Six ichnogenera,Chondrites,Palaeophycus,Planolites,Rhizocorallium,Skolithos,and Thalassinoides,were identified from the drilling cores near the Global Stratotype Section and Point(GSSP) of Permian-Triassic boundary at Meishan,Zhejiang Province.The ichnological indices,including abundance,bioturbation index,and disturbed depth of trace fossils,show two episodes of intense changes near the Permian-Triassic Boundary(PTB).Episode I occurred in Beds 25-27b when the ecologically complicate forms such as Chondrites,Skolithos,Rhizocorallium,and Thalassinoides disappeared hereafter,the bioturbation index reduced from 1-5 to 1-3,and the disturbed depth declined from 5-66 to 2-5 cm.Episode II took place at the base of Bed 33 with the disappearance of Palaeophycus and Planolites,and subsequent absence of trace fossils and bioturbation till the middle-upper part of Bed 41 when the disturbed structures reoccurred,but they are only tiny Planolites and the bioturbation index was never higher than 3 and the disturbed depth less than 4 mm.Episode I shows an intense change,corresponding to the main stage of the end-Permian mass extinction,whereas Episode II is relatively weak,corresponding to the epilogue of the mass extinction of trace makers in the Early Triassic.Subsequently,ichnofossils were dominated by surface tracks in simple ecological habit and structures.This phenomenon indicates that the Early Triassic benthonic fauna is changed from sessile benthic system to mobile benthic system after the end-Permian mass extinction.In other words,the evolution of the trace fossils across the Permian-Triassic transition had an episodic process similar to the body fossils.In addition,the change of ichnofabrics is well coincided with the negative excursion of carbon isotopes and the expansion of cyanobacteria.As the results of physical and biogenic processes,trace fossils provided unique materials for the study of the biotic and environmental events,as well as their coupling evolution through the great Permian-Triassic transition.
ZHAO XiaoMing1,2* & TONG JinNan1 1 Key Laboratory of Biogeology and Environmental Geology of Ministry of Education,China University of Geosciences,Wuhan 430074,China
The stratigraphic sequence of calcimicrobialite facies at the Permian-Triassic (P/Tr) boundary has well recorded the biotic and environmental transition across the end-Permian catastrophic events. The biostratigraphy, microfacies, carbon isotopes, and fossil records across the P/Tr boundary have been studied at the Kangjiaping Section in Cili County, Hunan Province. Three biostratigraphic zones, Palaeofusulina-Colaniella Zone, Hindeodus parvus Zone, and Isarcicella staeschei Zone, are identified. The excursion of δ13Ccarb exhibits a sharp negative shift in the calcimicrobialite at the P/Tr boundary, which is roughly accordant with the abrupt bioclastic decline. In addition, five types of microfacies are recognized, including algal-foraminifer bioclastic limestone, algal-laminated calcimicrobial limestone, oolitic grainstone, vermiculate limestone, and intraclastic wackstone. The results indicate that the changeover of ecosystem from metazoan reef to calcimicrobialite in Cili is a classic case of marine ecological evolution during the Paleozoic-Mesozoic transition.
