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The shale-dominated hemipelagic succession exposed in the southwestern part of the Prague Synform preserves the most complete Ludfordian graptolite record so far encountered from peri-Gondwanan Europe. Four graptolite biozones -the Neocucullograptus inexpectatus, Nc. kozlowskii, Pseudomonoclimacis latilobus-Slovinograptus balticus and Pristiograptus fragmentalis biozones -are recognized in the middle and late Ludfordian, between the Bohemograptus tenuis Biozone and the base of the Pridoli Series. Conodont occurrences are restricted to scattered limestone beds, but enable tentative integration of the graptolite and conodont biozonal schemes. Particular attention was paid to faunal and sedimentary changes and the carbon isotope record across the middle Ludfordian Kozlowskii extinction Event. The Kozlowskii Event caused the almost simultaneous extinction of graptolites with ventrally curved rhabdosomes. The genera Bohemograptus, Polonograptus and Neocucullograptus, along with Pseudomonoclimacis dalejensis, disappeared from the fossil record. The offshore conodont fauna recorded in the section was not strongly affected and similarly the pelagic orthocerids and nektonic Ceratiocaris passed unaffected through the extinction interval. The abundant and widespread pelagic myodocopid ostracod Entomis, however, became extinct. The late Ludfordian graptolite recovery gave origin to a novel fauna of Pridoli type from taxa that emerged or just reappeared above the Kozlowskii crisis. In Všeradice and elsewhere in the Prague Synform, the recovery, manifested by the appearance of Pseudomonoclimacis latilobus and Slovinograptus balticus, closely postdates the end of the isotope excursion but pre-dates the first appearance of the conodont index 'Ozarkodina' snajdri. Here the graptolite recovery was delayed relative to the recovery of the benthic fauna. A canalized intraformational limestone conglomerate corresponds with a gap in the sedimentary record above the Kozlowskii extinction and just below the graptolite recovery. The benthic faunas from the conglomerate matrix and pebbles permit correlation with the shallower part of the basin indicating a distinct fall in sea-level. The present data demonstrate the coincidence of the graptolite crisis with benthic faunal change and eustatic fall in sea-level manifested by facies change and the carbon isotope excursion. Polonograptus chlupaci sp. nov., from the Nc. kozlowskii Biozone, is described and several other graptolite taxa are redescribed.

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Palaeoecology and palaeogeographic relations of the Silurian phragmoceratids (Nautiloidea, Cephalopoda) of the Prague Basin (Bohemia) TÌPÁN MANDAPhragmoceras and Tubiferoceras are discosorid nautiloid genera with endogastric cyrtoconic to orthoconic breviconic shells possessing a contracted aperture. Because of the constricted aperture phragmoceratids have usually been considered as microphages. The constriction of the aperture appears during the late ephebic stage. Preceding ontogenetic stages possessed a brevicone shell with an open aperture, usually considered to be indicative of nectobenthic predatory lifestyle. The apertural constriction probably improved hydrodynamic control and served as protection for the soft body. Attachment sites for distinct retractor muscles suggest evidence of potentially fast movement of the head-arm complex out of the aperture with consequent capture of larger prey. Phragmoceratids are a characteristic component of Silurian nautiloid faunas, which inhabited the mainly tropical carbonate platforms of Baltica and Laurentia. Phragmoceratids closely related to those of the Baltic and Avalonia occasionally appeared in the Prague Basin, which was located in the temperate zone at the northern margin of peri-Gondwana. The distribution pattern of phragmoceratids suggests that immigration into the Prague Basin occurred in three stages: (1) first appeared stray immigrants (or occasional visitors) from warmer seas, this stage of immigration reflecting activation of sea currents after the early Silurian widespread anoxia in peri-Gondwana (latest Llandovery-early Wenlock); (2) appearance of small endemic palaeo-populations of migrants (with evidence of local hatching of phragmocerids) that persisted only briefly during the early Homerian and early Gorstian low stands; (3) stable palaeo-populations appeared in the Early Ludfordian, with occurrence of endemic taxa related to the forms known from Baltica-Avalonia, suggesting stable conditions for nautiloid evolution. The Middle Ludfordian Kozlowskii Event caused the extinction of the last phragmocerid taxa in the Prague Basin as elsewhere. Nautiloid immigration to the Prague Basin (Perunica microplate) indicates that Perunica, in contrast to peri-Gondwanan areas, was within reach of the South Tropical Current since the latest Llandovery. Many other nautiloid families show a similar migration pattern although timing of the immigrations do not necessarily correlate. Thus the nautiloids provide useful data for palaeobiogeographic and climate reconstruction, such analysis can be made precise by comparison of nautiloid clades. •

Diversity evolution of 197 of the latest Ludlow-Lochkovian cephalopod species from Bohemia (representing more that 70 percent of all species known worldwide) was analysed and compared with changes in the global carbon cycle in the marine ecosystem. Our results show a distinct relationship between cephalopod species diversity and the global carbon cycle. A progressive increase in δ 13 C values in marine carbonates from the Monograptus lochkoviensis Zone to the end of the Monograptus transgrediens Zone reflects an increase of bioproductivity in the marine ecosystem. This increase was probably caused by a shallowing of the upwelling system, thus importing nutrient-rich water. A progressive cooling during the Přídolí and earliest Lochkovian triggered these changes in the upwelling system. Our data also show an increase in total diversity of cephalopod species, which may be explained by greater availability of new food sources for a period corresponding to the progressive increase in δ 13 C values. In the latest Silurian, δ 13 C reached its maximal values while the Lochkovian is characterized by steady conditions with a decreasing tendency in δ 13 C values. Strong, selective extinction started in the latest Silurian Monograptus transgrediens Zone and continued to the earliest Devonian Monograptus uniformis Zone. This extinction affected benthic and demersal cephalopod species with relatively large eggs and long incubation times. The change in dynamics of the global carbon cycle thus coincides with the beginning of this extinction. An anoxic or hypoxic event in the period between the LAD Monograptus transgrediens Zone (Transgrediens Event) in the latest Přídolí and the base of the Devonian Monograptus uniformis Zone (documented also from offshore sequences from Poland) is considered to be a cause for the extinction of non-pelagic cephalopods. Our study also revealed a need to revise and define all earlier established bioevents in the Silurian-Devonian boundary interval more clearly. •

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in the Middle Ordovician of the Prague Basin and its palaeobiogeographical significance TÌPÁN MANDANautiloids of the order Tarphycerida are a characteristic component of warm-water Ordovician faunas of Baltica and Laurentia but are usually absent from the cooler high latitude marine environments. The presence of the tarphycerid Trocholites, reported from the Middle Ordovician, Dobrotivian (late Darriwilian) strata of the Iberian Chain and Armorican Massif (peri-Gondwanan Europe), provide an exception. Lituites primulus Barrande, 1865, represented by a single poorly-preserved individual was reported from coeval strata in the Prague Basin (Perunica) and doubtfully assigned to Curtoceras or Trocholites. New biostratigraphicaly well-constrained specimens of Trocholites described and discussed from the early Dobrotivian of the Prague Basin, are conspecific with Trocholites fugax previously described from the Iberian Chain and Armorican Massif. The contemporaneous presence of Trocholites fugax in Perunica and peri-Gondwanan terranes is additional evidence for faunal connections between these microcontinents during the Dobrotivian. By comparison with the preceding Llanvirnian (early Darriwilian) fauna, the low diversity Dobrotivian cephalopod fauna of the Prague Basin suggests an increasing faunal separation between Perunica and Baltica. The occurrence of Trocholites in early Dobrotivian strata of Perunica, the Iberian Chain and Armorican Massif reflects an unusual dispersion event of tarphycerids into lower latitude seas coinciding with a distinct climate perturbation during the Ordovician.•

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