Preferencje help
Widoczny [Schowaj] Abstrakt
Liczba wyników

Znaleziono wyników: 34

Liczba wyników na stronie
Pierwsza strona wyników Pięć stron wyników wstecz Poprzednia strona wyników Strona / 2 Następna strona wyników Pięć stron wyników wprzód Ostatnia strona wyników

Wyniki wyszukiwania

help Sortuj według:

help Ogranicz wyniki do:
Pierwsza strona wyników Pięć stron wyników wstecz Poprzednia strona wyników Strona / 2 Następna strona wyników Pięć stron wyników wprzód Ostatnia strona wyników
For the past three decades, the Alvarez impact theory of mass extinction, causally related to catastrophic meteorite impacts, has been recurrently applied to multiple extinction boundaries. However, these multidisciplinary research efforts across the globe have been largely unsuccessful to date, with one outstanding exception: the Cretaceous–Paleogene boundary. The unicausal impact scenario as a leading explanation, when applied to the complex fossil record, has resulted in force−fitting of data and interpretations (“great expectations syndrome”). The misunderstandings can be grouped at three successive levels of the testing process, and involve the unreflective application of the impact paradigm: (i) factual misidentification, i.e., an erroneous or indefinite recognition of the extraterrestrial record in sedimentological, physical and geochemical contexts, (ii) correlative misinterpretation of the adequately documented impact signals due to their incorrect dating, and (iii) causal overestimation when the proved impact characteristics are doubtful as a sufficient trigger of a contemporaneous global cosmic catastrophe. Examples of uncritical belief in the simple cause−effect scenario for the Frasnian–Famennian, Permian–Triassic, and Triassic–Jurassic (and the Eifelian–Givetian and Paleocene–Eocene as well) global events include mostly item−1 pitfalls (factual misidentification), with Ir enrichments and shocked minerals frequently misidentified. Therefore, these mass extinctions are still at the first test level, and only the F–F extinction is potentially seen in the context of item−2, the interpretative step, because of the possible causative link with the Siljan Ring crater (53 km in diameter). The erratically recognized cratering signature is often marked by large timing and size uncertainties, and item−3, the advanced causal inference, is in fact limited to clustered impacts that clearly predate major mass extinctions. The multi−impact lag−time pattern is particularly clear in the Late Triassic, when the largest (100 km diameter) Manicouagan crater was possibly concurrent with the end−Carnian extinction (or with the late Norian tetrapod turnover on an alternative time scale). The relatively small crater sizes and cratonic (crystalline rock basement) setting of these two craters further suggest the strongly insufficient extraterrestrial trigger of worldwide environmental traumas. However, to discuss the kill potential of impact events in a more robust fashion, their location and timing, vulnerability factors, especially target geology and palaeogeography in the context of associated climate−active volatile fluxes, should to be rigorously assessed. The current lack of conclusive impact evidence synchronous with most mass extinctions may still be somewhat misleading due to the predicted large set of undiscovered craters, particularly in light of the obscured record of oceanic impact events.
Givetian and Frasnian stromatoporoid-coral limestone of the Kowala Formation in the southern Holy Cross Mts is subdivided stratigraphically, and correlated with strata elsewhere on the basis of identified sea-level cyclicity, with support from conodonts and other selected benthic fossils. After the Eifelian hypersaline sabkha phase, an extensive two-step regional colonization of the Kielce Region carbonate platform took place during the Eifelian/Givetian passage interval and the Middle Givetian. At least four deepening pulses resulted in intermittent drowning of the vast carbonate platform and sequential replacement of the undifferentiated Stringocephalus biostromal bank by the Sitkówka bank complex and, subsequently, by the Dyminy reef complex. The reef developed in the central Dyminy belt as result of the early Frasnian accelerated sea-level rise after some period of biotic stagnation near the Givetian-Frasnian boundary. Final demise of the reef resulted from combined eustatic and tectonic movements during the late Frasnian major crisis interval.
Brachiopod faunas from the Devonian stromatoporoid-coral series (Kowala Formation) of the southern Holy Cross Mts comprise at least 60 species, atrypids and ambocoeliid spiriferids being the most common. Largely monospecific bottom-level pioneer assemblages colonized intershoal and open shelf environments of the Late Givetian Sitkówka bank complex to the Frasnian Dyminy reef complex, and some lagoonal habitats of the older Givetian Stringocephalus bank. The associations dwelling organic buildups were more diverse and specialized. Faunal dynamics of the brachiopods were controlled primarily by eustatic cycles and the evolution of the carbonate shelf. Generally this was a four-step succession from the stringocephalid to the ambocoeliid, atrypid (or cyrtospiriferid), and rhynchonellid faunas. Twenty two species are reviewed,Praewaagenoconcha(?) sobolevi sp. n., Desquamatia globosa aequiconvexa subsp. n., and D. g. sitkowkensis ssp. n. are proposed. Two poorly-known species of Gürich (1896), Tenticospirifer lagoviensis and Ilmenia(?) elatior, are redescribed.
The article by Ptaszyński and Niedźwiedzki (2004) on vertebrate tracks from the well−known Tumlin Sandstone provides important documentation of the unique terrestrial ichnofauna of the Holy Cross Mountains in Poland. However, two of the major conclusions of this paper raise my objections. The authors propose a new position for the Permian–Triassic (P–Tr) boundary within the Buntsandstein succession of the regional lithostratigraphical scheme. In a conclusion of global significance, the authors find no signature of a mass extinction in the Late Permian land−dwelling tetrapod communities. Both of these issues are reviewed below.
Preliminary review of taxonomy of the brachiopod order Atrypida and its stratigraphic distribution in the late Frasnian Kellwasser Crisis of several regions of Laurussia, western Siberia and South China point to their moderate diversity and stepdown but irregular extinction pattern. The distinctive character of the late Frasnian atrypid fauna is emphasised by several relict genera, marked by recurrent and possibly aberrant characters (mainly in ornamentation types), tendency to size reduction and homeomorphy in some taxa. The transgressive/hypoxic Lower Kellwasser Event and preceding eustatic changes during the Palmatolepis rhenana Zone had only a regional destructive effect, and were linked rather to an enhanced dispersal of the last generic set of atrypids. The Variatrypinae, Spinatrypinae and Iowatrypa-group seem to belong to the latest surviving atrypids. The final demise of the remaining atrypids (and some other articulate brachiopods, e.g., gypidulids) coincided with the transgressive/hypoxic Upper Kellwasser Event, followed by catastrophic eustatic fall during the late Palmatolepis linguiformis Zone (F-F Event). This was probably exacerbated by accelerated submarine volcano-hydrothermal activity, and consequent progressive regional eutrophication, and climatic destabilization. The level-bottom rhynchonellid-inarticulate biofacies crosses the fatal F-F boundary horizon without major changes. No reliable data exist for the presence of atrypids in the Famennian survival and recovery biota, even for the smooth lissatrypid Peratos. Sustained competition from radiating and diversifying productid-cyrtospirifrid-athyrid faunas may have provide an additional biotic factor in the collapse of the Frasnian shelly benthos at the time of sfress, as well as in a post-extinction offshore repopulation from inner shelf habitats.
Each developmental phase of the Givetian to Frasnian carbonate complex of the southern Holy Cross Mts is marked by distinctive calcareous microbiota. The Stringocephalus bank deposits contain a very rich, well-preserved microproblematica (of chiefly ?algal origin) dominated by calcispheroids, and many calcified cyanobacteria and green algae with filamentous Bevocastria, tubiform Devonoscale, and charophyte Trochiliscus. In the late Givetian biostromal complex more sparse microfossil associations occur, with the exception of locally abundant semitextulariid foraminifera (mainly Nanicella) and tubiform dasyclad(?) Jansaella. Also in the Frasnian back-reef facies, only limited and poorly preserved calcispheroids are identified. Contrarily, reef- and fore-reef microbiotas were present in great profusion. Microbial mats (including calcified cyanobacteria Renalcis and Sphaerocodium), associated with locally frequent solenoporids and multichambered foraminifera (Nanicella, also many nodosariids in the later Frasnian) played a significant depositional role and evidence progressive shoaling conditions within the Dyminy reef-complex.
Disarticulated fish microremains from the Middle Givetian to early Frasnian of the Holy Cross Mts include representatives of the most groups known from the Devonian: placoderms, elasmobranchs, holocephalians, acanthodians, osteichthyans (mostly osteolepidids) and actinopterygians. Taxonomic identifications are possible usually only in very general terms and only in two cases the generic level has been reached: Phoebodus among euselachians, and Moythomasia among paleoniscids. Many of these groups, regarded traditionally as typical dwellers of lacustrine to resticted marine nearshore biotopes, apparently flourished also in the offshore, open shelf carbonate (peri-reefl settings. The sequential replacement of the fish faunas was primarily influenced by eustatic events, in like manner to invertebrate communities.
Variable bioclastic-biostromal-marly Posłowice facies within the Late Givetian open-shelf Jaźwica Member in the SW part of the Holy Cross Mts contain palaeocopid-podocopid ostracod associations of moderate diversity (18 species) dominated by Microcheilinella fecunda and Fellerites tuimazensis. More deepwater and stagnant environments of the Bolechowice micrite-marly facies, supported similarly differentiated (12 species) but less equitable associations with platycopids Uchtovia rejrathensis and palaeocopids Buregia jazwicensis as a main component. Other late Givetian, and Frasnian reef and lagoonal microfaunas are mostly impoverished (at least in generic terms) and strongly predominated by podocopids, mostly Bairdiocypris. Late Givetian associations from the Kostomłoty basin are marked by the metacopid Polyzygia symmetrica, and the planktic entomozoid Ungerella torleyi Polenovula beckeri sp. n., Clavofabellina poslovicensis sp. n., Buregia jazwicensis sp. n., and Bairdia zbikowskae sp. n. are proposed.
Late Famennian brachiopods are described for the first time from the Holy Cross Mts. (Góry Świętokrzyskie), Poland. Six taxa belong to two families, the Rhynchonellidae and the Cranaenidae; the new genus Pugnaria and three new species (Rozmanaria magna, Pugnaria plana and Cranaena lgaviensis) are proposed. The brachiopods are dominated by the smooth and comparatively large sulcate R. magna and the uniplicate P. plana. This macrobenthic assemblage, called Rozmanaria magna assemblage, is interpreted here as having successfully colonized deep-water habitats typical of the Chęciny-Zbrza intrashelf basin on the rising slope of the submarine ridge in the Kielce region.
Late Frasnian Atrypida (Brachiopoda) from the Holy Cross Mountains, Poland, include 15 taxa and were widely distributed in foreslope habitats of the declining Dyminy Reef complex. The Palmatolepis semichatovae transgression, followed by the transgressive/hypoxic Lower Kellwasser (KW) Event during the Palmatolepis rhenana Zone did not have catastrophic effects for atrypid faunas, but were rather associated with the appearance of a new species group comprising Iowatrypa, Waiotrypa, Costatrypa, Spinatrypina, Desquamatia and Radiatrypa. Stepdown demise of the biota started during the inter-KW regression, and culminated as a result of increasing stress during the Upper Kellwasser Event in the late Palmatolepis linguiformis Zone, mainly due to catastrophic sea level changes and anoxia, possibly linked to oceanic thermal changes (cooling) and nutrification pulses. The extinction pattern was diachronous and facies-controlled in this area, and the last atrypid survivors reached the Frasnian-Famennian (F-F) boundary. Increasing expansion from the adjacent deeper-water environment of the more resistant assemblages, with productids, cyrtospiriferids, athyridids and schizophoriids, occurred in the final crisis interval. This brachiopod fauna profusion characterized the earliest Famennian survival and early recovery phases of the mass extinction in this part of the Laurussian shelf, as well as the continuity of the deeper-water rhynchonellid-inarticulate biofacies across the F-F boundary. Spinatrypina (Exatrypa) relicta sp. n. is proposed as new.
Late Frasnian representatives of the order Athyridida from the Holy Cross Mountains, Poland, support the idea that the Laurussian basins were the places of origin and radiation of the subfamilies Athyridinae and Meristinae during the middle and early late Paleozoic. At least three new species have been identified from two localities (Łgawa Hill and Kowala) in the Gałęzice Syncline. Of these, one was probaby endemic (Merista rhenanensis sp. n.; maybe also ?Zonathyris sp. A), and two (Athyris postconcentrica sp. n. and Pachyplaxoides postgyralea gen. et sp. n.) were more widely distributed in this part of the Laurussian shelf, being known also from the East European Platform and Rheinisches Schiefergebirge, respectively. This confirms an intermediate biogeographic position of the Holy Cross Mountains area, belonging to an important centre of brachiopod origin and diversification. In contrast to other articulate brachiopods, athyridids reveal a higher rate of diversification, especially at the species (and partly also generic) level, during the global Kellwasser Crisis.
Pierwsza strona wyników Pięć stron wyników wstecz Poprzednia strona wyników Strona / 2 Następna strona wyników Pięć stron wyników wprzód Ostatnia strona wyników
JavaScript jest wyłączony w Twojej przeglądarce internetowej. Włącz go, a następnie odśwież stronę, aby móc w pełni z niej korzystać.