Biotic Turnover Research Articles

Evolution, biogeography, and the utility of mitochondrial 16s and COI genes in phylogenetic analysis of the crab genus Austinixa (Decapoda: Pinnotheridae)

This study used molecular data (mitochondrial 16s and COI) for the first time to explore evolutionary relationships among species of the pinnotherid crab genus Austinixa. Low levels of phylogenetic signal were detected for COI. High levels of phylogenetic signal were detected for 16s, indicating it is a more useful marker for inferring species level phylogenies in Austinixa. Phylogeographic patterns among species of Austinixa are consistent with allopatric speciation due to numerous climatic and oceanographic fluctuations during the last 5–6 my. In addition, all but two species have been derived since the closure of the Isthmus of Panama, a pattern consistent with hypotheses that the marine biota of the Caribbean and southeastern North America underwent a pulse of biotic turnover within the last 2–3 my. Austinixa aidae and Austinixa hardyi had identical 16s sequences, and differed by only 2 bp in COI, raising questions about the validity of A. hardyi as a distinct species.

An integrated approach to taphonomy and faunal change in the Shungura formation (Ethiopia) and its implication for hominid evolution.

Environmental and faunal changes through time have been recorded for many African Plio-Pleistocene sites. Fossil evidence suggests that there is a continuous, if not uniform, transformation of the fauna and flora from the Pliocene through the end of Pleistocene. However, discerning major biotic turnovers and linking them to global and regional climatic changes have been complicated by many factors, notably taphonomy and discontinuity of the fossil evidence, notwithstanding the considerable work of some researchers (e.g., Vrba, E.S., 1988. Late Pliocene climatic events and hominid evolution, in: Grine, F. (Ed.), Evolutionary History of the "Robust" Australopithecines. De Gruyter, New York, pp. 405-426, Vrba, E.S., 1995. The fossil record of African (Mammalia, Bovidae) in relation to human evolution and paleoclimate, in: Vrba, E.S., Denton, G.H., Partridge, T.C., Burkle, L.H. (Eds.), Paleoclimate and Evolution, with Emphasis on Human Origins. Yale University Press, New Haven, pp. 385-424). A sample of over 22,000 fossils collected by the French Omo Expedition, from the Shungura Formation of Ethiopia, was analyzed using an integrated approach to investigate taphonomic and faunal change patterns. The following results are obtained: (1) Univariate and multivariate studies support continuous faunal change from Member A through Member G of the Shungura sequence; (2) Correspondence analysis (CA) on extant bovids in African game parks shows that bovid tribes and genera are generally characterized by habitat specificity; (3) Taphonomic studies demonstrate that the relative abundance of different skeletal elements varies according to depositional environment; (4) CA on 73 localities of the Shungura Formation and 19 mammalian taxa points to a major faunal change around the base of Member G dated to ca. 2.3 Ma. This transformation is characterized by a change to open and edaphic grassland as a dominant type of environment; (5) This major faunal change correlates in time with the appearance of A. boisei. It is tentatively suggested that this major biome change is associated with an anagenetic speciation from A. aethiopicus to A. boisei.

System analysis of links interactions and development of ecosystems of different types

The anthropogenic impact on the Earth's ecosystems are leading to dramatic changes in ecosystem functioning and even to destruction of them. System analysis and the use of heuristic modeling can be an effective means to determine the main biological interactions and key factors that are of high importance for understanding the development of ecosystems. Cycling of limiting substances, induced by the external free energy flux, and trophic links interaction is the basis of the mathematical modeling studies presented in this paper. Mathematical models describe the dynamics of simplified ecosystems having different characteristics: 1) different degrees of biotic turnover closure (from open to completely closed); 2) different numbers. of trophic links (including both “top-down”, “bottom-up” regulation types); 3) different intensities of input - output flows of the limiting nutrient and its total amount in the system. Adaptive values of the changes of lower hierarchical levels (populational, trophic chain level) are to be estimated by integrity indices for total system functioning (e.g. NPP, total photosynthesis). The approach developed can be used for evaluating the contributions of lower hierarchical levels to the functioning of the higher hierarchical levels of the system. This approach may have value for determining biomanipulation management and their assessment.

History of oceanic front development in the New Zealand sector of the Southern Ocean during the Cenozoic—a synthesis

The New Zealand sector of the Southern Ocean (NZSSO) has opened about the Indian‐Pacific spreading ridge throughout the Cenozoic. Today the NZSSO is characterised by broad zonal belts of antarctic (cold), subantarctic (cool), and subtropical (warm) surface‐water masses separated by prominent oceanic fronts: the Subtropical Front (STF) c. 43°S, Subantarctic Front (SAF) c. 50°S, and Antarctic Polar Front (AAPF) c. 60°S. Despite a meagre database, the broad pattern of Cenozoic evolution of these fronts is reviewed from the results of Deep Sea Drilling Project‐based studies of sediment facies, microfossil assemblages and diversity, and stable isotope records, as well as from evidence in onland New Zealand Cenozoic sequences. Results are depicted schematically on seven paleogeographic maps covering the NZSSO at 10 m.y. intervals through the Cenozoic. During the Paleocene and most of the Eocene (65–35 Ma), the entire NZSSO was under the influence of warm to cool subtropical waters, with no detectable oceanic fronts. In the latest Eocene (c. 35 Ma), a proto‐STF is shown separating subantarctic and subtropical waters offshore from Antarctica, near 65°S paleolatitude. During the earliest Oligocene, this front was displaced northwards by development of an AAPF following major global cooling and biotic turnover associated with ice sheet expansion to sea level on East Antarctica. Early Oligocene full opening (c. 31 Ma) of the Tasmanian gateway initiated vigorous proto‐circum‐Antarctic flow of cold/cool waters, possibly through a West Antarctic seaway linking the southern Pacific and Atlantic Oceans, including detached northwards “jetting” onto the New Zealand plateau where condensation and unconformity development was widespread in cool‐water carbonate facies. Since this time, a broad tripartite division of antarctic, subantarctic, and subtropical waters has existed in the NZSSO, including possible development of a proto‐SAF within the subantarctic belt. In the Early‐early Middle Miocene (25–15 Ma), warm subtropical waters expanded southwards into the northern NZSSO, possibly associated with reduced ice volume on East Antarctica but particularly with restriction of the Indonesian gateway and redirection of intensified warm surface flows southwards into the Tasman Sea, as well as complete opening of the Drake gateway by 23 Ma allowing more complete decoupling of cool circum‐Antarctic flow from the subtropical waters. During the late Middle‐Late Miocene (15–5 Ma), both the STF and SAF proper were established in their present relative positions across and about the Campbell Plateau, respectively, accompanying renewed ice buildup on East Antarctica and formation of a permanent ice sheet on West Antarctica, as well as generally more expansive and intensified circum‐Antarctic flow. The ultimate control on the history of oceanic front development in the NZSSO has been plate tectonics through its influence on the paleogeographic changes of the Australian‐New Zealand‐Antarctic continents and their intervening oceanic basins, the timing of opening and closing of critical seaways, the potential for submarine ridges and plateaus to exert some bathymetric control on the location of fronts, and the evolving ice budget on the Antarctic continent. The broad trends of the Cenozoic climate curve for New Zealand deduced from fossil evidence in the uplifted marine sedimentary record correspond well to the principal paleoceanographic events controlling the evolution and migration of the oceanic fronts in the NZSSO.

Causes of mass extinctions

Both bolide impacts and  ood basalts have been suggested as the cause of major perturbations in the history of the biosphere, and the external (cosmic) and internal (Earth-bound) factors of mass extinctions were usually presented as competing paradigms (‘impactor’ contra ‘volcanist’ debate: Glen 1994, Frankel 1999). We present below some new evidence from the literature that both these ideas are not conclusive in their pure form and that both groups of factors (terrestrial and extraterrestrial) might have been interrelated, with some extinctions at least being the result of a cumulative effect of volcanism and impact. In a recent review of extinction problematics, Olsen (1999) presented the new data on the giant igneous event across the Triassic–Jurassic (T–J) boundary, and considered a bolide impact as a ‘viable alternative’ for a factor responsible for the T–J biotic turnover. On the other hand, many authors see both the Earthbound (volcanism, sea-level change, oceanic anoxia, etc.) and extraterrestrial mechanisms as jointly participating in a catastrophic environmental disturbance on a global scale; an impact is discussed and modelled as a trigger for paroxysmal magmatism (e.g. Boslough et al. 1996; Jones 2000). From this standpoint, even the end-Permian Siberian Traps might be induced by a violent impact (Frankel 1999, p. 155; see also new evidence from extraterrestrial noble gases in fullerenes in Becker et al. 2001). With respect to the best-studied extinction, at the Cretaceous– Tertiary (K–Pg) boundary, authors of the impact hypothesis (Alvarez et al. 1984) have already envisaged the ‘terminalCretaceous extinctions on two time scales: a slow decline unrelated to the impact and a sharp truncation synchronous with and probably caused by the impact’. As emphasized by MacLeod (1998), the K–Pg multicausal scenario is suggested by (1) the high number of bio-crises known to have occurred prior to the onset of impact debris, (2) diverse ecological characters and differing fates of known survivors, and (3) the group-speciŽ c biogeography of the biotic change. This opinion is supported more and more now by independent data on high-resolution biotic and climatic patterns (e.g. Li & Keller 1998; Zinsmeister 1998; but see Cadee 1999). Of major signiŽ cance is the recognition of the relative timing of impact and the Deccan Trap volcanism, due to identiŽ cation of an Ir-enriched K–Pg boundary layer sandwiched between basalt  ows (see summary in Courtillot et al. 2000). The presence of three lava  ows below this layer makes it clear that the overall brief period of volcanism (three chrons only) had already been active when the ejecta layer was deposited and the impact of the K–Pg bolide evidently did not trigger the Deccan volcanism, which started 0.5– 0.1 m.y. earlier. In fact, this observation can be expected in light of the current understanding of impact cratering processes and a minimal probability of impact-induced volcanism (Melosh 2000). Although some authors believe the K–Pg extinction could have resulted from environmental stress even without signiŽ cant extraterrestrial addition (e.g. Li & Keller 1998), others (Sutherland 1996; Chatterjee 1997; Hallam & Wignall 1997; Courtillot 1999; Keller 2000) emphasize contemporaneity of endogenous and exogenous events precisely at the boundary. Notably, many workers believe the mantle plume event is not a terrestrial alternative for the end-Cretaceous extraterrestrial catastrophe scenario. A combination of longer-term abnormal hot-spot/hotline deep-plume volcanism, catastrophically Ž nalized in possibly multiple K–Pg impact episodes (see Keller 2000), is favoured in Sutherland’s (1996) integrative model. The end-Cretaceous global crisis is seen as a cumulative effect of gradually induced extinction pressures, in response to a prolonged vanishing of the Aptian to Maastrichtian greenhouse period, promoted by the mantle superplume activity (Pele hypothesis of Landis et al. 1996). Similar conclusions are being formulated for some other Phanerozoic mass extinctions (see Walliser 1996; Hallam & Wignall 1997):

Regional biotic turnover dynamics in the Plio-Pleistocene molluscan fauna of the Wanganui Basin, New Zealand

Sampling gaps and variable preservation greatly complicate attempts to interpret patterns of biotic change from the fossil record. This study investigates whether observed patterns of first and last occurrences of mollusc species can be explained by poor or uneven sampling of a diverse (>800 species) Plio-Pleistocene marine fauna from the Wanganui Basin, North Island, New Zealand. Estimates of sampling completeness are high, ranging from 78 to 98% (mean 88%) despite several sampling gaps in the succession. Stratigraphic permutation tests suggest that most of the apparent peaks in the local first and last appearances of species are caused by uneven sampling. However, peaks of last occurrences of species during the Late Pliocene (2.5–2.0 Ma), and of first occurrences in the Middle Pleistocene (0.75–0.5 Ma), cannot be classified as artefacts, and appear to represent real biological events. A wide Early Pleistocene gap in the stratigraphic section in the basin prevents the precise estimation of the magnitude, duration or timing of these biotic transitions. Mechanisms for the biotic change remain elusive. Better palaeoenvironmental control and detailed ecological study of selected molluscan clades are required to test alternative causal hypotheses. However, similar turnover episodes have been documented from the temperate and tropical Western Atlantic, suggesting a global biotic perturbation on shallow shelves in response to Plio-Pleistocene global environmental change.

Did the Late Paleocene thermal maximum affect the evolution of larger foraminifers? Evidence from calcareous plankton of the Campo Section (Pyrenees, Spain)

The larger foraminifer turnover (LFT), which marks the base of the Ilerdian stage, may be related to the Late Paleocene Thermal Maximum (LPTM), or be at least nearly coeval with that climatic event. Thus, the impact of the LPTM may have been greater than hitherto realised, having also affected mid-latitude shallow-marine biota. This conclusion has been reached after a re-study of the calcareous plankton of the uppermost Paleocene and lowermost Eocene interval of the Campo section in the central southern Pyrenees. Campo is an important reference section because it contains larger foraminifers, planktic foraminifers and calcareous nannofossils, and their co-occurrence was used to intercalibrate their respective zonal schemes. Previous studies at Campo placed the onset of planktic foraminiferal Zone P5 near the base of the Ilerdian, and the calcareous nannofossil NP9/NP10 chronal boundary (sensu Bybell, L.M., Self-Trail, J.M., 1995. Evolutionary, biostratigraphic and taxonomic study of calcareous nannofossils from a continuous Paleocene/Eocene boundary section in New Jersey. US Geol. Surv. Prof. Pap. 1554, pp. 1–36) not less than 150 m above the Ilerdian lower limit. By these estimates, the LPTM (known to have occurred in the middle part of Zone P5 and just before the NP9/NP10 boundary) would be an event much younger than the LFT. However, our reexamination of planktic foraminifers suggests that the base of the Ilerdian is probably situated at the middle of Zone P5 (a possibility proposed by Hillebrandt in 1965, but denied by later authors). For instance, Morozovella occlusa has been found for the first time in the Campo section. Its Last Appearance Datum (LAD), which in the Pyrenees was approximately coeval with that of Morozovella velascoensis (event used to place the top of Zone P5), has been identified in beds situated less than 70 m above the base of the Ilerdian. Such thickness represents a time span of a similar magnitude as the one which separated the LPTM and the LAD of M. occlusa in the deep-water hemipelagic succession of the Basque Basin, in the western Pyrenees. Autochthonous calcareous nannofossils are neither abundant nor well preserved in most of the studied interval, with Rhomboaster bramlettei (the marker of the base of Zone NP10) being extremely rare in lower and middle Ilerdian beds, a fact that makes it very difficult to fix the position of the NP9/NP10 boundary in the Campo section. However, the bases of zones NP9 and NP11 have been located, and they support the zonation with planktic foraminifers. These new data suggest that the LFT and the LPTM may have been coeval or nearly so, a possibility reinforced by correlation with sections of the Basque Basin. Specialists of larger benthic foraminifers can easily delineate the LFT in shallow water carbonate successions of the Tethys domain, and they propose to place the Paleocene/Eocene boundary at the base of the Ilerdian stage. On the other hand, the deep benthic extinction event (BEE) is a major global biotic turnover in the bathyal and abyssal realms, while the ∂13C excursion (CIE) is an excellent tool for correlation between marine and terrestrial records. Therefore, the synchrony or near synchrony postulated here between the LFT, BEE and CIE (all of them probably related to the LPTM) would argue strongly in favour of these events as the criterion to officially redefine the Paleocene/Eocene boundary.

Computer modeling of the biotic cycle formation in a closed ecological system.

The process of biotic turnover in a closed ecological system (CES) with an external energy flow was analyzed by mathematical modeling of the biotic cycle formation. The formation of hierarchical structure in model CESs is governed by energy criteria. Energy flow through the ecosystem increases when a predator is introduced into a "producer-reducer" system at steady state. Analysis of the model shows that under certain conditions the presence of the primary predator with its high mineralization ability accelerates the biotic turnover measured by primary production. We, therefore, conclude that for every system it is possible to find a suitable predator able to provide the system with a higher biotic turnover rate and energy consumption. Grant numbers: 99-04-96017/2000.

Closed Artificial ecosystems as a means of ecosystem studies for Earth and space needs.

Closed Artificial ecosystems (CAES) have good prospects for wide use as new means for quantitative studies of different types of both natural ecosystems and man-made ones. The paper deals with the discussion of three points of CAES applications. The first one is of importance for theoretical ecology development and is connected with bringing together "holistic" and "merological" approaches in ecosystems studies. Using CAES, we can combine both approaches, taking into account the biotic turnover of limiting substrates which few in number even for complicated natural ecosystems. The second CAES use concerns the development of "ecosystems health" concept and application of a key-factor-approach for the indication and measurement of healthy unhealthy state and functioning of ecosystems or their links. The third use is more of an applied nature, oriented to the intensification of bioremediation or biodepollution processes in different types of ecosystems, including the global biosphere. Grant numbers: N 99-04-96017, N25.

Can long-distance dispersal be inferred from the New Zealand plant fossil record?

New Zealand is generally thought to have been physically isolated from the rest of the world for over 60 million years. But physical isolation may not mean biotic isolation, at least on the time scale of millions of years. Are New Zealand’s present complement of plants the direct descendants of what originally rafted from Gondwana? Or has there been total extinction of this initial flora with replacement through long-distance dispersal (a complete biotic turnover)? These are two possible extremes which have come under recent discussion. Can the fossil record be used to decide the relative importance of the two endpoints, or is it simply too incomplete and too dependent on factors of chance? This paper suggests two approaches to the problem—the use of statistics to apply levels of confidence to first appearances in the fossil record and the analysis of trends based on the entire palynorecord. Statistics can suggest that the first appearance of a taxon was after New Zealand broke away from Gondwana—as long as the first appearance in the record was not due to an increase in biomass from an initially rare state. Two observations can be drawn from the overall palynorecord that are independent of changes in biomass: (1) The first appearance of palynotaxa common to both Australia and New Zealand is decidedly non-random. Most taxa occur first in Australia. This suggests a bias in air or water transport from west to east. (2) The percentage of endemic palynospecies in New Zealand shows no simple correlation with the time New Zealand drifted into isolation. The conifer macrorecord also hints at complete turnover since the Cretaceous.

Foreshadowing the Cretaceous–Tertiary extinction: none if by land, two if by sea

There is broad consensus that the end of the Cretaceous period [144–65 million years ago (Mya)] was punctuated by a major bolide impact, which unleashed global environmental catastrophe. However, paleontologists are still scuffling over the biological consequences of that impact. Was everything rosy until that fateful Cretaceous day? Or, were earth-based processes, such as climate change and volcanism, destabilizing biological communities and setting the stage for mass extinction? The timing and rate of extinctions are crucial to these mechanistic hypotheses. Two recent papers bring perspectives from among the biggest and smallest organisms to suffer Cretaceous–Tertiary (K–T) extinctions.Peter Sheehan et al.1xDinosaur abundance was not declining in a ‘3 m gap’ at the top of the Hell Creek Formation, Montana and North Dakota. Sheehan, P.M. et al. Geology. 2000; 28: 523–526CrossrefSee all References1 argue that the abundance of dinosaur fossils in southwestern North Dakota and adjacent areas of Montana was not declining in the upper meters of Cretaceous sediments. The team used 200 volunteers to search more than 11 000 000 m2xLate Cretaceous sea-level changes in Tunisia: a multi-disciplinary approach. Li, L. et al. J. Geol. Soc. 2000; 157: 446–458Crossref | Scopus (71)See all References2 of Cretaceous outcrop at 1.5-m and 3.0-m intervals, and to map the distribution of dinosaur fossils. Although dinosaur fossils are not evenly distributed within the latest Cretaceous sediments, the upper meters of the Cretaceous were not among the most dinosaur-poor zones. This result contrasts with previous reports of a ‘3-m gap’ below the K–T boundary, in which vertebrate fossils are uncommon or absent. The ‘gap’ has led some to speculate that dinosaurs were rare or extinct millennia before the terminal-Cretaceous impact and to propose that other forces led to their demise. Sheehan et al. conclude that dinosaurs were regionally abundant until the K–T impact, thus supporting the causal role of the bolide in their extinction.By contrast, Gerta Keller and colleagues have long argued that patterns of extinction in marine foraminifera do not support a single, catastrophic extinction mechanism. New results, which combine paleontological data with geological and geochemical perspectives, show that climate-related changes in the marine system foreshadow biotic turnover. In previous work using stable oxygen isotope data from a series of globally distributed marine cores, Li and Keller showed that both intermediate water and sea surface temperatures increased by 3–4°C between 450–200 Ka (thousand years) before the end of the Cretaceous. Warming was followed by a cooling of 2–3°C during the past 200 Ka of the Cretaceous. In their most recent paper, Li et al.2xLate Cretaceous sea-level changes in Tunisia: a multi-disciplinary approach. Li, L. et al. J. Geol. Soc. 2000; 157: 446–458Crossref | Scopus (71)See all References2 detailed the analysis of a series of sections in Tunisia showing that the warming and cooling events were paralleled by rising and falling sea level2xLate Cretaceous sea-level changes in Tunisia: a multi-disciplinary approach. Li, L. et al. J. Geol. Soc. 2000; 157: 446–458Crossref | Scopus (71)See all References2. They conclude that these events played an important role in marine extinctions across the K–T boundary.It does not seem surprising that lineages as disparate as foraminifera and dinosaurs would respond differently to the same series of geologic events. Rather, these papers highlight that the end of the Cretaceous was a complex time, for which multifaceted hypotheses will be required. Sorting out the complexity of the biological signal is the first step to such comprehensive hypotheses of extinction and survival across the K–T boundary.

Enregistrement des événements remarquables de la limite Crétacé–Tertiaire dans la coupe d'Ellès (Tunisie)

Abstract The review of the Cretaceous–Paleogene interval deposits of the Elles section based on a detailed sampling gives a good characterization of the K/T boundary and reconstitution of the geological events underlining this boundary. Thus, the discovery of a thin Ir-rich layer, with Ni and Cr-rich spinel and shocked quartz is a well preserved record of the known cosmic event that occurred at that time. This sudden event is also corroborated by the quantitative and qualitative organic components distribution along the K/T interval. Added to some other long-term events (e.g. climatic, eustatic), it generated a mass extinction, at the K/T boundary, of specialists among the Globotruncanids and Heterohelicids planktic foraminifera species. Following this biological crisis, the biotic turnover into the Danian is slow. The underlining of all the Cretaceous–Tertiary interval biozones attests that the Elles section is complete. If compared to the K/T stratotype and El Kef II sections, the Elles section is found to display a better exposure of the K/T interval and shows more expanded zones and subzones. Such characteristics promote the Elles section to be considered as a parastratotype.

Silica-secreting biota and mass extinctions: survival patterns and processes

High survival and at least regional blooms of siliceous marine groups, with coeval retreat of calcareous organisms, is known from the Frasnian–Famennian (radiolarians, silicisponges) and end-Cretaceous (diatoms, radiolarians) mass extinctions. A strictly actualistic approach to the palaeoecology of silica-secreting biota is of limited significance, especially for silica-enriched and oligotrophic Palaeozoic epeiric seas and, in particular, during biotic turnovers. Adaptation among silica-secreting marine plankton has led toward more efficient utilization of shrinking dissolved silica resources within surface waters of Cenozoic oceans, as shown by both radiolarians and diatoms. This biosiliceous signal during the major biotic crises is mostly explained by a large-scale increase in volcano-hydrothermal activity during major plate-boundary re-arrangements, triggering global ecosystem perturbations. The cumulative stimulus favoured siliceous versus calcareous biota growth, deposition and preservation because of (1) a higher rate of input of silica and other nutrients, promoting eutrophication pulses, (2) a punctuated (or reversed) greenhouse climatic effect, episodically coupled with (3) vigorous oceanic circulation due to a variety of volcanogenic upwellings and turnovers. Siliceous communities, adapted to more eutrophic conditions, have thrived in the stressed niches. On the other hand, expanding anoxia and nutrification may lead to a stepwise loss of deep-water niches and consequent selective decline of more specialized oligotrophic radiolarians and other pelagic biota. By contrast, siliceous faunas experienced severe non-selective losses during the end-Permian mass extinction (recorded as a `chert gap'). Thus, the greatest environmental disaster of the Phanerozoic was driven by a unique set of killing factors, extremely effective for radiolarian productivity, e.g., due to the combination of a drastic volcanic winter with expanding superanoxia.

The K/T stratotype section of El Kef (Tunisia): Events and biotic turnovers

Abstract At El Kef (NW Tunisia), the absence of bioturbation and the high sedimentation rate preserve a high resolution record of the Cretaceous-Tertiary transition. In the upper part of the Maastrichtian (Mayaroensis zone) planktonic foraminifer assemblages show a clear marine regression while the variations of spore-pollen contents show a trend related to a climatic cooling. In addition to these gradual changes, one can see a sharp discontinuity at the top of the Mayaorensis zone. This discontinuity is marked by a thin millimeter-scale brownish goethite layer containing Ir and Ni-rich spinels, the remains of the K/T cosmic catastrophe. This event occurred in coincidence with a drastic reduction (90–95%) of calcareous microfossils (coccoliths, foraminifers, ostracods,…). In contrast, the noncalcareous algae (dinoflagellates) seem to have been insensitive to the K/T event. Moreover, spores and pollen grains from the nearby lands do not indicate such a change in the continental vegetation. The selectivity of the extinctions can be explained neither by the absorption of solar radiations by stratospheric dusts produced by the K/T impact nor by an heat flash burning everything on the Earth. All species requiring solar light for their development should have been affected. The severity of the crisis for calcareous marine species seems due to the, difficulty of microorganisms to fix carbonate. It is well known that trace elements play a fundamental role in the development of micro-organisms, particularly in the construction of their carbonate shells and tests. A significant infall of meteoritic material in the oceans could have severely disturbed the normal processes. The disappearance of Coccoliths should have produced disastrous consequences in surface oceanic warm waters, where they are the main providers of oxygen. Judging from El Kef data, the crisis was fatal to all highly specialized taxa; the others, more adaptable, could find refuge in less disturbed places and recovered when normal conditions prevailed again. In the continental vegetation, it seems that the angiosperms, newly arrived in the vegetal realm and consequently having greater adaptative ability, cross the K/T boundary without very catastrophic extinctions.

Is famine a common factor in marine mass extinctions?

A comparison of biotic, sedimentary, and stable isotope patterns across the mass extinctions at the Cenomanian-Turonian and Cretaceous-Tertiary boundaries shows that despite differences in initial trigger, duration, and scale of the two events, reduction of marine primary productivity is a common factor. Our model predicts that zooplankton and suspension feeders will be most severely affected during marine mass extinctions, survivors will be small, and biotic turnover will be concentrated during significant δ 13 C excursions. The model is thus testable by using data from other mass extinctions.

Late Pliocene Paleoecologic Reconstructions Based on Ostracode Assemblages from the Sagavanirktok and Gubik Formations, Alaskan North Slope

Shallow-marine ostracode assemblages from upper Pliocene sediments of the upper part of the Sagavanirktok Formation and lower part of the Gubik Formation record the last warm period that occurred before the onset of significant cooling of the Arctic Ocean and the initiation of Northern Hemisphere continental glaciation. The informally named Colvillian and Bigbendian transgressions represent the oldest deposits of the Gubik Formation and are dated, based on various lines of evidence, between 2.48 and 3 Ma. Ostracode faunas from the lower part of the Gubik Formation indicate a cold-temperate to subfrigid marine climate with summer bottom temperatures 1-4 C warmer than today. Deposits of the upper part of the Sagavanirktok Formation at Manning Point and Barter Island are older than Colvillian sediments but are believed to be late Pliocene in age and contain an ostracode fauna that has many species in common with the lower part of the Gubik Formation. The Sagavanirktok ostracode faunas indicate a cold-temperature to subfrigid marine climate, similar to that inferred for the lower part of the Gubik Formation, with summer bottom temperatures 1-3 C warmer than today. The opening of Bering Strait at about 3 Ma altered Arctic Ocean assemblage composition as Pacific species migrated into the Arctic and North Atlantic oceans. The admixture of evolutionarily distinct faunas from the Atlantic and Pacific oceans identifies Colvillian (and younger) faunas and provides a convenient reference horizon in the Alaskan fossil record. The marine climatic deterioration that followed the Bigbendian appears to have been abrupt and is documented by biotic turnover, with large numbers of species extinctions and first appearances of new species. The change in species composition can be attributed to the cooling of the Arctic Ocean during the late Pliocene.Key words: Pliocene, ostracode, Sagavanirktok Formation, Nuwok Member, Gubik Formation, Alaska, North Slope, shallow marine

Climatic and biotic change in the late Triassic

Generally arid conditions that pervaded much of Europe and North America during the late Triassic were interrupted by a wet monsoonal climatic phase during Middle and Late Carnian times. Extensive fluviatile sandstones deposited at this time throughout the region, occur within a thick sequence of playa-lake mudstones. The sandstones occasionally contain kaolinite, suggesting a humid climate. An extreme δ 13 C depletion in a shallow marine sequence of this age in Israel has been interpreted as evidence for an influx of freshwater. A widespread change from carbonates to clastics in marine sequences at this time may also be climate-related. Water-course cave systems in limestone areas exposed during the late Triassic indicate high levels of runoff during the Middle and Late Carnian. The marine invertebrate fauna shows a significant turnover at the end of the Early Carnian. The terrestrial fauna and flora were relatively unaffected at this time but subsequently diversified prior to a major biotic turnover at the Carnian-Norian boundary. These periods of biotic change appear to be synchronous with the onset and cessation of a Carnian humid phase. The change to a monsoonal climate during this interval has been documented over more than 90° of longitude between 5° and 50° north of the Triassic equator. It may have been caused by rising atmospheric CO 2 levels due to volcanism associated with the incipient dispersal of Pangaea.

Soft-Bottom Paleocommunity Dynamics in the Cenomanian - Turonian Boundary Extinction Interval of the Western Interior, United States

Extensive species-level, marine biotic turnover is recorded across the Cenomanian-Turonian (C-T) Stage boundary in the western interior of the United States (Fig. 1) and intercontinentally (e.g., Wright and Kennedy, 1981; Elder, 1987; Jarvis et al., 1988). Widespread intensification and expansion of the oxygen minimum zone have been widely proposed as a causal mechanism for the organic-rich deposits and biotic extinctions found in strata of this age (e.g., Schlanger et al., 1987; Jarvis et al., 1988). Accompanying these extinctions were large-scale reorganizations of community compositions and trophic structures. In the western interior, repeated community replacement was associated with widespread cyclic sedimentation, and long-term community turnover was related to extended periods of dysaerobia and possibly low salinity conditions. Community compositions and distributions were therefore largely controlled by physical rather than biological factors. This paper will document how these oceanographic and sedimentologic factors affected community patterns across the C-T boundary.

Biotic turnover in benthic foraminifera across the cretaceous/tertiary boundary at El Kef, Tunisia

The Cretaceous/Tertiary boundary section exposed near El Kef, Tunisia is the most complete boundary sequence known to date. It contains nearly 1 m of black and gray boundary clay containing planktic foraminiferal Zone POa, b, 2 m of clayey shales of P1a ( Globigerina eugubina) Zone and 4 m of shales and marls of P1b ( G. taurica) Zone. Quantitative analysis of the benthic foraminiferal fauna suggests that deposition during the latest Cretaceous occurred in an upper slope to outer shelf environment which shallowed at the K/T boundary to an outer to middle shelf depth and shallowed further by P1b time to mid-shelf depth. A major reduction in benthic diversity occurred near the K/T boundary with about 50% of the fauna disappearing. Diversity remained an average of 37% lower during deposition of the first 3 m of sediment above the boundary (POa, b-P1a) and productivity was very low. Surviving and thriving foraminifers during this interval were primarily low oxygen tolerant epifaunal and infaunal species. A sharp decrease in the low oxygen tolerant fauna and appearance of a shallow mid-shelf fauna at about 4 m above the boundary (P1b Zone) signals a second regression, return to higher oxygen levels and higher productivity. Although the environmental effects of the K/T boundary event can be inferred from benthic faunas, the ultimate cause remains elusive. Faunal changes prior to and the long recovery period after the K/T boundary are difficult to explain by a single impact hypothesis.

Abrupt Climate Change and Extinction Events in Earth History

Slowly changing boundary conditions can sometimes cause discontinuous responses in climate models and result in relatively rapid transitions between different climate states. Such terrestrially induced abrupt climate transitions could have contributed to biotic crises in earth history. Ancillary events associated with transitions could disperse unstable climate behavior over a longer but still geologically brief interval and account for the stepwise nature of some extinction events. There is a growing body of theoretical and empirical support for the concept of abrupt climate change, and a comparison of paleoclimate data with the Phanerozoic extinction record indicates that climate and biotic transitions often coincide. However, more stratigraphic information is needed to precisely assess phase relations between the two types of transitions. The climate-life comparison also suggests that, if climate change is significantly contributing to biotic turnover, ecosystems may be more sensitive to forcing during the early stages of evolution from an ice-free to a glaciated state. Our analysis suggests that a terrestrially induced climate instability is a viable mechanism for causing rapid environmental change and biotic turnover in earth history, but the relation is not so strong that other sources of variance can be excluded.