Cycadodendron galtieri gen. nov. et sp. nov. - A lower Permian gymnosperm stem with cycadalean affinity

Abstract

Premise of research. In our modern flora, the Cycadales represent one of the oldest-known gymnosperm clades, with their evolutionary roots tracing back to the late Paleozoic. Their radiation and wide distribution in the Mesozoic are well documented by numerous fossils. In contrast, the fossil record of late Paleozoic forms is restricted to a few specimens, thus leaving open questions on the origin and early evolution of cycads. A petrified stem with cycadalean affinity has been found in Saxony (central-eastern Germany) in Holocene gravel deposits of the Zwickauer Mulde river as a result of fluvial transportation from its original lower Permian locality. Methodology. The specimen was sectioned transversely, radially, and tangentially. Its provenance was clarified by comparing fossil woods of various localities in the upstream catchment area. Samples of these localities were exposed to ultraviolet light of different wavelengths, revealing distinctive fluorescence patterns related to specific geochemical conditions during the fossilization process. Plant anatomical studies were performed by macroscopic and microscopic documentation, including photo scanning, microscopic photography, and anatomical measurements. Pivotal results. Cycadodendron galtieri gen. nov. et sp. nov. originates from the Chemnitz Fossil Lagerstätte, an autochthonous floral assemblage of early Permian age (291 Ma; Sakmarian–Artinskian) that has been buried and conserved by pyroclastics in a geological instant. Several anatomical characteristics provide evidence for its cycadalean affinity: (1) a wide pith with numerous scattered medullary bundles, (2) a pith-peripheral vascular system with endarch primary xylem bundles, (3) at least nine successive vascular segments with secondary xylem and phloem, (4) araucarioid-type pitting of secondary xylem tracheids, and (5) primary rays and medullary bundles traversing the vascular segments. Conclusions. Cycadodendron galtieri gen. nov. et sp. nov. provides insights into stem anatomical characteristics of the oldest-known cycads, for example, revealing that polyxyly was an early-derived feature in cycad evolution. Its provenance reveals that the arborescent but generally small taxon was part of intramontane forested landscapes thriving on a well-drained mineral substrate in an alluvial plain setting and experiencing seasonally dry paleoclimate.