SILICIFIED GYMNOSPERMOUS ROOT FROM THE UPPER WUTONGGOU LOW ORDER CYCLE (CHANGHSINGIAN-INDUAN) IN BOGDA MOUNTAINS, NORTHWESTERN CHINA

Anatomy of the root reflects its origin, its subterranean environment, and its function. Fossil roots of seed plant from the upper Palaeozoic have received more and more attention. However, most of them are found from the Devonian and the Carboniferous. Gymnospermous roots with anatomical characteristics from the Permian and the lower Mesozoic are much less known. Here we present new data on the silicified roots collected from the upper Wutonggou low order cycle (Changhsingian-Induan) in Bogda Mountains, northwestern China. Root bear clusters of rootlets. Roots are protostelic, composed of xylem, phloem, and periderm. Primary xylem is exarch, diarch, with spiral thickenings on radial tracheidal walls. Secondary xylem is composed of radially arranged tracheids with uni- to triseriate araucarian pitting on the walls, and uniseriate rays. Cross-field pitting is araucarioid, with 6-12 bordered pits in each field. Phloem is commonly compact, filled with opaque contents. Primary xylem of the rootlet is commonly diarch. Phloem and cortex are preserved on the outside of the xylem. The presence of coprolites and borings within roots suggest well developed plant-arthropod interactions during that time. Root growth partly reflects the productivity of the plant, but is strongly influenced by substrate conditions. Therefore, the absence of growth ring in the roots and rootlets from the Bogda Mountains indicates a stable subterranean environment in the northeastern Pangaea during the deposition of the upper Wutonggou low order cycle.