A Study On Fossils From The Background Taphofacies Of The Chengjiang Biota

According to sedimentary characteristics,the strata that bears Chengjiang biota can be divided into two taphofacies,i.e.the event(E)taphofacie and the background(B)taphofacie.The taphofacie E is a process of rapid burial and deposition.In this taphofacie,many beautiful fossils with soft body parts are preserved.While the taphofacie B represents a slow sedimentation process.In this taphofacie,although some fossils are preserved,but most of them are sponge,fragments of arthropods skeletons et.al.Recent discoveries revealed that soft-body fossils can also be preserved in the taphofacie B.However,little is known about the diversity and the underlying prevservational path of the soft body fossils in taphofacie B of the Chengjiang biota.This thesis aims to study the taxonomic composition and preservation of the soft body fossil from the Chengjiang background mudstone,mainly based on the investigation of the last three decades' collections in Yunnan Key Laboratory of Palaeobiology,Yunnan University.Analysis on the taxonomic composition revealed that pelagic swimming taxa,such as Ixosys,radiodonts et al.,are the dominant groups in the Chengjiang background mudstone,with some representants of benthic groups,such as sponges and brachiopods.While taxa that swam in water column or bottom water are rare.It also reveals the taxonomic composition of the soft body fossils have same pattern as the shelly fossils,domainatied primarily by pelagic and free swimming taxas,such as arthropods and radiodonts.SEM-EDX analysis on the taxa with different habitat reveal a variable preservation of pyritization and carbonaceous compression,i.e.animal groups that lived in different water depth have different ratio of pyrite/organic carbon.It is inferred that this might due to the different way of burying when the they were preserved as fossils.In addition,Tauricornicaris is taken as an example to investigate the biological significance of the fossils from the background mudstone of the Chengjiang biota.By combining the morphological information of the fossils both from the taphofacies B and E,it is revealed that Tauricornicaris has typical arthropod tergite,biramous appendages,a head shield with possible stalked eyes and an annulated antenna.It is thus concluded that Tauricornicaris is an euarthropod,rather than a radiodont.The work of this thesis confirmed that the fossils(including the soft body ones)from the taphoface B of the Chengjiang biota bear abundant information of animal morphology and fossil preservation.Thus they are very important to understand the early evolution of animal morphology and to study the preservation and palaeoecology of the Chengjiang biota.