The Evolution of Early Vascular Plant Complexity

Premise of research. The concept of biological complexity is deeply embedded in evolutionary theory and empirical enquiry. Yet it is largely used intuitively and rarely quantified. Here, building on a previous phylogenetic analysis of 36 early Paleozoic tracheophytes and the extant genus Equisetum, we discuss the evolution of character states based on the consensus tree topology and the first appearance of derived character states in the fossil record in a preliminary attempt to quantify the tempo and mode of the evolution of complexity during a critical evolutionary period in land plant history. Methodology. Thirty-seven fossil taxa were selected (predominantly from the Silurian-Devonian time interval) to provide as many characters and character states as possible for each taxon to reduce the negative effects of missing characters in phylogenetic analyses and to most accurately represent phenotypic diversity. In addition, the character states of the extant taxon Equisetum were added to evaluate the phylogenetic position of this genus and its effect on the consensus tree. Pivotal results. Using the best-preserved taxa representative of their respective lineages, the consensus tree juxtaposed with the first recorded appearance of each fossil taxon showed that anatomical, morphological, and reproductive complexity increased throughout the time interval examined in this study and that this trend was attended by increasing body size within a well-defined zosterophyllophyte-lycophyte clade and a well-defined trimerophyte-euphyllophyte clade, distal to a rhyniophyte clade. Conclusions. The data indicate that (1) the taxa sampled segregate into two major tracheophyte mega clades (i.e., a trimerophyte-euphyllophyte clade and a zosterophyllophyte-lycophyte clade), (2) complexity (defined on the basis of four criteria and computed on the basis of the first occurrence of derived character states in the fossil record) and body size increased within each of the two mega clades, albeit at different tempi, and (3) substantial evolutionary convergence occurred (e.g., leaves, vascular cambia, roots, and aggregated reproductive structures).