Megaporoxylon sinensis sp. nov., a new coniferous trunk from the Upper Triassic of northern Bogda Mountains, northwestern China

Abstract

A coniferous trunk, Megaporoxylon sinensis sp. nov., is described from the Carnian–Norian (Triassic) Huangshanjie Formation in the Dalongkou section, Jimsar County, Xinjiang, northwestern China. The trunk is composed of pith, primary and secondary xylem. The pith is solid, circular, heterocellular, with pitted parenchyma and secretory cells and canals. The primary xylem is endarch, with spiral and scalariform thickenings on tracheidal walls. The secondary xylem is pycnoxylic, consisting of tracheids and rays. Radial tracheidal pitting is araucarian. Tangential pitting and axial parenchyma are absent. Uniseriate pits with oval apertures are commonly present on radial tracheid walls. Biseriate pitting occurs only on radial walls of the first to fifth tracheids outward the primary xylem. Rays are parenchymatous, uniseriate, and 1–7 cells high. Cross-field pitting is window-like or phyllocladoid. Previous records of species of Megaporoxylon Kräusel were limited to the Southern Hemisphere during the Carboniferous to Triassic. Its occurrence from the Upper Triassic of northern Xinjiang is the first report in the Northern Hemisphere and suggests a floristic exchange between Gondwana and Laurasia. The new trunk and two previously described fossil stems, including Medulloprotaxodioxylon triassicum Wan et al. and Xenoxylon junggarensis Wan et al., demonstrate that conifers were important elements of Carnian–Norian terrestrial ecosystems and had a higher diversity in northern China than previous thought. The growth ring pattern of the trunk and palaeogeographic reconstruction of the research area at high latitudes suggest that the growth of the trees was limited by the seasonality of the light regime.