Wood decay of Xenoxylon yunnanensis Feng sp. nov. from the Middle Jurassic of Yunnan Province, China

A permineralised gymnosperm wood, Xenoxylon yunnanensis Feng sp. nov., is described from the Middle Jurassic Shaximiao Formation in Huize County, Yunnan Province, Southwest China. The pycnoxylic wood consists of thick-walled tracheids and thin-walled rays. It is characterised by uniseriate, contiguous, compressed bordered pits on the radial walls of tracheids, and one large simple pit in each cross-field. Growth rings exhibit narrow latewood. The mean sensitivity of analysis of 25 growth rings is 0.2, suggesting that a climate with abundant rainfall and weak seasonality could have prevailed in Yunnan, at least locally, during the Middle Jurassic. The outer portion of the wood is largely decayed, and shows poorly preserved cells lacking details. Decayed tracheids displaying various features, including removal of middle lamellae, separations of secondary walls, presence of thickened corners and appositions, are recognised as cell reactions to fungal activity. Irregularly distributed micro-cavities are formed by complete decomposition of tracheids and rays in the wood. Abundant fungal remains occurring in the wood generally consist of multiple branched hyphae. These fungal hyphae are either confined to individual cell lumens or extend through adjacent cells. Although the probable reproductive structure and typical clamp connection are very rarely observed with few septate hyphae, the affinities of these fungal hyphae are somewhat obscure. Nevertheless, our study sheds light on the palaeoecology of Middle Jurassic terrestrial ecosystems of Southwest China. [Display omitted] •The first Mesozoic wood is described with details from Yunnan, China.•The wood is featured by compressed tracheid pits and window-like cross-fields.•It indicates a subtropical or tropical palaeoclimate with abundant rainfall.•A decay pattern and cell reactions of fungal infection are recognised in the wood.•Fungal remains contained in the wood may represent different fungal lineages.