Petr Štorch

Graptolite biostratigraphy of the Lower Silurian (Llandovery) shelf deposits of the Western Iberian Cordillera, Spain

Eight Lower Silurian graptolite biozones (triangulatus, convolutus, linnaei, turriculatus, cris- pus, griestoniensis, tullbergi and spiralis) and three subzones (runcinatus-gemmatus, palmeus and hispani- cus) have been recognized in the Sierra Menera, Nevera, and Tremedal massifs in the Castilian Branch of the Iberian Cordillera (Western Iberian Cordillera). Early Silurian, Rhuddanian low-diversity normalograp- tid faunas are also present and these, together with the rich graptolite faunas of the Aeronian triangulatus and convolutus biozones, come from black, shaly intercalations within the quartzose sandstones of the upper part of the Los Puertos Quartzite. Telychian graptolite biozones have been recognized in the succeeding black-shale sequence of the Badenas Formation. Diachroneity of the transition from sandstones to black shales is dated by graptolites. It ranges from about the base to at least the top of the linnaei Biozone. We sug- gest that the Lower Silurian black shales of the Western Iberian Cordillera were deposited in a shallow, shelf environment, not much deeper than that of presumably storm-influenced sandstones of the Los Puertos Quartzite.

The graptolite, conodont and sedimentary record through the late Ludlow Kozlowskii Event (Silurian) in the shale-dominated succession of Bohemia

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 Vseradice 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.

The late Aeronian graptolite sedgwickii Event, associated positive carbon isotope excursion and facies changes in the Prague Synform (Barrandian area, Bohemia)

Study of the lower Silurian black shale succession of the Prague Synform has enabled detailed insight into graptolite faunal dynamics and diversity trends from the mid-Aeronian diversity maximum through to the late Aeronian crisis. Graptolite diversity decreased from 33 taxa in the Lituigraptus convolutus Biozone to 17 taxa in the upper part of the Stimulograptus sedgwickii Biozone and newly erected Lituigraptus rastrum Biozone. The graptolite assemblages of the latter biozones exhibit low species richness along with high dominance. Many graptolite species that became extinct in the early part of the sedgwickii Zone were promptly replaced by new forms. In the later part of the sedgwickii Zone, however, replacement of extinct species by new forms considerably decelerated. The increased rate of graptolite extinction recorded in the convolutus–sedgwickii biozone boundary beds coincided with subtle changes in black shale lithologies and a positive shift in δ 13 C org (of 2.2 ‰) compared to baseline values. Sea-level drawdown can be inferred from siltstones and/or temporary nondeposition in the middle sedgwickii Zone. This level also sees total organic carbon (TOC) fluctuations and a strong positive δ 13 C org excursion with a peak shift of at least 7 ‰. The sedgwickii Event exhibits substantial reorganization of the graptolite fauna, its taxonomic impoverishment and concomitant increase in species dominance rather than a sudden collapse of the pre-extinction assemblage. Associated changes in lithology, TOC and the pronounced δ 13 C org excursion suggest a relatively extended and probably multi-phase period of stressed conditions that affected the pelagic realm inhabited by graptolites in the course of the late Aeronian interval.

Integrated stratigraphy of the Ludfordian in the Prague Synform

The stratigraphic correlation of the Ludfordian in the Prague Synform is summarized using refined biostratigraphic data from updated conodont and graptolite faunal records. The seven graptolite and seven conodont biozones and faunal intervals are correlated. The biozonations are then integrated with the generalized eustatic and carbon isotope curves. Levels of major extinction and radiation events are plotted against an integrated and refined Ludfordian stratigraphy of the Prague Synform. The proposed correlation chart is a basis for further high-resolution correlation of that region.

Sporophytes of polysporangiate land plants from the early Silurian period may have been photosynthetically autonomous

The colonization of land by vascular plants is an extremely important phase in Earth’s life history. This key evolutionary process is thought to have begun during the Middle Cambrian1 period and culminated in the Silurian/Early Devonian period (interval about 509–393 million years ago (Ma)), and is documented primarily by microfossils (that is, by dispersed spores, phytodebris including fragments of algae, tissues, sporangia and cuticles), tubes and rare megafossils2. A newly recognized fossil cooksonioid plant with in situ spores from the Barrandian area, Czech Republic, is of the highest importance because it represents extremely ancient megafossil evidence of land plant diploid generation: sporophytes (~432 Ma). The robust size of this plant places it among the largest known early polysporangiate land plants and it is probable that it attained adequate size for both aeration and effective photosynthetic competence. This would mean not only that sporophytes were photosynthetically autonomous but also that the they might have been able to sustain a relatively gametophyte-independent existence.Ancient fossil Cooksonia plants from the Czech Republic are among the largest known early polysporangiate plants from when vascular plants were colonizing the land. They were of sufficient size to support effective photosynthesis.

Cyrtograptus sakmaricus Koren' from Langao, Shaanxi Province, China

Specimens of Cyrtograptus sakmaricus found in the Qiaoxi section, Langao County, Shaanxi Province, China, preserve different developmental stages of the species ranging from sicula through immature single whorl primary stipe to mature rhabdosomes with a number of cladia. Langao specimens of C. sakmaricus are characterized by having about two whorls before the origin of the first cladium and very small diameter of the first whorl. Cladia spaced at the rate of three, occasionally four, thecae; stipe width increasing rapidly from 0.5 mm to a maximum of 2 mm, exclusive of the, up to, 1 mm long apertural spines. Two thecae repeat distance stable 2 mm throughout the length of the primary stipe.