogenetic history is surely one such variable in many studies. In order to test this possibility for these variables, a phylogenetic analysis of Great Basin pterocephaliids was performed and the results used to test for phylogenetic effect. As has been documented in other clades, the distribution of geographic range sizes showed strong dependence on phylogeny. In addition, the amount of intraspecific variation expressed by each species through its lifetime showed weaker but significant phylogenetic signal. Duration, however, showed no significant phylogenetic signal. These results were consistent across different cladograms, methods for estimating branch lengths, and subsets of taxa.

The association between duration and each of the two other variables is best described as “polygonal”. Because of this as well as the lack of phylogenetic signal in species duration, traditional statistical methods may still be the best methodology to apply to the investigation of the relationship between these variables, at least in this clade. Nonetheless, results from phylogenetic least squares analysis were consistent with the results found using traditional statistical methods.