A New Species of Metacladophyton from the Late Devonian of China
The Late Devonian (earliest Frasnian) plant Metacladophyton ziguinum sp. nov. is described from Hubei Province, south China. Sterile main axes and first‐ and second‐order lateral axes possess linear spines that are sometimes dichotomous. Paired second‐order axes have a common base and occur oppositely and decussately on first‐order axes to form four vertical rows. With paired tips curving in opposite directions, sterile ultimate appendages are dichotomous, attached irregularly to first‐order axes and alternately/suboppositely to second‐order ones. Fertile ultimate appendages bifurcate equally three to five times and terminate in ovoid and erect sporangia in pairs. Metacladophyton ziguinum combines a protostele in the lateral axis and a dissected stele in the main axis. The primary xylem is U shaped and radially five ribbed in the protostele and consists of four V/U‐shaped or straplike ribs radially arranged in dissected stele. The mesarch primary xylem bears one or two protoxylem lacunae near the rib tip and is surrounded by secondary xylem containing uniseriate rays observed in transverse, tangential, and radial sections. The tracheid wall bears elliptical or circular bordered pits. In the protostele, a slightly ribbed column bearing an oval central hole originates from a nearly circular column via radial elongation of the primary xylem; through further radial elongation of the xylem and radial separation of the xylem between two adjacent ribs, the slightly ribbed column becomes a highly ribbed and U‐shaped column. In this process of stelar development, the secondary xylem occurs much earlier, gradually occupies the stelar center, and envelops the primary xylem. Metacladophyton ziguinum is assigned to Cladoxylopsida (sensu Berry and Stein), comprising Iridopteridales and Pseudosporochnales because of deeply bifurcate primary xylem ribs. In terms of a whole‐plant scenario of M. ziguinum, the dissected stele is suggested to be a synapomorphy of Cladoxylopsida, many members of which need better understanding in the morphological and/or anatomical characters. Based on anatomical evidence in M. ziguinum, one model is proposed explaining origination of the ribbed primary xylem, and an unequivocal secondary xylem is shown for first time in Cladoxylopsida. Metacladophyton ziguinum shows similarities with Iridopteridales in spiny axes, ultimate appendages with recurved tips, the protostele, permanent protoxylem strands, and larger metaxylem tracheids occurring along midplanes of ribs.