A new conifer stem, Ductoagathoxylon wangii from the Middle Jurassic of the Santanghu Basin, Xinjiang, Northwest China

Abstract

A new permineralized stem, Ductoagathoxylon wangii Gou et Feng, is described from the Middle Jurassic Xishanyao Formation in Naomaohu Town of Yiwu County, Hami City, Xinjiang Uygur Autonomous Region, Northwest China. The fossil stem is preserved with pith, primary xylem, and secondary xylem. The solid pith is heterocellular, and characterized by the presence of regularly arranged clusters of secretory cells at the periphery. The primary xylem is endarch to mesarch, showing annular, helical, and scalariform thickenings on the tracheid walls from the protoxylem to the metaxylem. The secondary xylem is pycnoxylic and exclusively composed of tracheids and parenchymatous rays. The secondary xylem tracheids have mostly uniseriate, partially biseriate, and rarely triseriate alternately arranged contiguous bordered pits on their radial walls. The rays are uniseriate and up to ten cells high. There are five to nine cupressoid pits in each cross-field, arranged in the araucarioid type. Growth rings are distinct and are characterized by a narrow band of latewood. This is the third fossil stem described from the Middle Jurassic of China that is preserved with anatomical structures of pith, primary, and secondary xylem, thus shedding new light on the diversity and evolution of conifers. The occurrence of a traumatic scar in the stem indicates that the tree was probably damaged by an abiotic cause, but it successfully survived through physiological responses to the wound.