Medulloprotaxodioxylon triassicum gen. et sp. nov., a taxodiaceous conifer wood from the Norian (Triassic) of northern Bogda Mountains, northwestern China

A permineralized coniferous wood, Medulloprotaxodioxylon triassicum Wan, Yang, Tang, Liu et Wang gen. et sp. nov., is described from the Norian (Late Triassic) Huangshanjie Formation in the Dalongkou Section, Jimsar County, Xinjiang Uygur Autonomous Region, northwestern China. The fossil wood is composed of the pith, primary and secondary xylem. The pith is solid, circular, heterocellular, with numerous isolated or clustered secretory cells, and parenchyma. Secretory cells commonly form a network in a radial view. The pith is surrounded by numerous primary xylem strands and about 22 distinctive leaf traces. The primary xylem is endarch. Tracheids of primary xylem have helical and scalariform thickenings. The secondary xylem is pycnoxylic, composed of tracheids, rays and axial parenchyma. The general aspect of the tracheids and rays, presence of taxodioid cross-field pits, and abundant axial parenchyma, indicate M. triassicum can be related to the taxodiaceous Cupressaceae sensu lato. By comparison with fossil and extant species of the Cupressaceae, M. triassicum is most comparable to Sequoiadendron giganteum (Lindley) Buchholz. It is hypothesized that M. triassicum represents an ancestral form of the Sequoioideae Saxton based on the anatomical characteristics of the pith and secondary xylem. The occurrence of M. triassicum would indicate that the taxodiaceous conifers had already individualized within the Cupressaceae during the Late Triassic.