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Molluscan immigrations via biogeographical ecotone of the Middle Russian Sea during the Jurassic

Rogov M., Zakharov V., Kiselev D.

Volumina Jurassica

2008

Vol. 6, no. 1

143-152

Patterns of molluscan immigration via the biogeographical ecotone zone of the Middle Russian Sea during the Middle – Late Jurassic have been studied in detail. Firstly, the migration routes and ranges of recent molluscs are briefly reviewed, with some comments about their applicability to the Jurassic. Secondly, the migrational events have been classified by their direction, duration and intensivity. We recognize two main types of immigration by their direction (unidirectional and multidirectional) and by their intensity (mass immigrations and isolated strayings). Recent progress in infrazonal ammonite biostratigraphy leads us to the more precise recognition of paleobiogeographical events and to the understanding of the immigration patterns that can produce more precise correlations.

The Volgian Stage and Jurassic/Cretaceous boundary in the Panboreal Superrealm

Zakharov V., Rogov M., Kiselev D.

Volumina Jurassica

2006

Vol. 4, no. 1

224-226

Correlation between the Volgian and Tithonian stages has been the subject of controversy for many decades. But comparison of the zonal succession within the single stage also encounters some problems. Here we propose new correlation between the Volgian Stage and Portlandian Stage. The correlation chart shows possible relations between the Volgian and basal Cretaceous zones and these of the different provinces (Fig. 1). Important for correlation records of ammonites are as follows: 1. Paracraspedites sp. and Epivirgatites cf. variabilis Shulg.occur together with E. lahuseni (Nik.) and E. cf. nikitini (Mich.) in Gorodoschi (Kiselev & Rogov 2005). 2. Laugeites sp. nov. aff. parvus Donovan occurs with Subcraspedites sowerbyi Spath, S. cf. preplicomphalus Swinn. in Gorodischi (Kiselev & Rogov 2005). 3. Craspedites okensis (d'Orb.) appears in Russia, Svalbard and Siberia nearly simultaneously. 4. Laugeites cf. planus Mesezhn. co-occurs with Epivirgatites variabilis Shulg. in Nordvik section. 5. Epilaugeites ranges into the upper part of Variabilis Zone of the Nordvik section. 6. Laugeites sp. nov. aff. parvus Donovan is recorded together with Praechetaites exoticus Shulg. and P. tenuicostatus Shulg. in Nordvik. 7. Dorsoplanites maximus Spath and D. gracilis, typical for Subpolar Urals, are known from faunas 42 and 43 of East Greenland (Callomon & Birkelund 1982). 8. Epipallasiceras in the Albani Zone of England and French coast (Wimbledon 1984), includes so-called "Epivirgatites" forms. 9. Crendonites spp. ranges from Glaucolithus to Kerberus zones of England (Wimbledon 1984) and from Anguinus to Groenlandicus zones in East Greenland (Callomon & Birkelund 1982); ammonites close to Siberian Crendonites subleslie are known from the Glaucolithes/Okusensis boundary interval of England (Wimbledon 1984). 10. Epilaugeites vogulicus (Ilov.) was recorded in the Oppressus Zone of England; subsequent ascribing of this ammonite to Okusensis Zone by Wimbledon (1984) seems not well proved. 11. Craspedites pseudonodiger Shulg. occurs in the Taimyrensis Zone of Siberia and in the uppermost Nodiger Zone of Kashpir. 12. Recent records of Volgidiscus in the Yaroslavl region permit to correlate this level with uppermost Volgian of Siberia and uppermost Portlandian. Existence of the detached zone with Subcraspedites above Volgidiscus is in Arctic (Mesezhnikov et al. 1983) and it appears in the basal Cretaceous of the Russian Platform (Mitta 2005) and England (Casey 1973). 13. Praetollia spp. is widely distributed in Arctic (Mesezhnikov et al. 1983) and appears in the basal Cretaceous of the Russian Platform (Mitta 2005) and England (Casey 1973). 14. The key event was a spreading of Boreal Hectoroceras from Siberia through Central Russia to England (Casey 1973) and East Greenland.