Major Biotic Events Research Articles

Evidence for local and global redox conditions at an Early Ordovician (Tremadocian) mass extinction

Profound changes in environmental conditions, particularly atmospheric oxygen levels, are thought to be important drivers of several major biotic events (e.g. mass extinctions and diversifications). The early Paleozoic represents a key interval in the oxygenation of the ocean–atmosphere system and evolution of the biosphere. Global proxies (e.g. carbon (δ13C) and sulfur (δ34S) isotopes) are used to diagnose potential changes in oxygenation and infer causes of environmental change and biotic turnover. The Cambrian–Ordovician contains several trilobite extinctions (some are apparently local, but others are globally correlative) that are attributed to anoxia based on coeval positive δ13C and δ34S excursions. These extinction and excursion events have yet to be coupled with more recently developed proxies thought to be more reflective of local redox conditions in the water column (e.g. I/Ca) to confirm whether these extinctions were associated with oxygen crises over a regional or global scale.Here we examine an Early Ordovician (Tremadocian Stage) extinction event previously interpreted to reflect a continuation of recurrent early Paleozoic anoxic events that expanded into nearshore environments. δ13C, δ34S, and I/Ca trends were measured from three sections in the Great Basin region to test whether I/Ca trends support the notion that anoxia was locally present in the water column along the Laurentian margin. Evidence for anoxia is based on coincident, but not always synchronous, positive δ13C and δ34S excursions (mainly from carbonate-associated sulfate and less so from pyrite data), a 30% extinction of standing generic diversity, and near-zero I/Ca values. Although evidence for local water column anoxia from the I/Ca proxy broadly agrees with intervals of global anoxia inferred from δ13C and δ34S trends, a more complex picture is evident where spatially and temporally variable local trends are superimposed on time-averaged global trends. Stratigraphic sections from the distal and deeper part of the basin (Shingle Pass and Meiklejohn Peak) preserve synchronous global (δ13C and δ34S) and water column (I/Ca) evidence for anoxia, but not at the more proximal section (Ibex, UT). Although geochemical and paleontological evidence point toward anoxia as the driver of this Early Ordovician extinction event, differences between I/Ca and δ13C–δ34S signals suggest regional variation in the timing, extent, and persistence of anoxia.

Landforms as extended composite phenotypes

ABSTRACTBiotic influences on geomorphology (and vice‐versa) are ubiquitous. This paper explores whether landforms may be extended (composite) phenotypes of biota, based on four criteria: process–form relationships between biota and landforms; evolutionary synchrony; selective pressure via ecosystem engineering and niche construction; and positive feedback benefitting the engineer organism(s). Coral reefs, peat bogs, biomantles, insect mounds, grassland soils, salt marshes, mangrove swamps, and some vegetation‐dependent sand dune types clearly meet these criteria. Karst landforms, meandering rivers, and tree uprooting pit‐mound systems meet the first three criteria, but positive feedback to engineer organisms has not been established. Research in biogeomorphology will surely identify other extended phenotypes. Implications are that biological evolution will continue to drive landscape metamorphosis, the appearance of new landform types, and presumably the disappearance of extended phenotypes associated with extinct species. Independently of extended phenotypes, tightly‐coupled geomorphological–ecological interactions such as coevolution, and biogeomorphic forms of ecosystem engineering and niche construction are common. The toposphere, encompassing Earth's landforms, is partly a biotic construct. Some elements would be present in an abiotic world, but the toposphere would not exist in anything resembling its contemporary state without a biosphere. This raises important questions with respect to Earth system evolution. The bio, litho‐, atmo‐, hydro‐, topo‐, and pedospheres coevolve at the global scale. Major biotic events have driven revolutions in the other spheres, but the atmosphere and the global hydrological system seem to have been relatively steady‐state at the global scale. The toposphere and pedosphere have not. This suggests that perhaps landforms and soils provide the major mechanisms or degrees of freedom by which Earth responds to biological evolution. Landforms and soils may thus be the ‘voice’ of the biosphere as it authors planetary change, even if clear biotic signatures are lacking. Copyright © 2015 John Wiley & Sons, Ltd.

A new procedure to assess dissolution based on experiments on Pliocene–Quaternary foraminifera (ODP Leg 160, Eratosthenes Seamount, Eastern Mediterranean)

Abstract This study aims to reveal the effects of differential dissolution on the quantitative composition of foraminiferal assemblages. Through this, we develop a procedure to evaluate dissolution in foraminiferal assemblages in order to reduce taphonomic bias in paleoenvironmental reconstructions. To achieve this, we carried out experimental dissolution studies on foraminiferal assemblages from Pliocene–Quaternary sapropel-bearing sequences of Eratosthenes Seamount, Eastern Mediterranean (ODP Leg 160, Site 966). Our experiments refine two general observations on modern foraminifera, which are in fact only applicable for size fraction of 125–630 μm: 1) Dissolution increases the relative abundance of fragmentation in planktic assemblages (%Fp) more rapidly than in benthic assemblages (%Fb); 2) with moderate to severe dissolution, planktic foraminiferal numbers decrease faster than benthic foraminiferal numbers. Besides, our experiments show that dissolution susceptibility differs between architectural types. As dissolution progresses, the proportions of uniloculars, uniserials, biserials, then tri-multiserials and miliolines gradually decrease and benthic foraminiferal assemblages become relatively enriched in planispirals and trochospirals. These findings allow re-evaluation of commonly used foraminiferal dissolution indices. Accordingly, %F can be used to assess dissolution in weakly to moderately dissolved planktic assemblages (size fraction > 125 μm) in non-lithified sediments; P/B ratios are a better indicator of dissolution in moderately to severely dissolved assemblages, but only in the size fraction > 125 μm and not in the smaller size fraction. Anomalously low foraminiferal numbers (compared to background numbers in a sequence) can also hint at dissolution. A dominance of planispiral, trochospiral and large taxa in benthic assemblages may be used as an additional dissolution indicator. Based on these results, we propose a new procedure to evaluate dissolution in foraminiferal assemblages. Application of this procedure to some published Pliocene–Quaternary records reveals a good agreement between our experiments and expected consequences of dissolution for these records. In order to achieve more robust interpretations based on quantitative foraminiferal data, the dissolution procedure proposed should be tested elsewhere and could develop into a standard micropaleontologic procedure. This is useful in not only open ocean records, but also continental margin studies dealing with major biotic events or studies employing P/B ratios for sea-level reconstructions are expected to benefit from this approach.

Color/facies changes and Global Events, a hoax? A case study from the Lochkovian (Lower Devonian) in the Spanish Central Pyrenees

Five Pyrenean Lochkovian sections are studied aiming at the recognition of the influence of facies changes in Biotic Global Events. The wealthy conodont record permits precise age control of strata in the different sections, and high-precision correlations, both regional and intercontinental; it also permits identification of major Global Biotic Events for this group of fossils. The comparison of the bio- and chronostratigraphic framework with the different Facies identified in the Pyrenean rocks shows that contrasting color (and facies) changes are not synchronous in the studied sections and that the major Biotic Events are not directly related to relevant facies shifts. These results question the opinion of direct tie relations between strong color changes and Global Events in the Lower Devonian, and reinforce the value of palaeontological tools for Global studies and correlations.

Ordovician and Silurian sea–water chemistry, sea level, and climate: A synopsis

Following the Cambrian Explosion and the appearance in the fossil record of most animal phyla associated with a range of new body plans, the Ordovician and Silurian periods witnessed three subsequent major biotic events: the Great Ordovician Biodiversification Event, the end-Ordovician extinction (the first animal extinction and second largest of the five mass extinctions of the Phanerozoic), and the Early Silurian post-extinction recovery. There are currently no simple explanations for these three major events. Combined extrinsic (geological) and intrinsic (biological) factors probably drove the biodiversifications and radiations, and the appearance and disappearance of marine habitats have to be analysed in the frame of changing palaeogeography, palaeoclimate and sea-water chemistry. The present paper reviews the relationships of the three biotic events to chemical and physical processes occurring in the ocean and atmosphere during the Ordovician and Silurian, including sea-level changes, geochemical proxies (δ 13C, δ 18O, 87Sr/ 86Sr) of the ocean waters, and the evolution of the atmosphere (oxygen and carbon dioxide content).

Experimental dissolution of a fossil foraminiferal assemblage (Paleocene–Eocene Thermal Maximum, Dababiya, Egypt): Implications for paleoenvironmental reconstructions

Dissolution experiments were carried out on a foraminiferal assemblage from the Paleocene–Eocene Thermal Maximum (PETM) at Dababiya, Egypt, in order to: 1) reveal the effects of differential dissolution on the composition of the foraminiferal assemblage and 2) develop objective criteria for the evaluation of dissolution in foraminiferal assemblages used in early Paleogene paleoenvironmental reconstructions, particularly with respect to neritic Midway-type assemblages from the Paleocene/Eocene transition. Our results confirm two general observations on modern foraminifera: 1) planktic foraminifera are much more vulnerable to dissolution than benthic foraminifera, leading to depressed P/B ratios and 2) dissolution susceptibility differs between size fractions, with the smaller specimens dissolving more rapidly than the bigger ones, leading to a larger average size of the remaining assemblage. Within a size fraction, wall structure and thickness are considered to be the main factors controlling differential dissolution susceptibility. We propose a ranking scheme for taxa with respect to dissolution resistance. Among the benthic taxa, Lenticulina is most resistant, followed by the agglutinated Gaudryina cf. ellisorae and Alabamina midwayensis. Biserial and triserial hyaline taxa and the porcelaneous Spiroloculina sp. are most susceptible to dissolution, whereas rotaliines, such as Cibicidoides and Anomalinoides have an intermediate susceptibility. This implies that mild dissolution of a Midway-type benthic assemblage leads to a relative enrichment in Lenticulina, Gaudryina and rotaliines. Amongst planktic foraminifera, the muricate taxa Acarinina and Morozovella are most resistant, followed by the cancellate Subbotina. The smooth and generally small Globanomalina and Zeauvigerina are least resistant to dissolution. Our data enable to objectively evaluate various degrees of dissolution in benthic and planktic foraminiferal assemblages retrieved from the lower Paleogene Tethyan outcrops. In this way taphonomic artifacts can be readily distinguished from paleoenvironmental signals affecting the primary composition of the assemblages. More generally, we propose that the combined use of foraminiferal numbers, P/B ratio and relative abundances of non-calcareous agglutinated taxa and Lenticulina may provide a powerful proxy for assessing dissolution in hemipelagic assemblages from Cenozoic and upper Cretaceous continental margins. In order to achieve more robust pre-Quaternary paleoenvironmental reconstructions based on quantitative foraminiferal data, application of dissolution proxies, like proposed here, or in slightly modified form, should become a more widely used micropaleontologic procedure. Particularly continental margin studies dealing with major biotic events (e.g. PETM) or employing P/B ratios for sea-level reconstructions should benefit from such an approach.

Benchmark data of a changing sea — Palaeogeography, Palaeobiogeography and events in the Central Paratethys during the Miocene

The reconstruction and interpretation of terrestrial ecosystems and vegetational patterns in the Central European Miocene have to take into account the influence of a vast landlocked water body, namely the Paratethys Sea. As a northern appendage of the early Mediterranean Sea, it spanned a north–south gradient of at least 4° latitude and has been suggested to represent some kind of “palaeothermometer”, which reflected slight expansions or restrictions of climatic belts. Due to its vulnerable marine connections it was also highly susceptible to major (global) sea-level fluctuations which are reflected in phases of endemism. Hence, a fairly continuous record of marine nearshore assemblages throughout the Miocene reflects an extraordinary interplay of sea-level fluctuations, changes in climate, immigrations, and blooms in autochthonous elements. Whilst biostratigraphic implications of these patterns were recognised early in palaeontology, a biogeographic model is still lacking. The intermingling of palaeogeographic terms with those restricted to biogeography is still commonly used — a situation which can be overcome by integrating data from different biota into a new palaeobiogeographic scheme.Herein, a data-set of 1809 species-level taxa (Gastropoda and Foraminifera) serves as the base for interpretations. Gastropods in particular turned out to be of greatest value for the differentiation of palaeobiogeographic units due to sensitive reactions to all environmental parameters and occupying a wide range of ecological niches. Based on the excellent gastropod record the Proto-Danubian Province, Early Danubian P., Danubian P., Balatonian P. and a Proto-Caspian Subprovince are defined. The biogeographic reorganisations are associated with five major biotic events within the gastropod faunas. These “big five” comprise four extinctions – the late Ottnangian extinction event (LOEE), the mid-Badenian-extinction-event (MBEE), the Badenian–Sarmatian-extinction event (BSEE), the Sarmatian–Pannonian-extinction event (SPEE) – and one exceptional immigration/origination event termed the early Badenian Build-up event (EBBE).Foraminifera are less valuable for palaeobiogeographic purposes in the area and time-slice studied. Endemisms are generally lower and turn-over rates less dramatic. They are, however, very useful for stratigraphic correlations with the Mediterranean and Atlantic areas. This fairly well calibrated marine biostratigraphy of the Central Paratethys serves as stratigraphic backbone and control for continental stratigraphy.

Late Silurian—early Devonian biogeography, provincialism, evolution and extinction

There is increasing marine to continental regression from the latest Silurian until the latter half of the early Devonian, when a major transgressive trend is initiated which achieves its maximum in the later middle Devonian and late Devonian. Data suggest a relatively high climatic gradient, but no evidence favouring continental, sea-level glaciation during the late Silurian-early Devonian. Arid climate evidence (marine evaporites, calcretes) shows a well-developed arid belt. Coal deposits are lacking before the late Devonian. Palaeogeography of the time interval is disputed, largely owing to the use of different classes of data - remanent magnetic, lithological, biogeographical. I employ a pangaeic reconstruction because it fits the available lithological and biogeographical data comfortably, but I am under no illusions about its being the ‘correct’ palaeogeography. Rate of phyletic evolution of marine benthos speeds up during the time interval owing to a steadily increasing level of provincialism, that is, cutting up biogeographical entities into smaller entities with consequent smaller populations. There are no major marine adaptive radiations, nor evidence for any marked extinction events during the interval. Few family level and higher taxa become extinct during this time interval; such units as the halysitid corals, pentamerinid brachiopods, and graptoloid graptolites are exceptional. Few adaptive radiations, such as those of the ammonoids and terebratuloids occur during the interval. The absence of other major biotic events during the interval is consistent with its position well within ecologic—evolutionary unit VI (A. J. Boucot,J. Paleont. 57, 1-30, 1983).