Protocupressinoxylon baii sp. nov., a gymnospermous fossil trunk from the Upper Shihhotse Formation (Permian) of Yangquan City, Shanxi Province, North China

A new permineralized gymnospermous fossil trunk with only secondary xylem being preserved, Protocupressinoxylon baii sp. nov., is described from the Upper Shihhotse Formation (Permian) in Yangquan City, Shanxi Province, North China. The pith, primary xylem, and bark are not preserved. The wood is pycnoxylic, composed of tracheids, rays, and axial parenchyma. True growth rings are absent. However, growth interruptions are well-developed. In cross section, tracheids in the growth interruption vary gradually in diameter from thin-walled, larger cells to thick-walled, smaller cells, and change at an opposite direction, forming a symmetrical boundary. Radial tracheidal pitting is uni- to biseriate. When uniseriate, pits are contiguous or separately arrayed. When biseriate, pits are mostly alternately arranged. Rims of Sanio and tangential pits are absent. The percentage of abietinean pitting of current fossil wood is 23.6%, and the araucarian pitting is 76.4%. Axial parenchymatous cells are irregularly distributed among the xylem tracheids with opaque contents. Rays are homogeneous, commonly uniseriate, and are 1–24 cells high. The cross-field pitting is mainly of the cupressoid type, and there are commonly 1–2, rarely 3–4 pits in each cross-field unit. The fossil trunk represents the first record of Protocupressinoxylon Eckhold from the Permian of the Cathaysia. Fungal hyphae present in tracheids and rays, demonstrating the association between the current trunk and fungi. The absence of true growth rings, and the presence of growth interruptions suggest suitable climatic conditions with episodic drought for the flourishing of plants during the time when the Upper Shihhotse Formation was deposited in the Permian.