The development of an Early Ordovician hard ground community in response to rapid sea-floor calcite precipitation

The Kanosh Shale (Upper Arenig, Lower Ordovician) of west-central Utah. USA. contains abundant carbonate hardgrounds and one of the earliest diverse hardground communities. The hardgrounds were formed through a combination of processes including the development of early digenetic nodules in clay sediments which were exhumed and concentrated as lags by storms. These cobble deposits. together with plentiful biogenic metrical. were cemented by inorganically precipitated calcite on the sea floor. forming intraformational conglomerate hardgrounds. Echinoderms may have -played a critical role in the development of hardground faunas since their disarticulated calcite ossicles were rapidly cemented by syntaxial overgrowths. forming additional cobbles and hardgrounds. The echinoderms thus may have taphonomically facilitated the development of some of the hard substrates they required. A significant portion of the hardground cements may have been derived from the early dissolution of aragonitic mollusk shells. Kanosh hardground species include the earliest bryozoans recorded on hardgrounds and large numbers of stemmed echinoderms. primarily rhipidocystid cocrinoids. Bryozoans and echinoderms covered nearly equal areas of the hardground surfaces. and there was a distinct polarization between species which preferred the upper. exposed portions of the hardgrounds and others which were most common on undercut. overhang surfaces. The Kanosh Shale hardground fossils combine elements of Late Cambrian assemblages and Middle Ordovician faunas, thus confirming predicted trends in hardground community evolution. especially the replacement of cocrinoids by bryozoans and. to a lesser extent, by other stemmed echinoderms, especially crinoids. The Kanosh community marks the transition from the Cambrian Fauna to The Paleozoic Fauna in The hardground ecosystem. *Carbonate hardgrounds, aragonite dissolution, calcite cement, Echinodermara, Trepostomata, Nicholsonclla. Dianulites. Porifpra. taphonomic facilitation, Utah. Pogonip Group, Kanosh Shale. Ordovician.     

Vegetation of pastoralist campsites within the coastal grassland of central Somalia

Seasonal occupancy of pastoralist campsites where livestock are bedded down at night has created islands of soil fertility within the coastal grassland of central Somalia. Soil organic matter, nitrogen, phosphorous, and potassium concentrations were greatest at the centre of the campsites and decreased towards the perimeter. The vegetation on the campsites differed from the surrounding grassland, presumably in response to soil fertility and intense grazing. The fast-growing, grazing-tolerant, stoloniferous grass, Cynodon dactylon , and the non-palatable, annual forbs, Cleome tenella and Gisekia pharnaceoides , occupied the centre of the campsites. The campsite perimeters were dominated by the slow-growing, grazing-sensitive, perennial plants Cencrhus ciliaris and Indigofera intricata .

Résumé

L'occupation saisonnière des lieux de campement où le bétail est rentré pendant la nuit a créé des ilots au sol fertile dans la partie côtière de la Somalie centrale. La matière organique du sol, les concentrations en azote, phosphore et potassium, étaient plus élevées au centre du campement et diminuaient vers la périphérie. La végétation à l'intérieur des campements différait de la prairie environnante, résultat sans doute de la fertilité du sol et du pâturage intense. L'herbe à la croissance rapide, résistante au pâturage et stolonifèhe, Cynodon dactylon , et les désagréables et annuelles Cleome tenella et Gisekia pharnaceoides , occupaient le centre des campements. Le périmètre des campements était dominé par les plantes à croissance lente, sensibles au pâturage et perennes, Cenchrus ciliaris et Indigofera intricata .     

Species-specific growth responses of favositid corals to soft-bottom substrates

Gibson, Michael A. & Broadhead, Thomas W. 1989 07 15: Species-specific growth responses of favositid corals to soft-bottom substrates. Lethaia , Vol. 22, pp. 287–299. Oslo. ISSN 0024–1164.
Species of favositid corals from the Upper Silurian and Lower Devonian of Tennessee, USA, exhibit structural modifications related to corallum geometry, interfacial skeletal material, and biotic associations that enabled them to survive in terrigenous mud rich environments. Favosites conicus Hall (Lower Devonian) had a flat, holotheca-covered base and a radial pattern of colony growth, but apparently had a short life span and may not have survived beyond the first reproductive cycle (monocarpous). It was adapted for living between major episodes of terrigenous mud influx. F. foerstei (Lower Devonian) had a convex, pseudoholotheca-covered base and a modified axial pattern of colony growth. Its large size, in comparison to that of F. conicus , suggests a longer lived colony (polycarpous), in which continued upward and outward growth enabled it to survive episodic sediment influx. F. forbesi (Upper Silurian) exhibited radial growth to form either (1) a globose corallum that was symbiotic with the stalks of living crinoids permitting the colony to live entirely above the substrate, or (2) a Gorallum with a steeply convex, holotheca-covered base that represents a bottom-dwelling colony in which the rate of growth probably only slightly exceeded the rate of sediment accumulation. * Functional morphology, astogeny, paleoecology, Tabulata .     

Some population consequences of variation in preference among individual predators

A number of foraging studies have demonstrated that populations of predators rarely consist of individuals with identical preferences for particular types of prey. Variation among predators can lead to frequency-dependent changes in population preference, because those predators mat prefer the rarer type of prey generally have the greatest influence on population preference. In this study we develop a series of theoretical models which demonstrate how anti-apostatic selection (i.e. selection against the rare form) can arise out of (a) bimodal and (b) normal variation in preference among individuals of the same species. We show that population level anti-apostatic selection can occur even when individual predators show pro-apostatic selection (i.e. selection against the common form). Furthermore, patterns of population prey selection that arise out of variation in preference can potentially be pro-apostatic over one range of relative densities and anti-apostatic over another range of relative densities. Finally, we examine a case study involving predation by female waterboatmen Notonecta glauca and show that the variation in preference in this species is large enough to generate higher anti-apostatic selection than would be expected from the diet selected by the average individual.