The Phylogeny, Speciation And Zonation Of Siphonodellid Group (Conodonts)

Study on the Devonian-Carboniferous boundary has always been one of the important stratigraphical and palaeontological issues since 1920s, and much work has been done by palaeontologists and geologists from different countries. In 1979, the International Working Group on Devonian-Carboniferous Boundary proposed that the first appearance of Siphonodella sulcata (eonodont) in the evolutionary lineage of Siphonodella praesulcata-S. sulcata could be taken as the definition of the Devonian-Carboniferous boundary. In 1989, the La Serre section (Montagne Noire, France) was chosen as the Global Stratotype Section and Point (GSSP) for Devonian-Carboniferous boundary. But the study on D-C boundary has never terminated because the La Serre section is not an ideal GSSP, it is problematic both in the strata and in the eonodont identify. Especially, it can not display the phylogenetic lineage from Siphonodella praesulcata to S. sulcata.In 2008, International Subcommission on Devonian Stratigraphy (SDS) and International Subcommission on Carboniferous Stratigraphy (SCS) jointly established the International Task Group on Devonian-Carboniferous Boundary. Most members of the task group still support the D-C boundary definition made by International Working Group on Devonian-Carboniferous Boundary in 1979. Because of the lost of the holotype of S. sulcata, the urgent tasks of the group is to choose the neotype of Siphonodella sulcata, classifying the later forms of Siphonodella praesulcata and the earlier forms of Siphonodella sulcata, and re-studying the phylogeny of Siphonodella praesulcata-S. sulcata.Based on the Siphonodellid materials discovered from North America, Europe and South China, and according to the former researchers’achievements, this paper mainly deals with the phylogeny, speciation and bio-zonation of siphonodellid group, which includes 4 genera: Eosiphonodella, Eusiphonodella, Protosiphonodella and Siphonodella. The following achievements are obtained:1. Identifying two morphotypes of Siphonodella praesulcata:S. praesulcata Morphotype land S. praesulcata Morphotype 2; recognizing two morphotypes of Siphonodella sulcata:S. sulcata Morphotype 1 and S. sulcata Moiphotype 2; giving the identification criteria to distinguish Siphonodella praesulcata from S. sulcata, and proving the phylogenetic relationship from Siphonodella praesulcata to S. sulcata.2. Taking the first appearance of Siphonodella sulcata Morphotype 1 in the evolutionary lineage of Siphonodella praesulcata-S. sulcata as the definition of the Devonian-Carboniferous boundary.3. Identifying a new species of Siphonodella transitans sp. nov., which is the earliest species of genus Siphonodella, giving rise to S. praesulcata Morphotype 1 at the beginning of Early praesulcata zone.4. Establishing a new genus:Eusiphonodella gen. nov., which contains 3 new species: Eusiphonodella intermedia gen. et sp. nov., Eusiphonodella guizhouensis gen. et sp. nov. and Eusiphonodella muhuaensis gen. et sp. nov.5. Identifying 2 new species of Protosiphonodella:Protosiphonodella sinensis sp. nov. and Pr. robusta sp. nov. and re-establishing the phylogeny of genus Protosiphonodella. According to current study, Protosiphonodella sinensis sp. nov. is the "root species"of Siphonodellid group.6. Subdividing the original sulcata zone into Early sulcata zone and Late sulcata zone based on the stratigraphic distributions of S. sulcata M.1 and S. sulcata M.2 and giving the definitions of Early sulcata zone and Late sulcata zone.7. Promoting the subgenus Siphonodella (Eosiphonodella) as a genus Eosiphonodella, and revising the Eosiphonodella zonation partly. According to current research, the levis zone in shallow-water areas is considered to be equivalent to the Lower sulcata zone in deep-water areas, and the first appearance of Eosiphonodella levis can be taken as the marker of Devonian-Carboniferous boundary in shallow-water areas.