Reconsidering Differences between Cladoxylopsida and Iridopteridales: Evidence from Polyxylon australe (Upper Devonian, New South Wales, Australia)

A new permineralized specimen of Polyxylon australe collected from the type locality at Barraba in New South Wales, Australia, represents a portion of stem showing a node in its proximal part. Analysis of its primary vascular system and pattern of branch trace emission indicates that the plant had a hierarchized architecture. The stem, ca. 2.5 cm wide, is radially symmetrical and bears a whorl of nine or 10 branches followed by a long internode. The bilateral symmetry of first-order branches, assessed from trace arrangement in individual branch bases, is a trait previously reported in three other nonpseudosporochnalean cladoxylopsid genera, Pietzschia, Cladoxylon, and Panxia. Xylem maturation in the stele of the new stem is exarch, but at node level, one mesarch strand of protoxylem differentiates around a cavity filled with thin-walled cells. The diagnosis of P. australe is emended, and the significance of architectural characters for distinguishing the Cladoxylopsida from the Iridopteridales is discussed. Polyxylon is the second and the best-known cladoxylopsid genus reported from the Devonian of Australia.