EXPERIMENT PHYLOGENY OF LATE PALEOZOIC CORDAITEANS AND THE CONIFER THUCYDIA BASED ON POLLEN, POLLEN CONES & SEEDS

Cordaiteans are extinct woody gymnosperm trees and shrubs with strap-shaped leaves that flourished in the Pennsylvanian and Permian of Euramerica and China. Phylogenetic analyses based on morphology place cordaiteans and conifers as sister groups. A number of seed and cone genera have been assigned to the cordaiteans; however most lack whole-plant reconstructions, limiting phylogenetic analysis of the group. In order to explore relationships within cordaiteans, we use experimental cladistics to create phylogenetic trees based on a character matrix of pollen, pollen cone, and seed traits. Our study includes all permineralized cordaitean pollen cones except for three taxa lacking descriptions of fertile scale shape: Cordaianthus ximinensis, C. thompsonii and C. compactus.

Phylogenetic trees assembled by parsimony analysis group taxa based on the distribution of shared characters. Our study yields three equally parsimonious trees with seed fern, Callistophyton, emerging as the outgroup. A Gothania/Mitrospermum clade forms the sister to a cardiocarpid clade. Within the cardiocarpid clade, Cordaianthus shuleri/Nucellangium forms the outgroup, and the remaining taxa form two lineages: a Cordaianthus/Cardiocarpus lineage; and a Cathayanthus lineage incorporating the two bilaterally symmetric secondary fertile shoots (Cathayanthus sinensis and Cordaianthus saportanus). These results support and extend cordaitean subgroup relations based on whole plant reconstructions and suggest that cladistic analysis of reproductive organs provides lucid results.

Analyses incorporating the conifer, Thucydia, yield 19 equally parsimonious trees with four patterns, in which Thucydia forms a second outgroup, or joins one of the following clades: Gothania/Mitrospermum, cardiocarpid, or Cathayanthus. The conifer Thucydia, a highly derived taxon, resembles an outgroup – a common problem in cladistic analysis. Unique shared derived characters (helical attachment of secondary fertile shoots and bilateral secondary fertile shoots) link Thucydia and Cathayanthus in four trees. Overall our results suggest that of the four cordaitean lineages (Gothania/Mitrospermum, Cathayanthus, Cordaianthus/Cardiocarpus, and Cordaianthus/Nucellangium) conifers most closely related to the Cathayanthus lineage.