Permineralized Conifer Stems From The Late Permian Of Western Guizhou And Eastern Yunnan

As an important part of gymnospermous,conifers have been reported in the Late Carboniferous,and reached an extremely prosperous and diverse form during the Jurassic period,after that,it began to decline in the Cretaceous and Tertiary.Compared to the reports of a large number of permineralized conifers from the Carboniferous and Early Permian,there are only a few reports of permineralized conifers in the Late Permian.In this paper,three permineralized conifers stems from the Xuanwei formation of the Late Permian in western Guizhou and eastern Yunnan are studied in detail by using the method of cellulose acetate peel technique,and two new species are established:Xuanweioxylon damogozense sp.nov.,and Panxianoxylon resinia gen.et sp.nov..Xuanweioxylon damogouense comprises a heterocellular pith,primary and secondary xylem,the phloem and cortex are absent.Pith is divided into two parts by the cell types.The primary xylem is mesarch and indistinct.The secondary xylem is pycnoxylic with uniseriate scalariform bordered pits on the radial tracheid walls.Rays are normally uniseriate and 1-13 cells high.Cross-field normally has 3-19 bordered pits.The "leaf gap" was absent.Panxianoxylon resinia only preserved the secondary xylem,secondary phloem and cortex,the pith and primary xylem are absent.The secondary xylem is pycnoxylic with biseriate alternate bordered pits on the radial tracheid walls.Cross-field has 2-14 bordered pits.Rays are normally uniseriate and 2-17 cells high.The single leaf trace horizontally throughout the secondary xylem and extend into the secondary phloem.The secondary phloem is poorly preserved with sieve cells,fibers,parenchyma cells,phloem rays and dilated phloem rays.The cortex is preserved only in some areas.The cortex composes of parenchyma cells,secretory cells,resin canals and sclerenchyma cells.The characteristics of secondary phloem and cortex in Panxianoxylon resinia are similar to those of the cycads,but the characteristics of the leaf trace that it crossing the secondary xylem into the phloem indicates that our specimen may still be in the growth stage,so the proportion occupied by the bark in the stem could still be in the process of change.Additionally,the dilated phloem rays in the secondary phloem of our specimen indicate that our specimen cannot belong to cycads.The research of Xuanweioxylon damogouense and Panxianoxylon resinia not only increased our knowledge of the classification diversity of conifers from the Late Permian,but also made us have a further understanding of the anatomical structure,evolutionary differentiation and ecology of the early conifers.And it also has a great significance to reveal the phylogeny and early evolution of conifers.Moreover,both of Xuanweioxylon damogouense and Panxianoxylon resinia without obvious growth rings indicate that the seasonal change in fossil origins was not obvious.The characteristics of the leaf trace continuously to pass through several growth rings indicating that the leaf has a relatively long life span,so Panxianoxylon resinia could be an evergreen plant.