Sprecher
Beschreibung
The medullosans depict widespread and diverse floral elements of late Paleozoic tropical landscapes on the Northern Hemisphere. In the phylogeny of the seed-plant clade the group holds a poorly constrained position, as its numerous ancient features complicate comparisons with modern relatives. Among these features, the medullosan stele is of key significance as it exhibits a complex and uncommon arrangement of vascular elements that varies distinctly among the known taxa. These include a variously dissected vasculature, clustered primary xylem zones, centripetally and centrifugally growing secondary xylem assigned to the manoxylic wood type, central accessory strands, and a kind of polyxyly recently documented for some taxa. Regarding the primary vascular organisation, there exist some parallels to siphonosteles of the Polypodiopsida, thus contradicting the concept of a eustele that is thought to be apomorphic to the seed-plant clade. Nevertheless, collateral bundles are present in the leaf vasculature of medullosans. By analysing new anatomical data on the diversity, ontogeny, and functionality of medullosan stem vascular architecture, we present new perspectives on the stelar evolution of ancient seed plants and their highly specialised growth models that were supposedly outcompeted at some point in transition to the modern seed-plant flora.