Revision of Silurian vertebrate biozones and their correlation with the conodont succession

摘要

The first vertebrate-based subdivisions of Silurian strata were mainly drawn on material from outcrops in Britain and drill cores from the southern Baltic. Nearly twenty years ago the first vertebrate biozonal scheme was developed on the basis of vertebrate distribution in several continuous drill core sections in the northern Baltic. This paper presents a new scheme in which many new data on vertebrate distribution from the Baltica (Baltic region, NW Russia), Avalonia (southern Britain, eastern Canada),Laurentia (northern Canada, Greenland, Scotland) and Kara (Arctic Russia) palaeocontinents have been used. All the zones have been defined, and the geographical distribution and the reference stratum and locality for each zone have been given. The Llandovery part of the succession contains the Valyalepis crista, Loganellia aldridgei and L. scotica zones; the Wenlock part is represented by the Archipelepis bifurcata/Arch. turbinata, L. grossi, Overia adraini, L. einari and Paralogania martinssoni zones. The Par. martinssoni Zone continues in the Ludlow and is followed by the Phlebolepis ornata, Phl. elegans, Andreolepis hedei,Thelodus sculptilis and T. admirabilis zones. The last zone continues in the lower P?idoli and is followed by the Nostolepis gracilis, Poracanthodes punctatus and Trimerolepis timanica zones. The L. aldridgei and Arch. bifurcata zones are new, and the Arch. turbinata and O. adraini faunas have been raised from 'faunal succession units' to zones. The geographically widely distributed L. grossi Zone in the upper Sheinwoodian and the Par. martinssoni Zone in the upper Homerian?lowermost Gorstian allow the integration of regional successions into one Generalized Vertebrate Zonal Scheme. Possible correlations of conodont and vertebrate biozones, and gaps in sedimentation in the northern East Baltic Silurian sequence have been discussed, the most extensive hiatus being between the Paadla and Kuressaare stages.