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Literatur/Abstracts - Paläoökologie, Diversität, Aussterben
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Gast





BeitragVerfasst am: Mo Aug 08, 2005 11:06 am    Titel: Literatur/Abstracts - Paläoökologie, Diversität, Aussterben Antworten mit Zitat

Denke dies ist eine spannende Rubrik, da Trilobiten aufgrund ihrer Faziesabhängigkeit viele paläoökologische Infos geben können.


Tim McCormick & Alan W. Owen (2001): Assessing trilobite biodiversity change in the Ordovician of the British Isles. - Geological Journal, Special Issue: History of Biodiversity, 36, (3-4): 279 - 290.

Abstract
The Ordovician Period saw the most sustained, steep rise in marine biodiversity in the history of life on Earth and set the ecological pattern for the rest of the Palaeozoic Era. The long history of research, wide variety of depositional settings and juxtaposition of terranes with very different palaeobiogeographical histories makes the Ordovician of the British Isles an excellent laboratory in which to study this change. A database approach to analysis of trilobite biodiversity change is described using a simple compilation of stratigraphical ranges of 663 species of Avalonian trilobites, and a much more sophisticated information-rich relational database based on the fundamental information of the fossil record-species at localities. The latter includes 2001 occurrence records of 617 trilobite species at 508 localities in the Welsh Basin. The first ever species-level diversity curves for the whole Ordovician of an entire region are presented. Both databases reveal an overall increase in species- and genus-level diversity through the Ordovician. Random resampling tests and environmental information are used to remove sampling effects from the Welsh Basin diversity curves. The main features of trilobite biodiversity change in Avalonia are the following: a late Arenig-early Llanvirn increase, during which time the highest species-to-genus ratios occur, is contemporaneous with a rise in global diversity and rifting of Avalonia from Gondwana; a late Abereiddian dip is followed by recovery during the Llandeilian-early Caradoc; decline during the late Caradoc-early Ashgill is at least partly attributable to lack of preserved rock and restriction of most of the preserved faunas to deep-water environments. The greatest diversity occurs in the palaeoenvironmentally differentiated Rawtheyan and is followed by the diversity crash seen in the Hirnantian throughout the world.


Obwohl Trilobiten im Abstrakt mit keinem Wort erwähnt werden, möchte ich doch darauf hinweisen, daß leider gerade die Trilobiten am Ende des Ordovizium zu den Verlieren gehörten und 2 Ordnungen (Ptychopariida, Agnostida) ausstarben und die Asaphida bis auf eine Familie reduziert wurden.

P. J. Brenchley, J. D. Marshall & C. J. Underwood (2001): Do all mass extinctions represent an ecological crisis? Evidence from the Late Ordovician. - Geological Journal, 36 (3-4), 329 - 340


Abstract
The high rate of species extinction in recent decades is seen by many ecologists as heralding an extinction of catastrophic magnitude in the near future. The ecological consequences of such a biodiversity crisis are hard to predict, but some indication of likely effects can be gained from the knowledge of mass extinctions in the past. The Late Ordovician extinction was one of the five great extinctions in the geological record. It occurred in two phases about 0.5-1million years (Ma) apart and resulted from climatic and related environmental changes associated with the rapid growth and decay of the large Gondwanan ice cap. Overall, an estimated 86% of species became extinct, 61% of genera and 12-24% of families, but few or no orders or higher taxa. The extinction severely affected both marine benthos and plankton. Using brachiopod data as a measure of ecological change, it can be seen that the number of within-habitat species (alpha diversity) was severely reduced and the number and distinctness of benthic communities (beta diversity) on marine shelves also declined sharply. Concurrently the number of palaeogeographic provinces fell from ten to five, possibly as a result of a loss of endemic species and preferential survival of cosmopolitan species. At the peak of extinction, following the second extinction phase, the ecological structure of both benthic and planktonic ecosystems had been severely disrupted and downgraded in complexity as a wide variety of niches were vacated. In spite of the profound biodiversity and ecological crisis within this survival interval, communities returned to their pre-extinction levels of alpha and beta diversity during the subsequent recovery interval. In spite of the large amount of vacant ecospace to be filled there was very little innovation in terms of adaptive strategy, so that the structure of the emerging Silurian communities was similar to that of the Ordovician. In these terms the ecological recovery was remarkable, but it was also prolonged over about 4-5 Ma. On a geological time scale the biosphere returned to normal, but on a human time scale the mass extinction produced a very severely degraded biosphere.

Grüsse,

Jens
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BeitragVerfasst am: Mi Aug 31, 2005 5:10 pm    Titel: Antworten mit Zitat

T. TOLMACHEVA*,, E. EGERQUIST*, T. MEIDLA, O. TINN and L. HOLMER* (2003): Faunal composition and dynamics in unconsolidated sediments: a case study from the Middle Ordovician of the East Baltic. - Geological Magazine, 140 (1): 31-44.

* Department of Earth Sciences, Historical Geology and Palaeontology, Norbyvägen 22, SE-75236, Uppsala, Sweden
Institute of Geology, University of Tartu, Vanemuise 46, 51014, Tartu, Estonia

Abstract:

The Volkhov Regional Stage (Middle Ordovician) in the East Baltic preserves diverse fossil assemblages dominated by epibenthic suspension feeders. Brachiopods, ostracodes, conodonts, echinoderms and bryozoans are the main components of palaeocommunities obtained from clay horizons in the Putilovo section (St Petersburg region, Russia), whereas trilobites, machaeridians, hyolithids, graptolites, benthic foraminifers and gastropods are rare or occur sporadically. Brachiopod bioclasts volumetrically dominate the debris of the studied sediments. Quantitative faunal data are used to assess species diversity patterns, as expressed by the species richness (total number of species in the standardized sample size) and by the evenness or equitability of the community. The numerical abundance of particular taxa in each standardized sample was used to evaluate the density of the fauna. The communities in the Volkhov Stage in Putilovo Quarry reveal a remarkable stability throughout the studied interval and are characterized by high density (1000–6000 specimens per 100 g), relatively moderate species richness (10–15 species) and a moderately variable equitability (0.3–0.7) for the dominant fossil groups (conodonts, ostracodes and brachiopods). Ostracodes significantly increase in numbers within the upper part of the section. This confirms a shallowing of the basin during the late Volkhov interval. Small-scale variability of the diversity estimates does not correlate with the small-scale sea-level changes reconstructed for this part of the basin. It may be connected with error in diversity measurement, or the result of undetected environmental parameters. Variability estimates for different faunal groups are poorly correlated because particular groups have different environmental tolerances.
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BeitragVerfasst am: Mi Aug 31, 2005 5:22 pm    Titel: Antworten mit Zitat

David C. Lees, Richard A. Fortey, and L. Robin M. Cocks (2002): Quantifying paleogeography using biogeography: a test case for the Ordovician and Silurian of Avalonia based on brachiopods and trilobites. - Paleobiology, 28: 343 - 363.

Abstract:

Despite substantial advances in plate tectonic modeling in the last three decades, the postulated position of terranes in the Paleozoic has seldom been validated by faunal data. Fewer studies still have attempted a quantitative approach to distance based on explicit data sets. As a test case, we examine the position of Avalonia in the Ordovician (Arenig, Llanvirn, early Caradoc, and Ashgill) to mid-Silurian (Wenlock) with respect to Laurentia, Baltica, and West Gondwana. Using synoptic lists of 623 trilobite genera and 622 brachiopod genera for these four plates, summarized as Venn diagrams, we have devised proportional indices of mean endemism (ME, normalized by individual plate faunas to eliminate area biogeographic effects) and complementarity (C) for objective paleobiogeographic comparisons. These can discriminate the relative position of Avalonia by assessing the optimal arrangement of inter-centroid distances (measured as great circles) between relevant pairs of continental masses. The proportional indices are used to estimate the "goodness-of-fit" of the faunal data to two widely used dynamic plate tectonic models for these time slices, those of Smith and Rush (1998) and Ross and Scotese (1997). Our faunal data are more consistent with the latter model, which we use to suggest relationships between faunal indices for the five time slices and new rescaled inter-centroid distances between all six plate pairs. We have examined linear and exponential models in relation to continental separation for these indices. For our generic data, the linear model fits distinctly better overall. The fits of indices generated by using independent trilobite and brachiopod lists are mostly similar to each other at each time slice and for a given plate, reflecting a common biogeographic signal; however, the indices vary across the time slices. Combining groups into the same matrix in a "total evidence" analysis performs better still as a measure of distance for mean endemism in the "Scotese" plate model. Four-plate mean endemism performs much better than complementarity as an indicator of pairwise distance for either plate model in the test case.
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BeitragVerfasst am: Mi Aug 31, 2005 5:38 pm    Titel: Antworten mit Zitat

J.G. Meert and B.S. Lieberman (2004): A palaeomagnetic and palaeobiogeographical perspective on latest Neoproterozoic and early Cambrian tectonic events. - Journal of the Geological Society, 161 (3): 477 - 487.


During the latest Neoproterozoic to Mid-Cambrian time (580–505 Ma ago), the Earth underwent significant changes in palaeogeography that included rifting of a possible supercontinent and the near simultaneous formation of a second, slightly smaller supercontinent. It is against this tectonic backdrop that the Cambrian radiation occurred. Although the general tectonic setting during this interval is fairly well constrained, models of the exact palaeogeography are controversial because of the lack of reliable palaeomagnetic data from some of the continental blocks. Palaeogeographical models based on palaeomagnetic data range from a high-latitude configuration for most continents, to a low-latitude configuration for most continents, or to rapid oscillations in continental configurations triggered by inertial changes within the planet. Palaeobiogeographical data can also be used to help constrain palaeogeographical models. To this end we use vicariance patterns in olenellid trilobites to determine their compatibility with three end-member palaeogeographical models derived from palaeomagnetic data for the Neoproterozoic and early Cambrian. The most congruent palaeogeographical model with respect to the palaeobiogeographical data described herein is the high-latitude configuration for most continents. Those palaeomagnetic models that predict inertial interchange true polar wander or multiple episodes of true polar wander differ significantly from the results from palaeobiogeography. The low-latitude palaeogeographical models also differ from the results from palaeobiogeography, but this may partly arise because of a lack of palaeomagnetic and palaeobiogeographical data from many parts of present-day South America and Africa.
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BeitragVerfasst am: Mi Aug 31, 2005 5:50 pm    Titel: Antworten mit Zitat

Jonathan M. Adrain, Stephen R. Westrop, Brian D. E. Chatterton, and Lars Ramsköld (2000): Silurian trilobite alpha diversity and the end-Ordovician mass extinction. - Paleobiology 26 (4): 625 - 646.

Following the end-Ordovician extinction, global clade diversity of Silurian trilobites dropped to about half of Ordovician levels. Although clade diversity failed to recover, this extinction had surprisingly little long-term impact on the number of trilobite species that occupied local habitats (alpha diversity). A new compilation of data from Laurentia and other continents indicates that Silurian trilobite alpha diversities in all major environments were comparable to those of the Late Cambrian and Ordovician; shallow subtidal diversity reached an all-time high during the Late Ordovician. The profound differences in patterns at local and global levels demonstrate the necessity for a hierarchical approach to analyses of diversity. Factors governing global clade diversity are lodged at hierarchical levels beyond those controlling local species richness and must be sought in studies of between-habitat (beta) or geographic (gamma) diversity.


Arnold I. Miller and Shuguang Mao (1995): Association of orogenic activity with the Ordovician radiation of marine life. - Geology, 23 (4): 305 - 308.



The Ordovician radiation of marine life was among the most substantial pulses of diversification in Earth history and coincided in time with a major increase in the global level of orogenic activity. To investigate a possible causal link between these two patterns, the geographic distributions of 6576 individual appearances of Ordovician genera around the world were evaluated with respect to their proximity to probable centers of orogeny (foreland basins). Results indicate that these genera, which belonged to an array of higher taxa that diversified in the Middle and Late Ordovician (trilobites, brachiopods, bivalves, gastropods, monoplacophorans), were far more diverse in, and adjacent to, foreland basins than they were in areas farther removed from orogenic activity (carbonate platforms). This suggests an association of orogeny with diversification at that time.
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BeitragVerfasst am: Mo Okt 10, 2005 4:33 pm    Titel: Antworten mit Zitat

J. Javier Álvaro, Olaf Elicki, Gerd Geyer, Adrian W. A. Rushton and John H. Shergold (2003): Palaeogeographical controls on the Cambrian trilobite immigration and evolutionary patterns reported in the western Gondwana margin. - Palaeogeography, Palaeoclimatology, Palaeoecology, 195 (1-2), 5-35.


Abstract
Southward drifting of the western Gondwanan margin during the Cambrian has been demonstrated by means of both palaeomagnetic methods and lithological indicators of climate (such as carbonates and evaporites). Recent improvements in biostratigraphical correlations permit an enhanced understanding of the climatic and palaeobiogeographical constraints that controlled the distribution of Cambrian benthic communities. Palaeogeographical and biogeographical reconstructions based on trilobites are reported in this paper in order to test interaction between migration, speciation and extinction rates. The variability of the documented biogeographical patterns is directly related to species diversity, in which wider distribution coincides with transgressive trends and subsequent connection of neighbouring platforms. Early Cambrian trilobite faunas show a high degree of both substrate control and endemicity, although transgressions led to parallel shifts in faunal compositions. By contrast, Mid-Cambrian trilobite faunas are relatively uniform across western Gondwana, and latest Mid- and Late Cambrian associations document influence of an increased similarity with Asian trilobite faunas.
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BeitragVerfasst am: Mo Okt 10, 2005 4:43 pm    Titel: Antworten mit Zitat

Matthew R. Saltzman, Robert L. Ripperdan, M. D. Brasier, Kyger C. Lohmann, Richard A. Robison, W. T. Chang, Shanchi Peng, E. K. Ergaliev and Bruce Runnegar (2000): A global carbon isotope excursion (SPICE) during the Late Cambrian: relation to trilobite extinctions, organic-matter burial and sea level. - Palaeogeography, Palaeoclimatology, Palaeoecology, 162 (3-4), 211-223.

Abstract
The Steptoean positive carbon isotope excursion (SPICE) marks a global oceanographic event that confirms intercontinental correlations between different biogeographic realms based on agnostids and other blue-water trilobites. The SPICE excursion is documented from sections in Laurentia, Kazakhstan, China, and Australia where it begins with the mass extinction at the base of the Pterocephaliid Biomere (Steptoean Stage) in Laurentia and at coeval extinction horizons in Gondwana and periGondwana terranes. The peak of SPICE (+5‰) coincided with a time of maximum regression in Laurentia. SPICE is similar in this regard to excursions that coincide with glacio-eustatic falls, such as in the Late Ordovician. A plausible scenario involves the transformation of ocean circulation between two states, which led to enhanced coastal upwelling and benthic extinctions. The lack of evidence for glaciation indicates that the coeval sea level fall (Sauk II–Sauk III event) resulted from tectonic or hydrologic changes that remain poorly understood at this time.
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BeitragVerfasst am: Mo Okt 10, 2005 4:46 pm    Titel: Antworten mit Zitat

J. Javier Álvaro, Daniel Vizcaïno and Emmanuelle Vennin (1999): Trilobite diversity patterns in the Middle Cambrian of southwestern Europe: a comparative study. - Palaeogeography, Palaeoclimatology, Palaeoecology, 151 (4): 241-254.


Abstract
This paper describes the pattern of trilobite diversity throughout the Middle Cambrian in two fossiliferous basins of the western Gondwana margin: the Iberian Chains (northeastern Spain) and the Montagne Noire (southern France). The documented fluctuations of species diversity allow us to recognize: (1) a substantial extinction event recorded in latest Early Cambrian times (named Valdemiedes event), which separates the culmination of a widespread decline of trilobites and the stepwise immigration of cosmopolitan invaders; (2) a major trilobite radiation occurring in the earliest Leonian and culminating across the Caesaraugustian/Languedocian transition, in which a peak in diversity of trilobites and carpoids was close to a major flooding surface; (3) a major reduction of trilobite taxa across the early/middle Languedocian transition related to a well-documented regressional trend; and (4) a second immigration of trilobite fauna in the late Languedocian which coincided with trangressional pulses and the establishment of suitable shaly substrates. In addition, we summarize the known trilobite occurrences from the Middle Cambrian of southwestern Europe. Maps of the distribution of some relevant taxa (genera and species) over the western Gondwana margin are documented.
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BeitragVerfasst am: Mo Okt 10, 2005 4:49 pm    Titel: Antworten mit Zitat

Mark L. Caplan and R. Mark Bustin (1999):Devonian–Carboniferous Hangenberg mass extinction event, widespread organic-rich mudrock and anoxia: causes and consequences. - Palaeogeography, Palaeoclimatology, Palaeoecology, 148 (4): 187-207.

Abstract
The Hangenberg Bio-event represents one of the largest Phanerozoic mass extinction events. It is not only recorded by changes in faunal composition affecting all trophic levels (in the terrestrial and marine environments), but is also recognized by lithological and geochemical changes just prior to the Devonian–Carboniferous (D–C) boundary. This global faunal crisis occurs at the base of a globally extensive black, organic-rich mudrock which sharply to gradationally overlies ramp and platform carbonates. Together with positive excursions to the S, O, C, and Sr isotope curves, dramatic lithological and faunal changes, just prior to the D–C boundary, imply an episode of global climatic perturbation. A low-latitude global continuum of organic carbon generation and burial is evidenced by the formation of organic- and phosphatic-rich black mudrocks, specific deviations to the isotope curves, disappearance of nektobenthic organisms, diversification of small-eyed or blind trilobites, and the appearance of primary producers and zooplankton accustomed to nutrient-rich conditions. It is proposed that oceanic overturn and subsequent global eutrophication resulted in heightened primary productivity, the development of benthic anoxia and subsequent biotic demise of benthic and nektobenthic forms. Gradual climatic cooling leading to the D–C ‘mini'-glaciation in Gondwana was the driving force leading to conditions of heightened oceanic mixing and eutrophication.
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BeitragVerfasst am: Do Nov 03, 2005 12:20 pm    Titel: Antworten mit Zitat

Es folgt Literatur zu Aussterbeereignisse bei Trilobiten.

Es wird im Artikel gezeigt, daß Änderungen der Umweltbedingungen zum Aussterben von etablierten Trilobitenfaunen führten (Verdrängung durch off-shelf Faunen), aber eine Erholung innerhalb eines "kurzen" Zeitraums (wenige Mio. Jahre) erfolgte.


Stephen R. Westrop, and Michael B. Cuggy (1999): Comparative paleoecology of Cambrian trilobite extinctions. - Journal of Paleontology, 73: 337 - 354.


Oklahoma Museum of Natural History, Norman, OK, United States
McMaster University, Canada

Abstract:


Analysis of 164 collections from shelf facies of Laurentian North America indicates that three successive trilobite mass extinctions at Late Cambrian stage boundary intervals ("biomere" boundaries) are characterized by a common pattern of change in distributional paleoecology and species diversity. In all cases, the extinction intervals are marked by a shift to biofacies that have broader environmental distributions than those prior to the onset of extinctions, implying a reduction in between-habitat (beta) diversity. Significant declines in within-habitat (alpha) diversity also characterize each extinction and the compositions of shelf biofacies record extensive immigration of taxa from off-shelf and shelf-margin sites. The nature and extent of ecologic disruption of the shelf appears to be comparable to changes associated with major mass extinctions, such as those at the end of the Ordovician and Permian. Unlike major mass extinctions, the Cambrian events are followed by a complete recovery of diversity and biofacies structure within a few million years.
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BeitragVerfasst am: Do Nov 03, 2005 12:22 pm    Titel: Antworten mit Zitat

Donald G. Mikulic and Joanne Kluessendorf (1999): Stasis and extinction of Silurian (Llandovery-Wenlock) trilobite associations related to oceanic cyclicity. - Journal of Paleontology, 73: 320 - 325.

Silurian trilobites of the central United States belong to a series of temporally-successive associations which appeared abruptly, maintained taxonomic stasis for a time, and then disappeared abruptly. Their disappearance resulted from global perturbations of short-term duration and moderate magnitude, which caused substantial taxonomic replacement but no reorganization of major ecosystems. The most significant extinction and replacement in Silurian trilobite associations in the study area occurs near the Llandovery-Wenlock boundary. This turnover in trilobite associations appears to correspond to Jeppsson's Ireviken Event in his model of oceanic and climatic cyclicity. Major sea-level changes earlier in the Llandovery did not have a similar impact on trilobite associations.
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BeitragVerfasst am: Do Nov 03, 2005 12:24 pm    Titel: Antworten mit Zitat

Na wenn das kein schöner Titel ist, "The rise and fall...", da gibts einen Song drüber. Confused


David K. Brezinski (1999): The rise and fall of late Paleozoic trilobites of the United States. - Journal of Paleontology, 73: 164 - 175.


Based on range data and generic composition, four stages of evolution are recognized for late Paleozoic trilobites of the contiguous United States. Stage 1 occurs in the Lower Mississippian (Kinderhookian-Osagean) and is characterized by a generically diverse association of short-ranging, stenotopic species that are strongly provincial. Stage 2 species are present in the Upper Mississippian and consist of a single, eurytopic, pandemic genus, Paladin. Species of Stage 2 are much longer-ranging than those of Stage 1, and some species may have persisted for as long as 12 m.y. Stage 3 is present within Pennsylvanian and Lower Permian strata and consists initially of the eurytopic, endemic genera Sevillia and Ameura as well as the pandemic genus Ditomopyge. During the middle Pennsylvanian the very long-ranging species Ameura missouriensis and Ditomopyge scitula survived for more than 20 m.y. During the late Pennsylvanian and early Permian, a number of pandemic genera appear to have immigrated into what is now North America. Stage 4 is restricted to the Upper Permian (late Leonardian-Guadalupian) strata and is characterized by short-ranging, stenotopic, provincial genera. The main causal factor controlling the four-stage evolution of late Paleozoic trilobites of the United States is interpreted to be eustacy. Whereas Stage 1 represents an adaptive radiation developed during the Lower Mississippian inundation of North America by the Kaskaskia Sequence, Stage 2 is present in strata deposited during the regression of the Kaskaskia sea. Stage 3 was formed during the transgression and stillstand of the Absaroka Sequence and, although initially endemic, Stage 3 faunas are strongly pandemic in the end when oceanic circulation patterns were at a maximum. A mid-Leonardian sea-level drop caused the extinction of Stage 3 fauna. Sea-level rise near the end of the Leonardian and into the Guadalupian created an adaptive radiation of stentopic species of Stage 4 that quickly became extinct with the latest Permian regression.
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BeitragVerfasst am: Do Jan 19, 2006 10:32 am    Titel: Antworten mit Zitat

Andrey Yu. Zhuravlev and Rachel A. Wood (1996): Anoxia as the cause of the mid-Early Cambrian (Botomian) extinction event. - Geology, 24: 311 - 314.

Russian Academy of Sciences, Palaeontological Institute, Moscow, Russian Federation
University of Cambridge, Department of Earth Sciences, Cambridge, United Kingdom

Abstract:

New and revised Early Cambrian biostratigraphic data allow a quantitative analysis of changes in biotic diversity and extinction rate. The mid-Early Cambrian extinction can now be resolved into two distinct events: the well-known early Toyonian Hawke Bay regression event, and a newly observed but more severe disruption during the early Botomian, here named the Sinsk event. During the Sinsk event, the shallow-water benthos of the so-called Tommotian fauna, together with archaeocyaths and some trilobites, underwent a rapid decline. The Sinsk event is characterized by the significant accumulation of nonbioturbated laminated black shales in tropical shallow waters. Lamination is due to the fine alternation of clay- and organic-rich laminae with calcite-rich laminae containing abundant monospecific acritarchs. These shales are enriched by pyrite and elements typical of anoxic conditions and support a benthic biota of dysaerobic character. Our observations suggest that the extinction during the early Botomian was caused by extensive encroachment of anoxic waters onto epicontinental seas, associated with eutrophication and resultant phytoplankton blooms.
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BeitragVerfasst am: Do Jan 19, 2006 10:42 am    Titel: Antworten mit Zitat

Dies erscheint mir ein wichtiger Artikel zu sein, da Larvalstrategien und Aussterben von Trilobiten am Ende des Ordoviziums miteinander in Zusammenhang gebracht werden können.

Leider habe ich keinen Abstract finden können.

Brian D. E Chatterton and Stephen E. Speyer (1989): Larval ecology, life history strategies,and patterns of extinction and survivorship among Ordovician trilobites. - Paleobiology; 15: 118 - 132.
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BeitragVerfasst am: So Feb 05, 2006 11:13 pm    Titel: Antworten mit Zitat

Trilobiten werden im Abstract zwar nicht erwähnt, sind aber von den Klimaschwankungen im Oberordovizium stark betroffen gewesen. Die Arbeit beschäftigt sich mit einer starken C13-Exkursion in den oberordovizischen Profilen von Lettland und Estland, die anzeigt, daß etwas passiert sein muß, was die Isotopensignatur von den untersuchten Brachiopoden und Ostrakoden-Schalen verändert hat.

Da wir am Baltikum besonders interessiert, führe ich diese Arbeit hier gerne auf.


P.J. Brenchley 1, G.A. Carden 1, L. Hints 2, D. Kaljo 2, J.D. Marshall 3, T. Martma 4, T. Meidla 5 and J. Nõlvak 6 (2003): High-resolution stable isotope stratigraphy of Upper Ordovician sequences: Constraints on the timing of bioevents and environmental changes associated with mass extinction and glaciation. - GSA Bulletin; 115 (1): 89-104.

1 Department of Earth Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 7GP, UK
2 Institute of Geology, Tallinn Technical University, 7 Estonia Avenue, 10143 Tallinn, Estonia
3 Department of Earth Sciences, University of Liverpool, 4 Brownlow Street, Liverpool L69 7GP, UK
4 Institute of Geology, Tallinn Technical University, 7 Estonia Avenue, 10143 Tallinn, Estonia
5 Institute of Geology, Tartu University, 46 Vanemuise Street, 51014 Tartu, Estonia
6 Institute of Geology, Tallinn Technical University, 7 Estonia Avenue, 10143 Tallinn, Estonia

Abstract:

The two phases of the Late Ordovician mass extinction are approximately coeval with the periods of rapid climate change associated with the onset and demise of the Gondwanan glaciation. In this paper we argue that the distinctive Late Ordovician carbon isotope profile provides a chrono strati graph ic "ruler" against which a sequence of environmental and biotic events may be located. The ruler also allows regional and global high-resolution correlation of successions representing very different environments.

Cores from the Upper Ordovician succession of Estonia and Latvia record a large 13Ccarbonate excursion (up to 6), with a similar profile shape. The consistent relationship between the chemostratigraphy and biostratigraphy in the Baltic region suggests that the isotope profile has a regional chronostratigraphic value. The presence of similar profiles in Nevada, United States, suggests that the excursion is a global chronostratigraphic signal. This interpretation enables a detailed correlation to be made between Upper Ordovician shallow-marine and basinal sequences that have wholly different faunas.

Successions in the Baltic area and in Canada that do not display the model profile are interpreted as incomplete. Reinterpretation of these important successions significantly modifies the global database used to assess the pattern of diversity change during the mass extinction.

Key levels of environmental change have been located against the carbon isotope profile. New oxygen isotope data from brachiopod and ostracode calcite set tight limits on the start of the glacial events. Cooling and sea-level fall started at the same stratigraphic level as the start of the carbon isotope excursion. The later rise in sea level and fall in oxygen isotope values record the end of the glaciation. These restrict the duration of the main glaciation to only 1.5 graptolite zones. We propose that models of the carbon cycle should be adapted to be consistent with the temporal relationships between carbon cycling, sea-level fall, and temperature change documented here.

The chronostratigraphic "ruler" provided by the carbon isotope profiles is used as a scale to determine the sequence of biotic changes and to allow high-resolution correlation of biotic events at different locations. This approach identifies regional similarities and differences in the patterns of extinction. The main phase of graptolite extinction in the Monitor Range, Nevada, for example, is synchronous with the chitinozoan extinction in the Baltic region, but chitinozoan taxa survive to higher levels in Nevada. The benthic faunas in the Baltic region demonstrate that the main extinction event corresponded with the beginning of the isotope excursion at the start of the Hirnantian—the level at which the marine environment started to change rapidly—but that there were further extinctions of species within the early Hirnantian. The cold-adapted Hirnantia fauna did not appear immediately after the extinction in this area.

The relationship between the second phase of extinction and the carbon isotope excursion is less clear, but the available data suggest that the extinction coincides with a time of rapid environmental change, but not at the inception of environmental change, as happened in the first phase.


Key Words: stable isotopes • Ordovician • Hirnantian • extinction • glaciation • Baltic region

Grüsse,

Jens
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