A new conifer stem, Yiwupitys elegans, from the Yiwu Jurassic Forest, Hami, Xinjiang, Northwest China

Abstract

Fossil wood not only provides a comprehensive understanding of the ancient floral composition and the evolution history of wood through time, but also sheds light on the paleoclimate, paleoenvironment, and paleoecology. Permineralized conifer wood commonly occurs in the Jurassic of China; however, the detailed anatomical study has been sparsely conducted on the Jurassic wood from Xinjiang Uygur Autonomous Region (Xinjiang), Northwest China. Here, we describe a unique permineralized stem, Yiwupitys elegans Gou et Feng gen. nov. et sp. nov. from the Middle Jurassic Xishanyao Formation in Naomaohu Town of Yiwu County, Hami City, Xinjiang, Northwest China. The fossil stem is preserved with pith, primary xylem, and secondary xylem. The solid pith is heterocellular and characterized mainly by clustered or isolated secretory cells and pitted parenchymatous cells at the pith periphery. The primary xylem is 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. Growth rings are distinct. The secondary xylem tracheids are featured commonly by bent ends overlapping each other. They have mostly uniseriate or biseriate, or partially triseriate alternately or oppositely arranged bordered pits on the radial walls and uniseriate or biseriate bordered pits on the tangential walls. The rays are uniseriate and up to sixteen cells high. There are four to eight cupressoid pits in each cross-field, arranged in the araucarioid type. Leaf traces initiate from the pith margin and possess a single vascular strand in each trace. The anatomical structures indicate that Yiwupitys elegans has a closer affinity with conifers, although it shows few characteristics of ginkgophytes. Our study shed new light on the plant diversity of the Yiwu Jurassic Forest.