Among the rich variety of trilobites inhabiting marine environments of the Ordovician Silurian and Devonian periods are those which form a small f,amily group known as the Odontopleuridae. The odontopleurid trilobites are distinguished from their contemporaries in having an elaborately spinose exoskeleton.

No description available.

The fire history in two geovegetation provinces in Yellowstone National Park was reconstructed to determine relations between postglacial climate change, fire frequency, and vegetation. Fire reconstructions were based on charcoal preserved in lake-sediment cores from the Central Plateau and Yellowstone-Lamar Provinces, and fire-related erosional events were inferred from changes in sediment magnetism. The fire records were compared with pollen data and paleoclimatic reconstructions for the past 17,000 years. In the Central Plateau Province, infertile soils restricted the vegetation response to Holocene climate changes, but fire frequency changed continuously as a result of variations in the intensity of summer drought. Fire frequency was highest (ten to 15 events/1000 years) when summer temperatures were high and effective precipitation was low in the early Holocene (ca. 10,000 cal yr BP). These conditions were caused by greater-than-present summer insolation and the expansion of die eastern Pacific subtropical high pressure system, which affected temperature and moisture conditions in the southern and central part of Yellowstone National Park. After 8000 cal yr BP, fire incidence decreased to present frequencies (two to three events/1000 years). The trend towards fewer fires coincided with decreasing summer insolation and cooler, effectively wetter conditions than before. In the Yellowstone-Lamar Province, fire frequency was moderate (six to ten events/l000 years) in the early Holocene. Pollen data indicate that summers then were warmer and wetter as a result of stronger-than-present monsoonal circulation that brought increased precipitation to the northern part of Yellowstone National Park during the summer insolation maximum. In the last ca. 2000 years, xerophytic vegetation has increased in the Yellowstone-Lamar Province, and fire frequency reached its highest levels (12 to 17 events/1000 years). The contrast in the two fire histories suggests that fire regimes reflect variations in the intensity of summer drought that resulted from variations in the seasonal cycle of insolation. Fire regimes in both provinces have been non-stationary during the Holocene as fire frequency changed continuously with millennial-scale climate variations. Fire incidence has also changed on submillennial-time scales. Several ca. 500-year periods were characterized by high fire occurrence in both provinces, including the period from ca. 500-1000 cal yr BP, which corresponded with the Medieval Warm Period.

No description available.

No description available.

The marine Lower Cretaceous sediments of northern, Boreal latitudes yield diverse and well-preserved calcareous nannofossil assemblages. The calcareous nannofloras of nine core and outcrop sections (of Ryazanian-Albian age) in England, Germany, the North Sea, and Barents Sea, are examined in detail (utilising 773 samples). Dense sampling of key outcrop sections (in particular, Speeton) enables calibration of the observed nannofossil events against ammonite zones. The Barents Sea sections, which are shown to be highly condensed, yield the northernmost Lower Cretaceous nannofossils to be described. The previously chaotic taxonomy of Lower Cretaceous nannofossils is revised, and a major rationalisation of genera proposed - this involves several generic emendations, and twenty five new combinations. Two new genera, Apertasphaera and Neoparhabdolithus, and nine new species are described - Apertasphaera jakubowskii, Calculites bumettiae, Clepsilithus maculosus, Cyclagelosphaera papilla, Nannoconus inomatus, Nannoconus pseudoseptentrionalis, Rucinolithus windleyae, Tegumentum bergeni and Tubodiscus parvus. In addition, one new subspecies - Rhagodiscus angustus parvus - is described, and a number of species are split into informal varieties, to avoid biostratigraphical ambiguity. Following these revisions, the suprageneric classification is reappraised, and a new family - Family Tubodiscaceae - is proposed. An updated nannofossil zonation scheme for the Ryazanian to Aptian of the Boreal area is presented; this comprises twenty two zones (labelled BC1 to BC22, in ascending order), and a number of additional subzones. Biostratigraphical resolution in the HauterivianBarremian is comparable with current ammonite zonations, and enables detailed sequence stratigraphical interpretation. Following the publication of new data from Tethyan sections (Bergen, 1994), the potential for direct inter-regional correlation is investigated; the nannofossil datums utilised generally support traditional, ammonite-based correlations but suggest that, in Boreal areas, the Hauterivian-Barremian boundary should be placed at the top of the variabilis ammonite Zone, slightly higher than its current position. Finally, the palaeoceanographical applications of Lower Cretaceous nannofossils are considered, in the light of new data on modem nannoplankton. Surface water temperature is believed to have been the principal factor controlling species distribution, throughout the history of nannoplankton; the importance of surface water fertility has probably been exaggerated in previous palaeoceanographical studies, which have been hampered by broad taxonomic concepts. There remains much to be done, in deducing the environmental preferences of Cretaceous nannofossils, but the potential for their application to palaeoenvironmental studies is good.

This thesis documents the response of calcareous phytoplankton assemblages to major changes in global climate during the Eocene-Oligocene transition (EOT) at three low-latitude sites Tanzanian Drilling Project (TDP) Sites 12 & 17 (southern coastal Tanzania), Deep Sea Drilling Site 242 (Mozambique Channel, western Indian Ocean) and St Stephens Quarry (Alabama, USA). High-resolution nannofossil count data from all three study sites clearly indicate a major change in coccolithophore assemblage compositions directly coincident with the first step change in the positive oxygen isotope (d180) shift into the early Oligocene Glacial Maximum (EOGM). This is followed by continued shifts in coccolithophore assemblages in the vicinity of the Eocene-Oligocene boundary to reach new "Oligocene" assemblage compositions before the second step in duO and peak glacial conditions of the EOGM. At all sites there is a marked increase in the abundance of Cyclicargolithus floridanus (>5um) and to a lesser extent Sphenolithus predistentus, combined with a general loss of warm-water and oligotrophic taxa including Calcidiscus protoannulus, discoaster and holococcolith species. This consistent pattern of biotic change, from sites with varying species compositions and preservation states, indicates a large and geographically widespread perturbation in the low-latitude surface water environment and is consistent with new trace metal paleothermometry that indicates a significant, 2-3 C, cooling of surface waters in the earliest stages of the EOT. An exceptionally-well preserved and highly diverse nannofossil assemblage from late Eocene-early Oligocene (nannofossil zones NP19/20 to NP21 Martini 1971) sediments of coastal Tanzania is described and its implications for the Cenozoic history of the coccolithophores is discussed. This consists of a total of 114 species-equivalent morphotypes, one new genus (Pocillithus), six new species (Reticulofenestra macmillanii, Blackites culter, Rhabdosphaera suptilis, Orthozygus occultus, Orthozygus arms, Pocillithus spinulifer) and a previously unknown Paleogene deep-photic zone assemblage including representatives of the extant genera Gladiolithus, Algirosphaera and Acanthoica.

No description available.

No description available.

No description available.