Cymastrobus, a new Paleozoic lycopsid from Australia investigated through a combination of old (thin-sections), less old (SEM), and new approaches (X-Ray Synchrotron microtomography).

International audienceThis paper addresses one of Bill Chaloner’s earliest center of interest, the reproductive structures of the Paleozoic lycopsids and their in-situ spores, through the analysis of a new cone genus, Cymastrobus. Cymastrobus is represented by a single anatomically preserved specimen collected from a marine deposit of Famennian age in northern New South Wales. The cone was closely associated with a small lycopsid stem comparable in dimensions to its central axis. To preserve its entirety, we choose to investigate the cone structure by means of propagation phase-contrast X-ray synchrotron microtomography at ESRF, Grenoble. Spores extracted from the sporangia were observed in SEM at the University of Montpellier. A thin section of the cone axis was made for comparison with a transverse section of the small stem in order to test their conspecificity.Cymastrobus is a large bisporangiate cone consisting of tightly packed sporophyll-sporangium units helically arranged around a narrow axis. This axis is characterized by the wavy outline of its primary xylem and an invaginate pattern of trace emission. The cone is megasporangiate in its proximal third. The sporophylls are comprised of a long narrow pedicel widening distally and of a delicate distal lamina. Pedicels show an abaxial keel, and a distal heel forming an hexagonal shield protecting the sporangium. Megaspores and microspores are represented by the casts of their central bodies. Megaspore casts show several rows of circular pores around the trilete mark. Microspore casts show one small pore between the rays of the trilete mark and are assignable to the genus of sporae dispersae Endosporites. Pores like those of the mega- and microspore casts are known to correspond to papillae or laminated zones of the exospore in extant spores (Grauvogel-Stamm & Lugardon, 2004).The vascular system of the cone axis differs from that of the associated stem which belongs to a different lycopsid. The structure of Cymastrobus is similar to that of the Carboniferous Flemingites cones (Brack-Hanes & Thomas, 1983) but its spores are different. It is also close to that of two Famennian genera of uncertain affinities, Clevelandodendron (Chitaley & Pigg, 1996) and Bisporangiostrobus (Chitaley & McGregor, 1988), the latter sharing a similar type of spores but differing by its size and shape. The possession of spores with pores/papillae/laminated zones appears to be plesiomorphic and unhelpful to precise the systematic position of Cymastrobus within the basal members of the rhizomorphic lycopsids