Fossil Wood Assemblage From The Xishanyao Formation Of The Santanghu Basin In Xinjiang And Its Paleoclimatic And Paleoenvironmental Implications

The Jurassic is a critical geological period for the diversification and evolution of gymnosperms.As an essential component of Palaeobotany,the study of fossil wood not only provides a comprehensive understanding of the floral composition in the Jurassic and the evolution history of wood,but also sheds light on the palaeoclimate,palaeoenvironment,and palaeoecology,as well as terrestrial ecosystems.Previous palaeobotanical studies have been focused on plant impressions and sporo-pollen assemblages from the Jurassic of the Santanghu Basin in Xinjiang Uygur Autonomous Region(Xinjiang),lacking a systematic study on fossil wood.Therefore,the floral composition,palaeoclimate,palaeoenvironmental conditions and palaeoecology remain poorly understood.Based on 2777 fossil wood specimens from 19 beds(including two beds with in-situ preserved tree stamps)of the Xishangyao Formation in the Naomaohu section of the Santanghu Basin,Xinjiang,a systematic anatomical study on fossil woods has been conducted in the current dissertation by using optical and SEM examinations.Subsequently,combined with field observations and analysing various parameters of the growth rings,quantitative studies have been applied on the palaeoclimate,palaeoenvironment,leaf phenology,plant–fungal interactions,forest productivity,and forest reconstruction.Based on the anatomical characteristics of 106 fossil specimens,seven species belonging to five genera(including two new genera and six new species)of fossil wood species(Agathoxylon sp.,Ductoagathoxylon wangii Gou et Feng 2021,Ductoagathoxylon naomaohuense Gou sp.nov.,Protophyllocladoxylon yiwuense Gou et Feng 2021,Santanghuoxylon mirablis Gou gen.et sp.nov.,Santanghuoxylon liaoi Gou sp.nov.,Yiwupitys elegans Gou gen.et sp.nov.)and one fossil root species(Amyelon sp.)were recognised.Statistic analysis indicates that Agathoxylon and Protophyllocladoxylon are the predominant components of the current fossil wood assemblage.Quantitative analyses on various parameters of growth rings in 58 wood specimens(>0.1 m in diameter)revealed that the current wood assemblage is characterised by a relatively wider mean value of the Growth-ring Width,a lower mean value of the Mean Sensitivity,and a higher value of the Ring Markedness Index.These data suggest the climate of the study region was warm,humid,and stable with regular and periodical change in the late Middle Jurassic.Analysis of the Growth-ring Width values indicates that forest productivity of the current wood assemblage was three times higher than those nowadays forests in the mid-latitude.CSDM values and the Ring Markedness Index analyses indicate the current wood assemblage possesses both deciduous and evergreen plants.Tree height estimation reveals that the trees were 10.47–43.95 m high,with an average of 28.64 m.The mean value of the Nearest Neighbour Index of in-situ preserved tree stamps in an area of 7656 m~2 is 1.1,indicating a random distribution.Wood decay structures were observed in the secondary xylem of a few fossil wood specimens,resembling the white-rot decay patterns caused by basidiomycetes in extant forest ecosystems.The common occurrence of septa-like structures in the tracheids is likely representing the host cell reactions to fungal affections.This is the first systematic study on fossil woods in the Xishanyao Formation of the Naomaohu section of the Santanghu Basin,Xinjiang.This study sheds new light on plant diversity,floral composition,forest palaeoecology,and palaeoclimatic and palaeoenvironmental conditions in the late Middle Jurassic.