SPECIES LEVEL PHENOTYPIC VARIATION IN LOWER PALEOZOIC TRILOBITES

Phenotypic variation within species provides the raw material acted up on by natural selection and other evolutionary mechanisms. As such, th e range and variation of morphology within a species can play an impor tant role in determining the tempo of evolution. The range and varianc e of aspects of cranidial morphology for nine lower Paleozoic trilobit es were measured to identify microevolutionary correlates of macroevol utionary patterns. Comparisons were made among sets of homologous land marks or upon partial warp vector matrices containing similar proporti ons of variance. Rarefaction and bootstrap analyses helped estimate th e effects of sampling. Levels of variance and range of morphology diff ered considerably within and among time periods. There is no significa nt temporal decline in the variance or range of morphology, suggesting that developmental or genomic constraints may not have been the prima ry factors controlling the tempo of trilobite macroevolution. The spat ial distribution of cranidial variance differed considerably among tax a, suggesting that a complex set of developmental processes governed t he morphogenesis of cranidia within trilobites.