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Major Episode (major + episode)
Selected AbstractsA macroevolutionary mosaic: episodic host-switching, geographical colonization and diversification in complex host,parasite systemsJOURNAL OF BIOGEOGRAPHY, Issue 9 2008Eric P. Hoberg Abstract Aim, To integrate ecological fitting, the oscillation hypothesis and the taxon pulse hypothesis into a coherent null model for the evolution of complex host,parasite associations. Location, Global. Methods, This paper reviews and synthesizes literature that focuses on phylogenetic analyses and reciprocal mapping of a model system of hosts and their parasites to determine patterns of host,parasite associations and geographical distributions through time. Results, Host-switching and geographical dispersal of parasites are common phenomena, occurring on many temporal and spatial scales. Diversification involving both co-evolution and colonization explains complex host,parasite associations. Across the expanse of Earth history, the major radiations in host,parasite assemblages have been preceded by ecological disruption, ecological breakdown and host-switching in a context that can be defined by the concept of ecological fitting. This cyclical process sets the stage for co-diversification during periods of relative stability, punctuated by host-switching during episodes of regional to global environmental disruption and climatological change. Main conclusions, Most observed host,parasite associations can be explained by an historical interaction between ecological fitting, oscillation (episodes of increasing host range alternating with isolation on particular hosts) and taxon pulses (cyclical episodes of expansion and isolation in geographical range). Major episodes of environmental change appear to be the main drivers for both the persistence and diversification of host,parasite systems, creating opportunities for host-switching during periods of geographical expansion and allowing for co-evolution and co-speciation during periods of geographical isolation. [source] The Devensian periglacial record on Thanet, Kent, UKPERMAFROST AND PERIGLACIAL PROCESSES, Issue 3 2003J. B. Murton Abstract The Devensian periglacial record on Thanet, Kent, is traced from c. 88 to 74,ka and from c. 24 to 12,ka by optical luminescence dating of aeolian sand and silt in the periglacial stratigraphy. The record commences before 88,ka with valley cutting at Pegwell Bay. Valley filling had begun by c. 88,ka and continued to at least 74,ka, coinciding with a major episode of loess deposition in Europe. Permafrost aggradation commenced before c. 21,ka, brecciating near-surface chalk by ice segregation in permafrost and the overlying active layer. Deposition of aeolian sand (coversand) occurred at c. 24,21,ka, correlating with the Older Coversand I in mainland Europe. Permafrost degradation commenced at c. 21,ka, probably due to climate warming during Greenland Interstadial 2. The resulting active-layer deepening through ice-rich permafrost initiated soft-sediment deformation and formation of large-scale patterned ground in an active layer c. 2,m deep. Renewed permafrost aggradation between c. 21.25 and 18,ka coincided with climate cooling during Greenland Stadial 2c and led to cryoturbation in a thinner active layer. Final permafrost degradation commenced no later than c. 14.7,ka, that is, the start of Greenland Interstadial 1e, and may have occurred to some extent during the climate warming associated with Greenland Stadial 2b (c. 19.5,16.9,ka). Renewed deposition of aeolian sand took place at c. 15.5,ka, coincident with loess deposition on Thanet. A final episode of aeolian sand deposition occurred at 12,ka, correlating with the Younger Coversand deposits that are widespread in northwest Europe and formed during Greenland Stadial 1. Copyright © 2003 John Wiley & Sons, Ltd. [source] Mitochondrial DNA sequence analysis in SicilyAMERICAN JOURNAL OF HUMAN BIOLOGY, Issue 5 2001G. Vona This study reports data on the sequences of the first hypervariable segment of a sample of the Sicilian population from Alia (Palermo, Italy). The results show the presence of 32 different haplotypes in the 49 individuals examined. The average number of pairwise nucleotide differences was 4.04, i.e., 1.17% per nucleotide. The distribution of the nucleotide differences matches the theoretical distribution and indicates only one major episode of expansion that occurred between 20,732 and 59,691 years ago, between the Middle Paleolithic and Upper Paleolithic. Compared with the other populations, parameters of the Sicilian sample lie in an intermediate position between the eastern and western Mediterranean populations. This is due to numerous contacts that Sicily has had with the Mediterranean area since prehistoric times. At the same time, the singularity of some of the haplotypes present in the sample studied indicates the persistence of some characteristics caused by genetic drift and isolation that the population has endured in the course of its history. Am. J. Hum. Biol. 13:576,589, 2001. © 2001 Wiley-Liss, Inc. [source] Linkage of Sevier thrusting episodes and Late Cretaceous foreland basin megasequences across southern Wyoming (USA)BASIN RESEARCH, Issue 4 2005Shao-Feng Liu ABSTRACT Deposition and subsidence analysis, coupled with previous structural studies of the Sevier thrust belt, provide a means of reconstructing the detailed kinematic history of depositional response to episodic thrusting in the Cordilleran foreland basin of southern Wyoming, western interior USA. The Upper Cretaceous basin fill is divided into five megasequences bounded by unconformities. The Sevier thrust belt in northern Utah and southwestern Wyoming deformed in an eastward progression of episodic thrusting. Three major episodes of displacement on the Willard-Meade, Crawford and ,early' Absaroka thrusts occurred from Aptian to early Campanian, and the thrust wedge gradually became supercritically tapered. The Frontier Formation conglomerate, Echo Canyon and Weber Canyon Conglomerates and Little Muddy Creek Conglomerate were deposited in response to these major thrusting events. Corresponding to these proximal conglomerates within the thrust belt, Megasequences 1, 2 and 3 were developed in the distal foreland of southern Wyoming. Two-dimensional (2-D) subsidence analyses show that the basin was divided into foredeep, forebulge and backbulge depozones. Foredeep subsidence in Megasequences 1, 2 and 3, resulting from Willard-Meade, Crawford and ,early' Absaroka thrust loading, were confined to a narrow zone in the western part of the basin. Subsidence in the broad region east of the forebulge was dominantly controlled by sediment loading and inferred dynamic subsidence. Individual subsidence curves are characterized by three stages from rapid to slow. Controlled by relationships between accommodation and sediment supply, the basin was filled with retrogradational shales during periods of rapid subsidence, followed by progradational coarse clastic wedges during periods of slow subsidence. During middle Campanian time (ca. 78.5,73.4 Ma), the thrust wedge was stalled because of wedge-top erosion and became subcritical, and the foredeep zone eroded and rebounded because of isostasy. The eroded sediments were transported far from the thrust belt, and constitute Megasequence 4 that was mostly composed of fluvial and coastal plain depositional systems. Subsidence rates were very slow, because of post-thrusting rebound, and the resulting 2-D subsidence was lenticular in an east,west direction. During late Campanian to early Maastrichtian time, widespread deposits of coarse sediment (the Hams Fork Conglomerate) aggraded the top of the thrust wedge after it stalled, prior to initiation of ,late' Absaroka thrusting. Meanwhile Megasequence 5 was deposited in the Wyoming foreland under the influence of both the intraforeland Wind River basement uplift and the Sevier thrust belt. [source] Neoproterozoic Mafic Dykes and Basalts in the Southern Margin of Tarim, Northwest China: Age, Geochemistry and Geodynamic ImplicationsACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2010Chuanlin ZHANG Abstract: Neoproterozoic rifting-related mafic igneous rocks are widely distributed both in the northern and southern margins of the Tarim Block, NW China. Here we report the geochronology and systematic whole-rock geochemistry of the Neoproterozoic mafic dykes and basalts along the southern margin of Tarim. Our zircon U-Pb age, in combination with stratigraphic constraint on their emplacement ages, indicates that the mafic dykes were crystallized at ca. 802 Ma, and the basalt, possibly coeval with the ca. 740 Ma volcanic rocks in Quruqtagh in the northern margin of Tarim. Elemental and Nd isotope geochemistry of the mafic dykes and basalts suggest that their primitive magma was derived from asthenospheric mantle (OIB-like) and lithospheric mantle respectively, with variable assimilation of crustal materials. Integrating the data supplied in the present study and that reported previously in the northern margin of Tarim, we recognize two types of mantle sources of the Neoproterozoic mafic igneous rocks in Tarim, namely the matasomatized subcontinental lithospheric mantle (SCLM) in the northern margin and the long-term enriched lithospheric mantle and asthenospheric mantle in the southern margin. A comprehensive synthesis of the Neoproterozoic igneous rocks throughout the Tarim Block led to the recognition of two major episodes of Neoproterozoic igneous activities at ca. 820,800 Ma and ca. 780,740 Ma, respectively. These two episodes of igneous activities were concurrent with those in many other Rodinian continents and were most likely related to mantle plume activities during the break-up of the Rodinia. [source] | |||||||||||||