Rooting System Of Arborescent Lycophytes From The Taiyuan Formation In Wuda Coalfield,Inner Mongolia

Arborescent lycophytes are important members of swampy forests during the late Paleozoic.Although the rooting system of arborescent lycophytes has been studied for three centuries,there remains numerous problems to be solved.So far,materials with both morphology and anatomic information are reported mainly from Euramerica.The present research is based on the rooting system of arborescent lycophytes preserved in two intervals from the Taiyuan Formation in the Wuda Coalfield,Inner Mongolia.The morphology and anatomical structures of these rooting systems are described systematically.Their ecological implications of downward axes and the evolution of rooting systems of arborescent lycophytes in Cathaysia is discussed.Stigmaria asiatica Jongmans et Gothan was erected for the rooting system of Carboniferous and Permian arborescent lycophytes in East Asia.It is characterized by relatively slender axes and loose rootlet scars.However,Stigmaria asiatica was neglected by most palaeobotanists.Most slender axis of stigmarian rooting axes in China are assigned to S.ficoides.Materials in this thesis come from a siltstone layer between Coal No.9 and Coal No.10 and a volcanic tuff between Coal No.6 and Coal No.7,respectively,in the Taiyuan Formation of the Wuda Coalfield.Similar lycopsid rooting systems have been found in these two beds.They are identified as Stigmaria cf.asiatica and Stigmaria asiatica Jongmans et Gothan,respectively.Both have downward axes,showing advantages in absorbing deeper water and nutrition comparing with those rooting systems only developing horizontal axes.This thesis describes the morphology and anatomical structures of the rooting system of Stigmaria asiatica in the volcanic tuff for the first time.Three types of anatomical structures of S.asiatica are figured out according to the features of stele and the growth pattern of rootlet trace.Type I:rootlet trace and its associated ray is broad in cross section;secondary xylem wedge-shape and distributed loosely in cross section;rootlet trace and associated ray appearing fusiform in tangential section,with a wide base and tapering top.Rootlet originates from the pith,passing through stele obliquely,bending at the edge of the inner cortex,extending to the edge of the middle cortex vertically,bending again and extending out of the outer cortex horizontally.The main difference between the Type ? and Type ? is that rootlet trace and associated ray are narrower in cross section,and the secondary xylem is relatively complete.The growth pattern of the rootlet trace of Type ? is similar to Type ?.The anatomical structure of stele Type ? is similar to the Type ?,but its rootlet traces pass through stele and cortex horizontally.From an anatomical point of view,differences between S.asiatica and S.ficoides are distinct.And the establishment of this species is necessary.In addition,three types of anatomical characteristics of S.asiatica indicate that there would be at least three different type of arborescent lycopsids after the falling down of the volcanic ash.Stigmaria Brongniart has four main axes arising from the base of the trunk.They bifurcate 2-3 times and spread only horizontally.The rooting systems discovered in Wuda Coalfield have developed downward axes,which implies a new evolutionary stage of the rooting system of arborescent lycophytes.This type of rooting system could have extended deep downward in the substrate.It helps to explain why arborescent lycophytes have almost been extinct in Euramerican but survived in Cathaysia when the Euramerican climate became drier near the Moscovian-Kasimovian boundary.Combine with stigmarian rooting systems in the upper Devonian in Xinhang,Anhui Province,and the extanding obliquely Stigmaria in the upper Taiyuan Formation in Fucheng and Benxi Formation in Yangquan of Shanxi Province.It shows that the rooting systems of arborescent lycophytes in Cathaysia are generally different from those in Euramerica in morphology,anatomy and ecology.These uniqueness of rooting system supports the existence of the oriental lycopsids in Cathaysia.