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

Abundant fossils of a single conifer species occur in a Pennsylvanian-age d eposit of eastern North America, providing the opportunity to describe a bi ological taxon of primitive conifers as well as to clarify the uncertain ta xonomy 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 cutic les; some also display internal anatomy. The plant has an orthotropic stem, with two orders of plagiotropic lateral branches that all bear helically a rranged simple leaves. Ovulate fructifications consist of compact zones of bracts with axillary ovuliferous dwarf shoots on otherwise vegetative branc hes. Pollen cones are compound shoots comprising helically arranged dwarf s hoots in the axils of bracts on a main axis. Polliniferous dwarf shoots pro duce sterile scales, sporophylls with a terminal pollen sac, and in situ Po tonieisporites grains. Stems have an endarch eustele with dense wood, perid erm, and resin canals in the pith. There are two adaxial stomatal zones on vegetative leaves, but stomata are distributed over the entire adaxial surf ace of bracts and sterile scales. Thucydia is the only conifer with ovulife rous fertile zones, compound pollen cones, and dissimilar stomatal distribu tions on vegetative and fertile leaves. This novel combination of features characterizes the Thucydiaceae fam. nov. The currently confused state of pr imitive conifer taxonomy is reviewed, nomenclature is clarified, and revise d approaches for inferring relationships are proposed. Thucydia provides a benchmark for developing sound taxonomic concepts and useful criteria for i dentifying specimens of walchian species and for resolving phylogenetic rel ationships among fossil and living conifers.