A structurally preserved cycad-like stem, Lioxylon liaoningense gen. et. sp. nov., from the Middle Jurassic in Western Liaoning, China

摘要

A cycad-like fossil stem, Lioxylon liaoningense gen. et sp. nov. is described based on structurally preserved material from the Jurassic in western Liaoning, China. The specimens consist of seven permineralized stems and occur from the Middle Jurassic Tiaojishan Formation in Changgao Town, Beipiao City, Liaoning Province. The stems are long and cylindrical to pyriform in shape. The apical part of the stem is covered with an armor composed of helically arranged petioles and cataphyll bases. The pith and cortex are large, consisting of medullary bundles, mucilage sacs, parenchyma, transfusion and sclerotic cells. They are surrounded by two rings of xylem cylinders, the inner one centripetal and the outer centrifugal. The vascular bundles in the petiole and cataphyll base consist of a two bundle series: one is arranged in an arc along the adaxial margin of the petiole and the cataphyll bases, and the other one is a poorly-developed omega or irregular shape and extends from the center to the abaxial side. This anatomical evidence indicates a close systematic affinity of Lioxylon with fossil and living Cycadales. This represents the first systematic description of an anatomically preserved fossil cycad-like stem in China.