Anatomy and morphology of select Devonian plants and the inference of macroevolutionary pattern and process during the radiation of early tracheophytes

This work provides new information concerning the anatomical and morphological form of Devonian vascular plants as well as new interpretations of land plant phylogeny and causal factors underlying the evolution of morphological disparity. Chapter one presents a new description of the anatomy of Cladoxylon dawsonii Read from the Upper Devonian of New York State. Evidence suggests the plant exhibits diagnostic traits of both the Devonian Cladoxylopsida and Iridopteridales, and thus challenges current phylogenetic interpretations of both groups. Based on the discovery of anatomical characters such as permanent protoxylem strands and whorled trace departure in C. dawsonii, a new cladistic analysis of Devonian euphyllophytes suggests systematic placement of the plant within the Iridopteridales and the recognition of a new iridopterid genus, Rotoxylon. Chapter two provides a description of the morphology of Coronapteris erectus gen. et sp. nov. from the Middle Devonian of Gaspé, Québec. This plant represents the earliest occurrence of complex morphological form typical of the Devonian Cladoxylopsida, involving a large, tree-sized main stem with an apical crown of helically arranged lateral branches, and offers an excellent hypothetical model for interpreting the growth form of early members of the group. In addition to large size, the plant exhibits a heterosporous condition. Analysis of the tree size/heterospory trait combination within an historical framework proves important in providing process explanations underlying large-scale trends in the evolution of early vascular plant form. Chapter three proposes a view of the phylogeny of Devonian euphyllophytes based on a new methodological approach for determining the polarity of cladistic characters. Macroevolutionary patterns of adaptive radiations involving (1) an initial phase characterized by populations in which variance of characters is homogeneous and modest, followed by (2) an intermediate phase characterized by increase in total variance accompanied by increases in taxonomic diversity and (3) a late phase characterized by increase in complexity of characters and the winnowing of form such that the distribution of form can no longer be considered homogeneous, are viewed as common features among major radiation events in the earth's history, suggesting a fundamental common cause that is useful in phylogeny reconstruction. The new methodology for determining character polarity involves measures between pairs of character states of: (1) relative stratigraphic age, (2) relative homogeneity of variance, and (3) relative complexity. Older states that are hypothesized to contribute more to character variance within groups and are less complex compared with their paired states are coded as primitive. Using the phylogenetic framework provided by chapter three, a historical comparative technique for testing hypotheses of underlying causal factors of high morphological disparity in early vascular plant clades is developed in chapter four. Heterospory and specialized photosynthetic appendages are hypothesized as key innovations (i.e. independent ‘causal’ variables) whose origins phylogenetically predispose, and are correlated with, high morphological disparity (i.e. dependent ‘caused’ variable). Implementation of the comparative techniques offers new perspectives concerning character correlations traditionally considered adaptive in our studies of land plant evolution. In addition, the analysis provides insight into the developmental flexibility of taxa whose origins lie near the base of major clades.