• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 46
  • 21
  • 9
  • 2
  • 2
  • 1
  • Tagged with
  • 405
  • 29
  • 15
  • 13
  • 10
  • 10
  • 8
  • 8
  • 8
  • 7
  • 6
  • 6
  • 6
  • 6
  • 6
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

A field-based and literature-based study of the lundgreni (Late Wenlock, Silurian) graptoloid extinction event

Muir, Lucy A. January 2003 (has links)
The extinction event at the end of the <i>lundgreni</i> graptoloid biozone was one of the most severe extinction events to affect graptoloids during the Silurian. There are many hypotheses of what caused the extinction, but little work has been done relating the extinction to graptoloid ecology. In this study, the <i>lundgreni</i> extinction event has been investigated in the field and from existing literature. Fieldwork has been conducted in the UK and Australia. Data gathered from the literature has been used to compile a global database of species occurrences before and after the event. Both the field-based and the literature-based studies have been used to test hypotheses of the ecological causes of the <i>lundgreni</i> extinction. The hypotheses considered include that extinction is random, that there is a correlation between geographical distribution and extinction probability, and that the life history strategy of a species (whether the species is K – or r-selected) affects extinction probability.

Some recent marine ostracods from the Mediterranean

Athersuch, J. January 1977 (has links)
This work is primarily a taxonomic revision of several genera of Ostracoda from the Mediterranean, belonging to the superfamily Cytheracea Baird, 1850 (i.e. Loxoconcha Sars, Xestoleberis Sars, Urocythereis Riuggieri, Cytheretta Muller and Loculicytheretta Ruggieri). In Chapters 1-6, the carapaces and appendages of forty-four species are described land illustrated, and seven new species and one new genus (Sagmatocythere) are introduced. The diagnostic features of each species are discussed, while the synonomy, ecology and distribution are reviewed in all cases. In addition, unusual morphological and anatomical features of several taxa are discussed, and their function and taxonomic significance are commented upon. (e.g. celation and fossal patterns in Urocythereis, Xestoleberis spots and labia in Xestoleberis, loculi in Loculicytheretta). Chapter 7 is a census of the G.W. Muller ostracod collection, and shows that most of the remaining specimens are badly decalcified. Chapter 8 is devoted to the autecology and faunal affinities of the Recent ostracods of Cyprus. This investigation indicates that the nature of thee biotope is a major factor controlling the distribution of living ostracods. There are four appendices which deal individually with ostracod thanatocoenoses around Cyprus, twenty additional species, preparative techniques and detailed sample data. The thesis is illustrated throughout with line drawings and S.E.M. photographs mainly in stereo-pairs.

The British Lithostrotiontidae

Nudds, John R. January 1975 (has links)
A revision and systematic study of the British species of the Carboniferous rugose coral family Lithostrotiontidae is described. No revision of this group has appeared since the middle of the last century. Results show that 15 described cerioid species of Lithostrotion can be lumped into four species. In addition L. decipiens depressum subsp. nov, is described in this group. The fasciculate species of Lithostrotion have been extended by the separation of L. variabile sp. nov., L. scaleberense sp, nov., L. junceum communicatum subsp. nov, L. martini praenuntium subsp. nov. and L. martini simplex subsp. nov. The genus piphyphyllum is shown to be polyphyletically derived and its species are referred back to Lithostrotion. The monospecific genus Nemistium is considered synonymous with Lithostrotion and ranoved. Its one species is an evolutionary intermediate between two species of Lithostrotion. The genus Orionastraea is reviewed and 0. sera sp. nov. is described. Two species of Orionastraea are referred to Hadsonia gen, nov. which, though similar morphologically to Orionastraea, has a different ancestral species. H. matura Sp. nov. is described in this genua. The genus Aulina is excluded from the Lithostrotiontidae as it is thought to have different ancestors. It is restricted to include only forms with massive coralla and A. botanica sp. nov. and A. rotif ormis aphroidia subsp. nov, are described. Fasciculate species of Aulina are referred to Easoioaulina gen, nov. as they are apparently unrelated to Aulina sensu stricto. The phylogeny of the Lithostrotiontidae is described and shown to proceed by following certain evolutionary trends which lead to an increasing level of colonial it within the group. The evolutionary centre and therefore the palaeomigration directions of the Lithostrotiontidae are shown to change during the phylogeny and this is related to the global palaeogeography of the Carboniferous period. Discovery of well preserved material has allowed the skeletal changes during the hystero-ontogeny of L. martini to be determined. This revision has led to a better understanding of the species in the Lithostrotiontidae so that their value as statigraphical zonal indices has been increased.

The stratigraphy and palaeontology of the Ordovician to Devonian rocks of the area north of Dornes (near Figueiródos Vinhos), central Portugal

Cooper, Anthony H. January 1980 (has links)
The Ordovician, Silurian and lower Devonian rocks of the Domes area (near Figueiro dos Vinhos), in central Portugal, form a continuous sedimentary sequence approximately 1450m thick. Folded Pre-Ordovician "Complexo xisto-grauvaquico" (CXG) is unconformably overlain by a transgressive quartzite, mudstone and greywacke succession of four Ordovician formations which are locally shelly and graptolitic. These formations are: Serra do Brejo Formation, Brejo Fundeiro Formation, Monte da Sombadeira Formation and Monte do Carvalhal Formation. The Ordovician rocks are conformably overlain by a Silurian sequence of graptolitic mudstones passing upwards into sandstones; these beds include the Vale da Ursa Formation, Foz da Serta Formation, Vale do Serräo Formation, Serra da Mendeira Formation and part of the Serra do Luacäo Formation. The Devonian rocks are represented by part of the Serra do Luagao Formation and the Dornes formation, a sandstone-madstone sequence passing up into Shelly limestones. Most of the Ordovician faunas are described and discussed including sixteen species of trilobite, five species of brachiopod and eleven ichnospecies of trace fossil. Faunal lists of material identified by specialists in the fields of Ordovician and Silurian graptolites, Ordovician echinoderms, Devonian brachiopods and Silurian microfossils are also given. The Ordovician faunas belong to the Selenopeltis province and show close links with those of Spain, France, Morocco and Bohemia. The correlation and palaeogeography of the Dornes area is discussed with emphasis on the Ordovician Period in the Ibero-Armorican area.

Studies on the palaeontology of the Lower Oxford Clay of southern England

Duff, Keith Leslie January 1974 (has links)
The Lower Oxford Clay (Middle Callovian, Upper Jurassic) of southern England has been studied in detail, in order to elucidate the stratigraphy, bivalve taxonomy and palaeoecology of the formation. Examination of four quarries in the Midlands, between Peterborough and Calvert (Bucks.), has allowed a detailed stratigraphy to be established, based upon that of Callomon (1968), and to which it has been possible to relate the range and occurrence of all the invertebrate species collected. The position of the Obductum-Grossouvrei Subzonal boundary at Stewartby (Bedford), which was not located by Callomon, has been established, on the basis of recurrent bivalve populations. The taxonomy of the bivalve fauna has been treated monographically, 48 species, 7 of which are new, having been described. One new genus, Byssentolium gen. nov. is introduced, with B. hudsoni sp. nov, as its type species. The subgenus Trautscholdia Cox , Arkell is elevated to full generic rank, and the generic assignations of several of the most well-known Upper Jurassic bivalve species are amended. The remainder of the Lower Oxford Clay invertebrate fauna has been considered in general terms, and its palaeoautecology inferred; the palaeo-autecology of the bivalve fauna has been dealt with in detail. By using the mass of detailed information collected at the four major quarries, a palaeoecological reconstruction of the environmental conditions established during the deposition of the Lower Oxford Clay has been made. Ten biofacies types have been defined and recognised at all the major quarries, and are characterised by 5 ecologically defined parameters. The benthonic fauna of several other Jurassic and Cretaceous Clay Formations has been compared with that of the Lower Oxford Clay, showing that only the Upper Lias of England is closely comparable. Consideration of the composition of deposit-feeder dominated communities since the Lower Palaeozoic has shown the major difference to be the replacement of the suspension-feeding brachiopods by suspension-feeding bivalves, a consequence of mantle fusion and siphon formation.

Late quaternary palaeoecology and phylogenetics in southern South America

Johnson, Kate January 2014 (has links)
Vegetation in South America has been impacted by multiple Pleistocene glacial events, climatic changes, volcanic eruptions, fire and human settlement on different temporal and spatial scales. Vegetation dynamics have been inferred from previous pollen studies, but some areas remain vastly understudied, despite the presence of suitable palaeoecological sites. Impenetrable volcanic tephra layers often restrict the ages of these sediments, limiting the age of palaeoecological studies. Combining palaeoecology with phylogenetics increases our understanding of the timing and spatial changes of vegetation movement. Two lake sediment sequences in central Argentina were analysed for pollen, and the phylogenetic history of two trees (the threatened Araucaria araucana and disjunctly distributed Lomatia hirsuta) were studied to better understand the postglacial movement of these species. Pollen indicated that vegetation has been relatively stable for the last 5,000 cal. yr BP in the region, with major changes (such as the introduction of Pinus sylvestris, and opening of the forest canopy) occurring only with the arrival of European settlers around 150 years ago. Populations of Araucaria araucana expanded after the Last Glacial Maximum, but also survived in situ at the edge of the Northern Patagonian glaciers. Preliminary analysis of ancient DNA from Araucaria araucana shows the species' range has not changed in 2,500 cal. yr BP. Lomatia hirsuta is disjunctly distributed due to long-distance dispersal, although research in this thesis was not able to refine the timing of this population split. This research successfully reconstructed the vegetation history from two lakes during the late Holocene, and highlights the complexity of vegetation response to natural and anthropogenic forcings.

An investigation into the microbial ecology of alpine paleosols

Pentlavalli, Prasanna January 2015 (has links)
Soil is the most complex microbiome on the Earth. Specifically, very little is known about the microbial ecology and functional diversity of alpine paleosols. This work focuses on the microbial ecology of pristine paleosol sediments from the Western Alps, at a location called the "Col de la Traversette". We found that: 1) Microbial ecology studies on the alpine paleosols with 16S rRNA pyrosequencing analysis revealed several relationships. There was a strong inverse correlation between the Acidobacteria and the Verrucomicrobia populations from diverse samples. There was also a positive correlation between Verrucomicrobia populations and pH. 2) Looking at functional genes in the sediments, the diversity of the ring hydroxylating dioxygenases (RHO) in the pristine paleosols showed the occurrence of unique enzymes that appear to have structural characteristics that are consistent with a role in the oxidation of phenolic substrates. 3) A comparative analysis of the RHO genes from pristine paleosols with a well-studied TOO-OF1 from Pseudomonas putida revealed a novel conserved residue Gly-264. 4) Furthermore, an ancient archaeological query about Hannibal crossing the Alps in 218BC to invade Roman Italy was addressed by finding microbiological evidence from this metagenomic study.

Silurian trilobites from the St Clair limestone (Arkansas) and Clarita formation (Oklahoma)

Holloway, David John January 1978 (has links)
Trilobites of Wenlockian age from the St Clair Limestone of northern Arkansas are described and assigned to 81 species (38 named) and 38 genera. Three of the genera - Dipharangus, Lygdozoon and Kerinion - and 15 of the species are new. The trilobite fauna of the Clarita Formation Wenlockian of south central Oklahoma is similar in composition to that of the St Clair and some Clarita specimens are described to supplement the St Clair material or for comparison with it. The new species are described from the Clarita Formation. The similarity of the St Clair trilobite fauna to those from the Wenlockian of Britain and Czechoslovakia reflects the relative cosmopolitanism of Silurian faunas in general, and the closest affinity seems to be with other Wenlockian trilobite faunas of eastern North America, particularly with those of the Niagaran dolomites of the Great Lakes region. This may be in part due to the occurrence of both the St Clair and Great Lakes faunas in pure carbonate facies.

Hydrodynamic assessment of graptolite morphotypes

Rantell, Lesley January 2001 (has links)
Physical and mathematical modelling techniques have been used to investigate the hydrodynamic significance of some aspects of the morphology of graptoloids. Functional structures are identified which could have had an impact on interpretations of graptolite taxonomy, oceanography and evolution. Experiments, testing isolated specimens in seawater and scale models in oil, show that the nema, cauda and virgula had a profound effect on the orientation of scandent graptoloids. These structures acted as trailing stabilisers, enforcing a stable orientation on the rest of the colony. Isolated monograptids with hooked and simple thecae have their orientation controlled by the nema, and would have presented their rhabdosomes to water sicula aperture first. This orientation control function was particularly marked in experiments on immature specimens, probably because these early growth stages are preserved with full length nemas. Oil tank modelling highlighted the importance of mass distribution for colony orientation, and observations of flow patterns over detailed models in a wind tunnel indicated that the increased surface area provided by secondary structures (e.g. vanes) further enforced this function. Mathematical modelling of distal structures such as the nema suggested that they would have functioned as stabilisers for all scandent graptoloids if they were of sufficient length or surface area (i.e. were thickened or bore vanes). The rate at which they would have acted to stabilise the colony was a function of this length or width and the distribution of colony mass, with longer or wider structures responding faster to changes in current orientation. The graptolite fossil record contains numerous examples of specimens bearing long virgulae and vane structures. Spines are a common feature of the proximal ends of biserial and uniserial scandent graptoloids. The virgella is evolutionarily conserved, while other processes have evolved more than once. Spines appear on the sicular aperture or on the first one or two thecae.

Palaeontological studies on Scottish Silurian fish beds

Ritchie, Alexander January 1963 (has links)
No description available.

Page generated in 0.1024 seconds