Thucydiaceae fam. nov., with a review and reevaluation of Paleozoic walchian conifers

Abundant fossils of a single conifer species occur in a Pennsylvanian-age deposit of eastern North America, providing the opportunity to describe a biological taxon of primitive conifers as well as to clarify the uncertain taxonomy and systematics of walchian conifers. Thucydia mahoningensis gen. et sp. nov. is represented by up to three orders of interconnected vegetative and fertile shoots that are preserved as coalified compressions with cuticles; some also display internal anatomy. The plant has an orthotropic stem, with two orders of plagiotropic lateral branches that all bear helically arranged simple leaves. Ovulate fructifications consist of compact zones of bracts with axillary ovuliferous dwarf shoots on otherwise vegetative branches. Pollen cones are compound shoots comprising helically arranged dwarf shoots in the axils of bracts on a main axis. Polliniferous dwarf shoots produce sterile scales, sporophylls with a terminal pollen sac, and in situ Potonieisporites grains. Stems have an endarch eustele with dense wood, periderm, and resin canals in the pith. There are two adaxial stomatal zones on vegetative leaves, but stomata are distributed over the entire adaxial surface of bracts and sterile scales. Thucydia is the only conifer with ovuliferous fertile zones, compound pollen cones, and dissimilar stomatal distributions on vegetative and fertile leaves. This novel combination of features characterizes the Thucydiaceae fam. nov. The currently confused state of primitive conifer taxonomy is reviewed, nomenclature is clarified, and revised approaches for inferring relationships are proposed. Thucydia provides a benchmark for developing sound taxonomic concepts and useful criteria for identifying specimens of walchian species and for resolving phylogenetic relationships among fossil and living conifers.