The Anatomically Preserved Marattiales Rachises From The Late Permian Of Western Guizhou And Eastern Yunnan

Marattiales is an important component of the Late Permian Euramerican-Cathaysian floras.At present,the study of Marattiales from the Euramerica flora has been more indepth,while the related research from the Cathaysian flora is still relatively weak.A large number of Marattiales fossils have been collected from the Xuanwei Formation of Late Permian,western Guizhou and eastern Yunnan.Such as stems,pinna rachises,petioles,reproductive pinnae,etc.These findings provide abundant fossil materials for studying the anatomical structures of various organs of Marattiales and the restoration and evolution of Marattiales.In this paper,three rachis fossils of Marattiales from the Xuanwei Formation of Late Permian,western Guizhou and eastern Yunnan,have been studied by the acetate peel method which is the commonly method to the permineralized plant fossils.Two of them were classified into Compsopteris: Compsopteris elliptica and Compsopteris sp.,And the other one was classified into Rothwellopteris: Rothwellopteris sp.,and its evolutionary significance is also discussed.The vascular bundles of rachis fossils which belong to Compsopteris are 4 cycles or 2 – 3 cycles.The primary xylem is endarch,protoxylem tracheid possesses spiral thickening,and metaxylem tracheid possesses scalariform thickening.Vascular bundles sheath is present.The ground tissue between the vascular bundles is composed of secretory cells and parenchyma cells with brown substances.The cortex could be divided into two zones: the outer zone consisting of parenchyma cells and secretory cells with slightly-brown substance,and the inner zone is composed of sclerenchyma cells.These anatomical characteristics are recognized as typical of the rachis of Marattiales.Compared with the rachis fossils with C-type and W-type monocyclic reported in the Carboniferous in Euramerican and the Early Permian in Taiyuan,Shanxi,China,the Marattiales fossils studied in this paper not only have more complicated structures but also have a later geological age,it may represent a more evolved type of Marattiales rachis.Rothwellopteris sp.are at least bipinnate,rachis robust,main rachis up to 5 mm in diameter and the pinna is lingulate.The type of bundles structure is multi-vascular(> 40),vascular bundles like a strip,the length decreases from the outside to the inside and have the typical characteristic of vascular bundles of Marattiales.This kind of multi-vascular bundles structure is already similar to the rachis structure of modern Marattiales,and it is an evolutionary type.Rothwellopteris sp.has the mixed characteristics of Late Paleozoic Marattiales Psaronius(tree-fern)and modern Marattiaceae.That is,it with more than two large pinnate compound leaves and multi-vascular bundle structures,etc.Which represents the transition type from the Late Paleozoic Marattiale to modern Marattiales,and it revealing the evolutionary relationship between the modern Marattiales and Late Paleozoic Marattiales tree-fern Psaronius.That is,the modern Marattiales is evolved from the Late Paleozoic Marattiales,and this development and evolution did not occur in the Triassic,but at least in the Late Permian.In the Late Permian strata of the western Guizhou and eastern Yunnan,there are not only abundant fossils of the various organs of Marattiales,but they also exhibit many unique features,such as different types of leaf traces,etc.At the same time,the primitive types and the different evolutionary types of Marattiales also be found in the meantime.So,it is speculated that the Late Permian period in the western Guizhou and eastern Yunnan area may have been a diversity evolution center of Marattiales.