Early And Middle Triassic Trace Fossils From South And Northwest China And Response For Biotic Recovery Of Marine Ecosystems

More and more studies have shown that the ecological proxies of trace fossils can accurately reveal the pattern and process of the recovery of marine ecosystems,and provide evidence for exploring the mechanisms of the co-evolution of organisms and environment in major geological catastrophes.Therefore,this dissertation aims at studying the Early and Midlde Triassic ichnocoenoses,according to more than 20,000 specimens or field pictures collected both from South and Northwest China.47 ichnospecies in 33 ichnogenera were described here.By using quantitative measurements of their ecological characteristics,such as burrow size,ichnodiversity,tiering level,behavioral complexity,and the bioturbation level of the host strata,we can assess the recovery stages of trace-making organisms.On this basis,we compared the new data with coeval ichnoassemblages reported from elsewhere in the world.We reported here for the first time two Indaun(early Early Triassic)ichnofossils sites from the Houzhougongmiao(HZGM)section and the Zhihema(ZHM)section,northwest China.The HZGM ichnoassemblage contains 16 ichnogenera and is dominated by shallow-tier Asteriacites and Biformites produced by ophiuroids,and the scratch marks or trackways Dimorphichnus,Diplichnites,and Monomorphichnus produced by arthropods.Several complex burrow forms such as Thalassinoides,Diplocraterion,and Rhizocorallium are excluded.The ichnofossils-bearing strata possess relatively low bioturbation level,and the burrow sizes are rather small,and a mixed layer is missing.When compared with coeval ichnoassemblages reported elsewhere in the world,the HZGM ichnofaunas differ clearly with others,except for the similar ichnodiversity,thus the HZGM ichnoassemblage indicates the initial recovery of the trace-maker ecosystem after the Permian-Triassic mass extinction.Trace-makers such as ophiuroids and arthropods were perhaps opportunistic organisms that proliferated in marginal marine settings.A total of 12 ichnogenera were identified from the lower part of the Xiahuancang Foration in the ZHM section.Of these,vertical burrows of Diplocraterion and Rosselia are very abundant and conspicuous in the field,as well as complex burrow Thalassinoides and Rhizocorallium.Burrow sizes are relatively large,Diplocraterion is particularly large,with tube diameters of 7?20 mm.Diplocraterion extended to a depth of 15?30 cm into the sediment,indicating an extremely high tiering level in the early Induan.The ichnofossils-bearing strata were highly bioturbated.A diverse ichnoassemblage is also identified from the upper part of the Xiahuancang Formation.Similar with the lower part,the vertical forms Diplocraterion and Skolithos are also commonly found in the upper part,as well as Rhizocorallium.Burrow sizes are smaller than those of the lower part,with more horizontal forms such as Gyrochorte,which differ from the lower ichnoassemblge.Thus the ZHM ichnoassemblages indicate the higher recovery of the trace-maker ecosystem.The diverse Induan ichnoassemblages from around the world,all proliferated in shallow marine environmental settings,indicating that the trace-makers may have an environmental preference for shallow marine settings.In few cases,the Induan trace-maker organisms proliferated in specific environmental settings,however,these specific environment types show obviously limitations,the similar environment settings do not ensure the proliferation of Induan ichnocoenoses,implying that environmental settings may not be the only factor affecting the post-extinction recovery of the trace-makers,and that other elements such as oxygen level,sedimentation rates,salinity fluctuations,eutrophication,ocean acidification,harmful temperatures,and taphonomic conditions may have been equally important.Clearly,the low ichnodiversity cannot be taken at face value as an indicator of environmental stress in all cases.Alternatively,the high ichnodiversity cannot always indicate environmental amelioration.Therefore,when treating ichnodiversity as a key proxy assessing the recovery levels of trace-makers,all of other ichnoecologic measures,as well as other factors should be taken into account.The post-extinction proliferation of ichnocoenoses appears no latitudinal preference and occurred in most latitudes of northern hemisphere.A late Smithian ichnoassemblage is reported from the Lower Triassic succession from the Qiyueshan section,Lichuan City South China.This ichnoassemblage comprises 13 ichnogenera,which includes simple,horizontal traces,vertical traces,oblique-or horizontal branching traces,and slightly complex burrow networks,suggesting a fairly high degree of behavioral complexity.The Lichuan ichnofauna shows an abrupt increase in ichnodiversity,burrow width,behavioral complexity,ecologic tiering level,as well as the highly bioturbated strata,which are indicative of higher recovery stages in the late Smithian.When compared with coeval ichnoassemblages from other sections in South China and coeval ichnoassemblages from elsewhere in the world,the Lichuan trace-fossil assemblage reaches similar level of diversity during this time interval.The late Smithian recovery pulse is typically confined to the offshore transition setting.The global ichnoassemblages in the Spathian show similar characteristics: ichnodiversity is always rathe high,which may be higher than those of the Smithian,several ecologic proxies,such as burrow size,complexity,tiering level and bioturbaiton are also higher than those of the Smithian.Differing from the several diverse Induan ichnoassemblages,most ichnoassemblages show similar high recovery stages,which indicate the overall improvement of the marine ecosystem.Two Anisian(Middle Triassic)marine ichnocoenoses are reported from the Boyun and Junmachang(JMC)sections,Yunnan Province,southwestern China.The Boyun ichnoassemblage contains 6 ichnogenera and is dominated by robust Rhizocorallium and Thalassinoides while the JMC ichnoassemblage is characterized by the presence of Zoophycos.Zoophycos is present in the early Anisian strata of the JMC section,and represents the oldest known occurrence of this ichnogenus following the latest Permian mass extinction.Its first reappearance in the early Anisian of South China has significance in terms of ichnodiversity and complexity of infauna in the earliest Mesozoic.The ichnofossil-bearing successions are usually highly bioturbated in both the Boyun and JMC sections.Rhizocorallium has maximum tube diameter up to 28 mm,while Thalassinoides maximum tube diameter is 22 mm.Complex burrow networks(i.e.,Rhizocorallium,and Thalassinoides)extended to a depth of 10 cm into the sediment,indicating a high level of tiering.Similar to coeval ichnoassemblages elsewhere in the world,the Yunnan ichnocoenoses embrace a relatively low ichnodiversity,but their burrows usually penetrate deeply into the sediment,and include large and complex Rhizocorallium and Thalassinoides.All of these ichnologic features are indicative of recovery stage 4 after the latest Permian crisis.When compared with some important Early Triassic(mainly Spathian)ichnoassemblages worldwide,the Anisian ichnocoenoses examined for this study are slightly less diversified,and possess more or less the same maximum burrow sizes,but the penetration depth of burrows and the distribution of burrow sizes are much larger than those from the Early Triassic.Rhizocorallium and Thalassinoides have much larger burrow sizes than the same ichnotaxa from the Lower Triassic,implying that ichnocoenoses may have recovered in Spathian,but did not stabilize until the Anisian.It is worthy of note that the lower ichnodiversity of the Anisian ichnocoenoses may have resulted from intense bioturbation by deeper tiers,representing a taphonomic product that is totally unrelated to environmental stress.Rhizocorallium is characterized by relatively complex spreite structures,indicating complicated behaviors of trace makers,and thus it has also been one of the most important ichnotaxa in quantifying recovery stages reached by biota in the aftermath of major mass extinctions.Rhizocorallium is most useful among Early–Middle Triassic ichnotaxa,not only because of its distinctive morphology and occupation of relatively deep ecologic tiers,but also due to its unique stratigraphic occurrence in Lower and Middle Triassic successions.This paper aims to document abundant Rhizocorallium ichnospecies from the Lower and Middle Triassic successions of 10 localities in South China.According to the data from both South China and the literature,we mainly analyzed the global distributions of Rhizocorallium in time and space during the Early and Middle Triassic,its ecologic features,as well as substrate preference of trace makers of Rhizocorallium.Globally,Rhizocorallium was sporadically present in the Griesbachian–Smithian interval,underwent specific proliferation and spatial expansion in the Spathian,and became very abundant during the Anisian.In general,the Spathian and Pelsonian(middle–late Anisian)saw ichnospecies richness peaks and morphologic complexity.Burrow sizes of Rhizocorallium generally all increase from the Griesbachian to Spathian,followed by a relatively steeper increase from the Spathian to Pelsonian.Rhizocorallium possesses the shallowest penetration depths of burrows prior to the Spathian and follows a pattern of occupying stepwise-increasing tiers from the Spathian to Pelsonian.The Rhizocorallium-bearing strata possess fairly high bioturbation levels in the Spathian and Anisian.The global Early–Middle Triassic Rhizocorallium ichnotaxa were distributed among seven habitat types in shallow seas.They,however,had an environmental preference for carbonate lagoon settings.Pre-Spathian Rhizocorallium possessed a relatively smaller size,shallower depth penetration,few ichnospecies,and limited ecologic/environmental and geographic distributions.Their occurrences in that time indicate early stages of recovery.This is consistent with other biota,which still suffered postextinction biotic depletion and environmental stresses.Although Spathian Rhizocorallium possessed elevated ichnospecies numbers and morphologic complexity,as well as environmental and geographic expansion,most ichnotaxa remained relatively small and thus may indicate stages 2–3 of ecologic recovery.In comparison,middle–late Anisian Rhizocorallium not only showed the highest ichnospecies richness and deepest penetration depth,but also exhibited the greatest morphologic complexity and largest burrow sizes,with some forms having tube diameters >20 mm.This indicates the final stage(4)of ecologic recovery following the end-Permian crisis.Measured by all ichno-ecologic proxies,Early–Middle Triassic Rhizocorallium ichnotaxa show a stepwise recovery pattern: unexpected rebound in the Griesbachian–Smithian,an increased ecologic recovery in the Spathian,and a full recovery in the middle–late Anisian(Middle Triassic).