The anatomy of Rotoxylon dawsonii comb. nov (Cladoxylon dawsonii) from the Upper Devonian of New York State

Cladoxylon dawsonii (Dawson) Read was reexamined by sectioning and analysis of a portion of Dawson's original specimen. The primary xylem consists of 18 radially oriented and elongate primary xylem ribs. Toward the center of the system, some xylem ribs are laterally continuous with adjacent ribs, forming elongate xylem segments, whereas others appear discrete. Order of xylem maturation is mesarch, with one to three protoxylem strands located near the tip of each primary xylem rib. Primary phloem immediately surrounds each primary xylem rib and consists of sieve cells. To the outside of the primary phloem, a conspicuous ground tissue is comprised of roughly isodiametric parenchyma with dark contents in their lumina as well as parenchyma cells that are longitudinally elongate with clear lumina. Beyond the vascular column, a fragmentary cortex contains numerous vascular traces embedded in parenchyma. These latter cells are surrounded by conspicuous masses of longitudinally elongate cortical thick-walled parenchyma. Vascular traces depart from the tips of the primary xylem ribs in a whorled phyllotaxis. Most traces emerge along the midplane of the primary xylem ribs. However, some depart at an oblique angle. This reinvestigation of an Upper Devonian specimen, known since 1882, provides significant new information on stelar architecture, trace departure, tissue histology, and phyllotaxis. Our study indicates systematic placement of the plant within the Iridopteridales and the recognition of a new iridopterid genus, Rotoxylon. New anatomical evidence indicates Rotoxylon is unusual, exhibiting diagnostic traits of both Cladoxylopsida and Iridopteridales, thus challenging current phylogenetic interpretation of both groups.