The Spatiotemporal Distribution Pattern Of The Devonian Spores And Acritarchs And Their Significance

The Devonian Period is characterized by important changes that took place in both terrestrial and marine biospheres.They were possibly triggered by the diversification of the earliest forests,and by the blooms of nektonic animals in the oceans.The Devonian is also a crucial time for the evolution of both terrestrial palynofloras and marine organic-walled phytoplankton(i.e.,acritarchs and prasinophytes).The diversities of the terrestrial palynoflora increase steadily,while the phytoplankton experienced a drastic declinethrough the Devonian,which has been called by some authors as the late Palaeozoic “phytoplankton blackout”.Both terrestrial palynofloras and marine phytoplankton have proven to be extremely useful in solving palaeoenvironmental,palaeogeographical,and biostratigraphical questions for the Palaeozoic strata.More than three hundred palynological samples have been collected from seven sections of Western Junggar and South China and have been macerated in the laboratory for microfossils during past years.In total,55 acritarch species belonging to 30 genera and 54 spore species belonging to 35 genera have finally been recognized(including ‘?',‘cf.',‘sp.' species and genus).A new combination is proposed: the acritarch species Cymatiosphaera enteichia(Wicander and Loeblich,1977)comb.nov.based on the main characters of the two genera.The thesis reports for the first time on the recovery of the Crassiangulina tesselita Jardiné,Combaz,Magloire,Peninguel and Vachey,1972,consideredhere to be the early synonym of the the Veryhachium improcerum Wicander and Loeblich,1977.Based on this understanding,the geographical distribution of Crassiangulina tesselita have been dispersed into Laurussia and the Kazakhstan Plate,rather than just restricted to the high palaeolatitude Perigondwana regions as previouslly reported by other authors;The spore assemblage can be compared with the East Europe spore assemblage zone,i.e.,Cyrtospora cristifer – Diaphanospora zadonica(CZ)Zone and Knoxisporites dedaleus – Diducites versabilis(DV)Zone from West Europe.The spore assemblages indicate that the lower part of Lower Member of Hongguleleng Formation is of the Famennian in age,and there is no Frasnian deposition in this formation;There may be a stratigraphical gapbetween the bottom of Hongguleleng Formation and the underlying top of Zhulumute Formation,and this stratigraphical gap is most likely caused by the eustatic regressionglobally at the latest Frasnian and earliest Famennian period.Therefore,there may beno deposition of the F-F events in such areas of western Junggar;Based on the appearance of Cf.Retispora lepidophyta(Kedo)Playford,1976,the top of the Middle Member of Hongguleleng Formationshould at least be of the late Famennian in age.Palynofacies analysis suggests that the Lower Member of the Hongguleleng Formation has experienced the evolution of the nearshore – offshore – deep sea – offshore environment.The thesis briefly reviews the development history of Chinese Devonian palynology and establishes a new database of the Devonian spores,very little pollen and acritarchs from China.The database of the plant microfossils contains 4568 occurrences belonging to190 genera in China,and 3363 occurrences assigned to 175 genera in South China,respectively.The sample-based rarefaction analyses show a pattern with an increasing trend until the Gevitian peak,followed by a plateau persisting through the Late Devonian.This trend is strikingly similar to both plant mega-and micro-fossil diversity patterns at global scales.Furthermore,the plateau in the Late Devonian of the Chinese plant microfossil diversity also confirms the observations of the absence of a F-F depletion of diversity like those that occurred in shallow water realms in one of the “Big Five” mass extinctions.The geochemical pattern of molybdenum(Mo)and Ceanom seems to be consistent with the terrestrial microfossil diversity trajectory,indicating that the Devonian evolution of the forests played a significant role in influencing the atmospheric oxygen levels and carbon cycle.The rise of plants appeared to have been essential for an oceanic oxygenation event during the mid-Devonian,rather than the Late Devonian.During the specific historical period,the fluctuation of sea-level affected the terrestrial palynofloras in two ways: on the longer term,sea-level rise increased the accumulation of terrigenous clastic rock,which was important to the discovery of the terrestrial palynofloras.On shorter timescales,sea-level rise,however,and the accumulation of carbonate platforms in lower latitudes may have caused a habitat loss,in turn,for the terrestrial palynofloras,and consequently,a diversity loss of the palynofloras.It seems like that the diversities of marine invertebrate and plant microfossils during the Devonian have strongly interacted with the climate changes.The rather hot climate during the Late Devonian(from the latest Frasnian to early Famennian)in the low latitudes makes equatorial miospore assemblages reached their maximum distribution range.To some extent,the hot climate may have limited the plant diversity increase during the Late Devonian.In addition to the above,a Late Devonian phytoplankton palaeobiogeography is established quantitatively.The results show that three phytoplankton palaeobiogeographical realms could be recognized,i.e.,the West Gondwana realm,East Gondwana realm,and Boreal realm.There is a high degree of similarity between acritarch assemblages in Euramerica and East Gondwana(especially those in Australia and Iran).Portugal(Iberian-Armorican block)was situated to the north of Algeria and acted probably as a stepping stone between East Gondwana and Euramerica.The acritarch province that had been typically confined to the higher latitudes was still existed,while its characteristic genera dispersed gradually into the lower latitudes during the Late Devonian.Current-and latitude-influenced palaeotemperature and oceanic circulation patterns are considered to have been the major determinants of the evolution of acritarchs and the geographical distribution in the Late Devonian.