MEDULLOPROTAXODIOXYLON TRIASSICUM GEN. ET SP. NOV., A TAXODIACEOUS CONIFER WOOD FROM THE NORIAN (TRIASSIC) IN NORTHERN BOGDA MOUNTAINS, XINJIANG, NORTHWESTERN CHINA

A permineralized coniferous wood, Medulloprotaxodioxylon triassicum Wan, Yang, Tang, Liu et Wang gen. et sp. nov., is collected from the Norian (Late Triassic) Huangshanjie Formation in Dalongkou section, Jimsar County, Xinjiang Uygur Autonomous Region, northwestern China. The fossil wood is composed of pith and, primary and secondary xylems. 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 many primary xylem strands and 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 tracheids and rays, presence of taxodioid cross-field pits, and abundant axial parenchyma, indicate M. triassicum is related to the taxodiaceous Cupressaceae sensu lato. By the comparison with fossil and extant species of the Cupressaceae, M. triassicum is mostly comparable to Sequoiadendron giganteum (Lindley) Buchholz. M. triassicum may represent an ancestral form of the Sequoioideae Saxton based on the anatomical characteristics of pith and secondary xylem. The occurrence of M. triassicum indicates that the taxodiaceous conifers had already individualized within the Cupressaceae in the Late Triassic.