New record of fossil wood Xenoxylon from the Late Triassic in the Sichuan Basin, southern China and its paleoclimatic implications

Fossil wood is one of the significant proxies for terrestrial paleoclimate and paleogeographical reconstruction in Earth history. Abundant and diversified Mesozoic fossil woods are well recorded in China; however, Triassic fossil wood is very scarce. Here, we report a new fossil wood from the Late Triassic Xujiahe Formation (Norian to Rhaetian) in Guangyuan of northern Sichuan Basin, southwestern China. The fossil wood material consists of two well-preserved specimens yielding secondary xylem with distinct growth rings. Bordered pits on the radial tracheid walls are mostly contiguous, biseriate alternate, locally uniseriate and strongly flattened. Cross-fields show a large window-like pore. This anatomy is typical for the important fossil wood morphogenus Xenoxylon Gothan, and, based on a novel combination of radial and cross-field pitting type and ray height, a new species, Xenoxylon guangyuanense sp. nov. is recognized. The finding of this new fossil wood taxon contributes to a better understanding of the yet poorly documented Xenoxylon early radiation during the Late Triassic, as well as of the origin of X. meisteri group, a peculiar endemic group which diversified in Far-East Asia from the Triassic to the Early Cretaceous. Xenoxylon is a paleobiogeographically significant genus, being bound to cooler and/or wetter climates of Northern Hemisphere throughout its Late Triassic-Late Cretaceous range. The occurrence of Xenoxylon in the Sichuan Basin of southern China may indicate a short-term cooling event, sandwiched within a period during which warm and wet climate conditions largely prevailed over lower latitude regions of the Northern Hemisphere. Such a cooling event in the Sichuan Basin which was located at a low paleolatitude on the eastern rim of Tethys may be in accordance in time with the temperature decline event in Norian-Rhaetian boundary uncovered by oxygen isotope (δ18O) record in Italy of the western rim of Tethys.